art conservation and restoration


art conservation and restoration
Maintenance and preservation of works of art, their protection from future damage, deterioration, or neglect, and the repair or renovation of works that have deteriorated or been damaged.

Research in art history has relied heavily on 20th-and 21st-century technical and scientific advances in art restoration. Modern conservation practice adheres to the principle of reversibility, which dictates that treatments should not cause permanent alteration to the object.

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Introduction
 any attempt to conserve and repair architecture, paintings, drawings, prints, sculptures, and objects of the decorative arts (furniture, glassware, metalware, textiles, ceramics, and so on) that have been adversely affected by negligence, willful damage, or, more usually, the inevitable decay caused by the effects of time and human use on the materials of which they are made.

      The term art conservation denotes the maintenance and preservation of works of art and their protection from future damage and deterioration. Art restoration, by contrast, denotes the repair or renovation of artworks that have already sustained injury or decay and the attempted restoration of such objects to something approaching their original undamaged appearance. The techniques and methods of art conservation and restoration go hand in hand and became the province of trained professionals in the 20th century. They have become an increasingly important aspect of the work not only of museums but also of civic authorities and all those concerned with works of art, whether artists, collectors, or gallerygoers. The methods of art restoration used in earlier periods were closely linked to and limited by the art production techniques known at the time. Advances in science and technology and the development of conservation as a profession in the 20th century have led to safer and more effective approaches to studying, preserving, and repairing objects. Modern conservation practice adheres to the principle of reversibility, which dictates that treatments should not cause permanent alteration to the object. Art conservation has become an important tool of research; it is standard practice among professional conservators to document treatments with photographs and written reports.

      The conservation and restoration of older architecture is an increasing modern preoccupation. The earliest buildings to have survived generally tend to be those that received religious veneration. When these structures were no longer venerated, they disappeared like other buildings. It was not until the early 19th century that the Forum of ancient Rome was uncovered and explored.

 Medieval builders treated the work of their forebears with a healthy lack of awe. Every new Gothic chapel or chantry and virtually every stage in the development of a single Gothic cathedral followed the style of its own day. With the Renaissance in Europe grew a new respect for classical antiquity and a new interest in its architectural forms. By the end of the 18th century a knowledge of archaeology had become an accepted accomplishment of the educated man. Architectural design itself became a matter of “correctness.” Old buildings everywhere began to be “restored” to the style of periods especially favoured. The French architect and writer E.-E. Viollet-le-Duc (Viollet-le-Duc, Eugène-Emmanuel) brilliantly restored the Sainte-Chapelle (1840–67) and the cathedral of Notre-Dame de Paris (1845–64). The ancient walls of Carcassonne in France and of Windsor Castle in England (United Kingdom) were not only repaired but also largely rebuilt.

      With the spread of the Industrial Revolution and the increasing reliance on mechanical processes, the labour of hands became more costly, and the value of craftsmanship gained a new significance. Old buildings, which often exhibited the personal touches of master craftsmen, began to command a new respect, and the English art critic John Ruskin (1819–1900) was even able to assert that “the greatest glory of a building is its age.” In 1877 the pioneers of the conservation movement, led by the English artist and writer William Morris (Morris, William) (1834–96), founded the Society for the Protection of Ancient Buildings (SPAB). Nicknamed Anti-Scrape, the society vehemently opposed the indiscriminate refacing of old stonework and the “conjectural restorations” still so fashionable, such as the new west front of St. Albans Cathedral in England (1880–83). The movement gathered force, and in the 20th century groups throughout the world devoted their efforts to architectural conservation.

      An added local impetus has been given by national pride; in countries such as Poland, postwar reconstruction became the symbol of national resurgence. Almost every country is increasingly conscious of its heritage of ancient buildings, while cultural bodies such as the United Nations Educational, Scientific and Cultural Organization (UNESCO) have lent to the conservation movement a powerful international impetus.

Effects of economic and social change
      The development of architecture can be read as a sensitive index of social change. The economic climate and social preoccupations of each age have combined to generate its own architecture and its own towns. Almost every decade of new building displays its own peculiar characteristics and modifies by constant adaptation the buildings and towns of yesterday. But the urban environment is society's investment in its future, and the cycle of renewal is continuous, if often slow. Thus, the problems of building maintenance and renewal are complicated by long-term economic and social change.

      Today the most marked trends are still those that brought about the conservation movement itself. First is the accelerated pace of physical growth. Old buildings have become not only relatively rare but often virtually irreplaceable in terms of labour and craftsmanship and sometimes of materials. In many cases, old buildings give to a locality much of its special character and identity, as, for example, in those English country villages where thatched roofs still predominate. Another and rarer asset is a sheer and intrinsic merit of architectural form. And alongside all these is the tangible evidence that any old building provides for its community a kind of social and environmental continuity—a reassuring reference point in a constantly changing world.

      Under the increasing pressure of the world's growing population, the value of urban land continues to climb steeply, with some curious effects on the fate of old buildings. Increased demand brings increased land values and, at first, better prospects for the repair and maintenance of old buildings. But as land values rise higher, the older building must also justify itself in terms of economic efficiency. All over the world, the town houses of the 19th century and earlier serve with varied grace in the 20th century as centres of modern industry and commerce. Their fabric is subjected to new strains, and their room shapes and capacity may become incompatible with new and changed demands. There comes a point at which the old building on a valuable urban site can compete no longer with the pressure for redevelopment. Then it is quickly overtaken, and financial subsidy is powerless to protect it from demolition. The old building in a deteriorating neighbourhood is at the same time likely to be in no better a situation. Its maintenance may become no longer economically worthwhile, condemning it to early death by neglect. The destructive effects of both over- and undervalue are clearly displayed side by side in a fine Georgian city such as Dublin or in once-distinguished neighbourhoods such as Bloomsbury in London or the Marais in Paris. The most successful neighbourhood conservation occurs where values have been held in pace with the architectural capacity of a community, as at Bath in England or in the Georgetown section of Washington, D.C.

      Another social change is the rapid increase in mobility. The automobile brought better roads and an incentive to use them. Old city centres, after centuries of essentially domestic life, began to be abandoned in favour of ring upon ring of suburbs. This peripheral accretion of cities is allied with their central decay as communities. As a universal result, the twice-daily thrombosis of the highways urges on a constant process of road widening, in which many an intervening historic area has been completely eroded away.

Role of law
      In all conservation of architecture, the first effective step is to decide and define what buildings or sites are worthy of protection. For most countries this has involved a systematic process of inventory and survey. In Great Britain, for example, the Royal Commission on Historical Monuments (RCHM) was set up in 1908, and the Civic Amenities Act of 1967 enabled local planning authorities to define special areas for “conservation and enhancement.” In France, the Commission des Secteurs Sauvegardés was set up in 1962 under André Malraux, minister for cultural affairs, to pursue an active program for public protection of historic areas. In the United States, the Historic American Buildings Survey was designed to assemble a national archive of historic American architecture.

      Criteria for conservation are rarely well defined. Architectural merit clearly must rank highly—especially in the case of any building that authentically exemplifies its period. Historical associations, such as the birthplace of a famous person, are less easily rated. One pernicious effect of all selection is the way in which the most outstanding architectural example of any period, rather than a truly typical example, is what in the end is chosen to remain as a representation of a particular period of architecture. Another is that defects as well as merits may be kept warm under the same blanket. This is particularly so in the larger groups of buildings that are coming to be recognized as worthy of conservation.

      Once a building has been targeted for preservation, its next defense is in specific legal powers for its protection. These may be of varied degree and effectiveness. The most obvious form of legislation is the restriction against demolition. A higher degree of legal sophistication occurs in powers for the annexation of property (property law) and its maintenance by the state. Covenanted rights and restrictions are a variant of this principle. Next in the scale of effectiveness comes positive encouragement to owners by means of grants, bringing a public share and interest in the work of repair. In this way, actual legal rights over private property may be confined to a minimum while finance is encouraged from private pockets. Probably the most effective ultimate defense is selective protection, exercised as a regular part of everyday town- and country-planning control.

      Negative legislation itself varies in degree. In Italy it is possible to insist upon the return even of certain pictures or chattels illegally dispersed from a building where these are adjudged to be of sufficient national importance. But negative powers are inherently weak. They convey no control over the philistine or intransigent owner and, at best, can only slow down neglect and demolition, whether deliberate or otherwise.

      The national acquisition of buildings for conservation in Britain has been carried out chiefly under the Ancient Monuments Consolidation and Amendment Act of 1913, by which suitable unoccupied properties can be “taken into guardianship.” A much more rigorous application of the principle is sometimes possible in the United States, whereby the owners of whole groups of buildings held to be of sufficient distinction can in fact be legally dispossessed. These erstwhile owners may then be allowed to remain in residence on condition of the repair and rehabilitation of their buildings to a specified standard. In this way, whole areas of buildings, such as Society Hill in Philadelphia, have been taken over, concentrated redevelopment by high-rise apartments being permitted in selected inner locations, while old buildings with frontage are restored in period styles.

  The most exhaustive of all restoration projects is in the United States, at Williamsburg, Virginia. This 170-acre (70-hectare) town, the colonial capital of Virginia from 1699 to 1780, has attracted the most expensive restoration program ever undertaken. Commenced in 1926, the project is dedicated to the purpose “that the future may learn from the past.” Careful and scholarly restoration has been completed on more than 500 buildings. Environmental management is of a high order. Tourist automobile traffic is excluded from the restored area in season, when a free bus service is provided. The emphasis is frankly educational. The enterprise not only owns its buildings but also staffs them, its employees wearing correct period costume.

      One of the most dramatic rescue operations has been in Egypt, where the ancient temples (temple) (c. 1250 BC) of Abu Simbel were threatened with destruction by the rising waters of the Aswān High Dam (Aswan High Dam). They were sawed into giant blocks and successfully reassembled 200 feet (60 metres) above the original site. This act of preservation was the result of intensive international negotiation and expertise.

      Another variant on public ownership may be found in acquisition by a private body, such as the National Trust in Great Britain. Founded in 1895, this property-owning body opens to the public several hundred of its properties. The trust receives no direct government subsidy and relies upon careful economic management, although certain legal preferences operate in its favour. In the United States the National Trust for Historic Preservation operates in a similar way.

      Among bodies devoted to grant aid, the Historic Buildings and Monuments Commission for England (as successor to the Historic Buildings Council) disburses grants within a modest annual budget, largely to help building owners penalized by heavy estate duties. These grants are administered to encourage owners to take pride in their own buildings. The commission is also responsible for the management of more than 400 monuments in the nation's care.

      A pioneer training program in architectural conservation has been established by the Faculty of Architecture of Rome University. Of six months' duration, the course provides specialist training in conservation for architects of all nationalities. In many countries, comparable courses are now available to meet the need for suitably qualified and experienced architects.

Techniques of building conservation
 The first requisite in conserving any building is a sensitive assessment of its history and merits. Every building has its own biography. The Parthenon in Athens, originally built (447 to 432 BC) as a temple, subsequently served as a Christian church, a mosque, and a powder magazine before it became one of the world's greatest attractions for the tourist and art lover. A knowledge of the whole life of a building brings an essential understanding of its features and its problems.

      Next, the conservator needs a thorough measured survey (surveying). Generally, this is prepared by hand, with tape and rod and level. Modern measuring techniques, including photogrammetry and stereophotogrammetry, are also used and are quick and remarkably accurate.

 Third, the architect or surveyor analyzes the structural (building construction) stability of the subject and its living pattern of movement. No structure is permanently still. Subsoil expands and shrinks, thrust moves against thrust, and materials move with heat and wind. Forceful exercises, such as English bell ringing, have an even greater effect on a building's stability. clay soil is the worst: the building protects the ground underneath but not around; and, with every downpour, a wall on saturated clay may vary the lean of the building. Many ancient buildings had piled foundations—at Winchester, the cathedral was supported on oak piles, which rotted over the centuries. In order to underpin the structure, a diver worked for months in the waterlogged soil. Framed structures can move a great deal. The skeleton of a timber-framed medieval house can be extremely crooked without losing strength if it is well triangulated and its joints are sound. A wall is theoretically safe until it leans far enough to develop tension on one side, yet even then it may be stiffened by structural cross walls. Generally, the old, evenly spread load will be stable, and any new point load or thrust will be suspect. The surveyors may check the observations over a period—e.g., by measurement with plumb lines or by simple “tell-tales” (marking devices) set across a crack, or now by electronic measuring devices of remarkable accuracy.

      The surveyor lastly tests all services, especially electrical wiring, with its risk of fire; gas lines, with their perils of seepage and explosion; and plumbing, with its danger of leaks. These services are frequently redesigned and simplified as well as improved. Lightning conductors and fire-fighting equipment are an important part of the protection of any ancient building.

      The conservator must analyze the good points and bad points of the building, in the context of its current and future use, and define remedies in terms of their relative urgency. He can then prepare a balanced and phased conservation plan, related to the available budget.

      The first remedial task is to stabilize and consolidate the structure. Ideally, this is best done by restraining, or tying, the point of active thrust and then by replacing, splinting, or in some way giving fresh heart to any failing or defective member. Adding heavy weights such as buttresses (buttress) can do more harm than good. A load can frequently be spread more widely or more evenly. A structure can, in effect, be corseted by inserting (for example, around a tower) a continuous beam or ring of concrete. This can be done even in delicate masonry and, as in underpinning, by removing alternate sections of a wall, threading in reinforcement, and casting successive sets of concrete stitches, which unite into one strengthening beam. Sometimes a metal rod or tie bar may be inserted along a direct line of thrust or weakness, linking structural elements in need of support.

      After structural movement, the next serious adversary in building conservation is damp (humidity). Not only of itself but also allied with almost every other trouble, damp accelerates decay. Weather may be penetrating through whole surfaces, such as porous brickwork, or finding its way through cracks or defects in the roofing. Especially vulnerable are gutters or any part of the rainwater-collecting system. Wet weakens walling, rots timbers, and spoils finishes. The remedy may involve renewing roof finishes. It may entail inserting a continuous moisture barrier, perhaps in a modern material such as stout polyethylene. Techniques of waterproofing wet walls include the insertion of high-capillary tubes, designed to draw the moisture to themselves and to expel it, and also the injection of silicone or latex and similar water-repellent solutions into the heart of the walling. Simple methods are best. The traditional ditch, or dry area, drained if necessary, disposes of the water before it reaches the wall. Double or cavity walls, with air between them, are another defense against damp.

      Again, dampness compounds decay, and the first attention should be to protective features such as copings. Both in stonework and in brickwork, much harm can be caused by damp, especially when allied with an overly hard mortar jointing. This traps moisture along the lines of the joints, bringing any harmful salts to the surface, where they crystallize and damage the facing. Mortar jointing should always be softer than the brick or stone of a wall.

      Much decay is the result of poor construction. Defects are almost always accelerated by the simple contravention of good building practice. In walling, a typical cause of structural instability is a double-skin construction with rough rubble between in which, by uneven loading, one skin has been caused to bulge and to release loose material in the core of the wall. Once on the move, this rapidly gains momentum as a live wedge, forcing apart its two faces. The conservator will insert temporary support, then remedy any uneven loading and rebuild the affected area. In some cases, after loose material is washed out, the unseen cavities can be grouted up, which strengthens a wall without disturbing the facing stonework.

      The roof is a building's first defense. It must be impervious and collect water clear of a building. Roof finishes are commonly either of unit materials such as tiles, slates, or stone or of boarding covered in sheet metal, such as lead. The failure of unit materials is usually caused by decay of fixings. Iron nails are especially destructive and are best replaced by nonferrous materials, such as copper. The battens that carry the tiles or slates have a longer lifespan but also need periodic renewal. leadwork failure is usually the result of sheer age. This material has a very long life but, if used in sheets of excessive size, has a tendency to buckle and creep as a result of expansion—especially in sunshine. Leadwork can readily be recast or can be repaired by lead burning a new patch to the original lead. Soldering is less reliable and tends to crack away.

      The chief enemies of timber (wood) are the natural predators of the forest—fungi (fungus) and wood-boring insects (insect). The most voracious fungus that attacks building timbers is dry rot (Merulius lacrymans). This can spread along infected wood to sound timber, carrying its own moisture supply. It extracts cellulose, which forms the chief part of plant cells, and leaves behind a tindery and useless shell. Stagnant air and warmth accelerate its spread. Eradication must be thorough, or the trouble will rapidly reestablish itself. Modern fungicides are highly effective.

      Wood-boring insects include the furniture and deathwatch beetles (deathwatch beetle). From eggs laid in cracks, the larvae tunnel into timber and damage it before emerging as beetles to lay more eggs. The deathwatch beetle inhabits mostly the outer sapwood of oak, when wet or softened by rot. The furniture beetle lives mostly in deal, especially when sappy or damp. Both of these species can be eradicated with modern pesticides.

      Regular maintenance is the key to building conservation; William Morris called this practice “daily care.” A building's life can be long, human tenancy relatively short. Yet the cumulative effect of neglect can be desperately damaging. Conversely, a sensitive awareness of a building's needs, with regular attention to them, will extend its life and promote its long enjoyment. The successful conservator identifies himself with a building's life, its structure and demands, with the special needs of an occupant, and with the skills of today's craftsmen. In this spirit, he can hand on to the future the best of the past.

Donald W. Insall

Paintings (painting)
      Broadly speaking, most paintings can be divided into (1) easel paintings, on either canvas or a solid support, usually wood; (2) wall, or mural, paintings; and (3) paintings on paper and ivory. The conservator of paintings aims above all at “true conservation,” the preservation of the objects in conditions that, as far as possible, will arrest material decay and delay as long as possible the moment when restoration is needed. The correct choice of conditions of display and storage is, therefore, of the first importance. Ideally, each type of painting requires its own special conditions for maximum safety, depending on the original technique and materials used to compose it.

      Portable paintings on canvas or panel are called easel paintings (easel painting). Basically, they consist of the support (the canvas or panel); the ground, ordinarily a white or tinted pigment or inert substance mixed with either glue or oil; the paint itself, which is composed of pigments held in a binding medium such as drying oil, glue, egg, casein, or acrylic; and, finally, the surface coating, usually a varnish, to protect the paint and modify its appearance aesthetically. These four layers have many variants but must be constantly borne in mind when considering the problems of conservation.

Paintings on wood (panel painting)
      Wood has been used as a support since the encaustic paintings of ancient Greece. Wood-panel supports were used almost universally in European art in devotional icons and other works before the 16th century, when the use of canvas became dominant. Wood has the disadvantage of swelling and shrinking across the grain when there are variations in the relative humidity of the atmosphere. In northern temperate climates, variations in humidity can be considerable. In England, for example, the seasonal variation in a museum that is centrally heated in the winter can be from 25 percent in midwinter to 90 percent in summer. Although paint has a certain elasticity, it cannot usually take up much movement, and in paintings on wood it generally cracks in a network referred to as “craquelure.” In continental landmasses, such as the United States, the average relative humidity in dry zones may be consistently low, so that European paintings with wooden supports “air-seasoned,” or accustomed, to a higher humidity may suffer considerably. In both Europe and the United States, the combination of an unsuitable environment of low or changing relative humidity with the restraining effect of the paint layer often produces a permanent bowing of the panel, which is convex at the front surface.

      To counteract both the shrinkage and the bowing (especially the latter), restorers in the past placed wooden strips called battens, or more complex structures called cradles, across the back of the panel as constraints. This solution, however, often produced internal stresses that led to severe distortion of the front surface, cracking of the panel along the wood grain, and in some instances extensive damage to the paint. This form of intervention has been largely abandoned in favour of an environmental approach that places the emphasis on the maintenance of a stable environment that fosters preservation. The ideal conservation solution is a form of air conditioning (air-conditioning) in which the relative humidity is maintained as much as possible at what is generally agreed to be the most reasonable level—i.e., about 55 percent. It is normal by modern standards to accept as inevitable some permanent convex curvature.

      When warping and cracking have already occurred or when the latter seems likely as a result of the mistaken application of secondary supports, such as cross battens, expert restoration treatment is required. In principle, this consists of removing the cross battens and applying a reinforcement to the back that imposes a uniform but gentle constraint over the whole surface. In the past, when the wood was badly worm-eaten or dimensionally unstable, the wooden support was occasionally removed from the paint and ground layers in the process known as “transfer.” This was accomplished by temporarily adhering (adhesive) a substantial support of paper and, possibly, canvas to the front surface and then cutting away the wood on the back. An entirely new support, of either panel or canvas, was then adhered to the back, and the temporary facing was removed. This treatment is very rarely done today and is generally considered to be an extreme form of intervention.

Paintings on canvas
      Painting on canvas became common in the 16th century, as aforementioned, and has been used largely in European and American painting traditions. A canvas support expands and contracts with variations in relative humidity, but the effect is not as drastic as with wood. Canvas, however, will deteriorate with age and acidic conditions and may be easily torn. In many cases, parts of the paint and ground will lift from the surface, a condition variously called “cleavage,” “flaking,” “blistering,” or “scaling.” The traditional method to address these problems is to reinforce the back of the canvas by attaching a new canvas to the old in a process called “lining,” also referred to as “relining.” A number of techniques and adhesives have been employed for lining, but with all methods there is a risk of altering the surface texture of the painting if the procedure is not carried out with the utmost care and skill. The most frequently used technique until the mid-20th century consisted of ironing a new canvas to the old, using an adhesive composed of a warm mixture of animal glue and a farinaceous paste, sometimes with the addition of a small proportion of plasticizer. This method, though less common today, is still used, especially in Italy and France. It has the advantage that the heat and moisture help to flatten raised (“cupped”) paint and local deformations and tears in the canvas. Another method, introduced after the mid-19th century, uses a thermoplastic wax-resin mixture. Originally executed with heated irons as in the glue-paste method, it increased in popularity by the introduction, around 1950, of the so-called “vacuum hot table.”

      With this table, the two canvases are coated with molten adhesive (at about 160 °F [70 °C]) and joined together on an electrically heated metal plate. They are covered with a membrane, enabling the air between the two canvases to be evacuated with a pump through holes at the corners of the table; adhesion then occurs on cooling. Excessive vacuum pressure and heat can drastically alter a painting's texture. In addition, during this process, wax penetration can darken canvas and thin or porous paint layers. To overcome this latter defect, “heat-seal” adhesives were introduced in the late 1960s. Formulations containing synthetic resins, including polyvinyl acetate and, increasingly, an ethylene-vinyl acetate copolymer, are applied in solution or dispersion to the surfaces and, after drying, are adhered on the hot table. Ethylene-vinyl acetate copolymer adhesives are also available as dry, nonpenetrating films. More recently, cold-setting polymer dispersions in water have been introduced by using a low-pressure suction table, from which the water is removed through spaced perforations in the table surface with a powerful downdraft of air. Pressure-sensitive adhesives have also been introduced as lining adhesives but have not been widely adopted. Although all these methods are currently in use, the trend has been to move away from lining and wholesale treatments in general in favour of more refined, precise, and limited treatments that address condition problems in a more specific way.

      The low-pressure suction table mentioned above and a smaller device used for localized treatment generally referred to as a “suction plate” have gained wide use at the turn of the 21st century. The more elaborate versions of this instrument are equipped with heating elements and humidification systems beneath the perforated table surface. These features make it possible to apply controlled humidity, heat, and gentle pressure to perform a variety of treatments, including tear realignment and repair, reduction of planar deformations, and the introduction of consolidating adhesives to reattach cleaving paint. The practice of edge lining (sometimes referred to as “strip lining”), which has been increasingly used as an alternative to overall lining, aims to reinforce weak and torn edges where the canvas is prone to give way. This treatment is often used in conjunction with local or overall treatments executed by using the suction table and suction plate.

      In the past, paintings have occasionally been transferred from wood to canvas by a variant of the treatments described above. The reverse of this—i.e., attaching a painting on canvas to a stable rigid support (a process known as “marouflage”)—is still sometimes done for various reasons.

      The ground (i.e., the inert paint layer covering the support below the painting itself) can ordinarily be regarded for conservation purposes as part of the painting layers. Occasionally, the ground may lose its adhesion to either the support or the paint layers, or the ground may fracture internally, resulting in cleavage and paint loss.

      The paint layers themselves are subject to a number of maladies as a result of natural decay, faulty original technique, unsuitable conditions, ill treatment, and improper earlier restorations. It must be remembered that, whereas housepaint usually has to be renewed every few years, the paint of easel paintings is required to survive indefinitely and may be already 600 years old. The most prevalent defect is cleavage. If the loss is not total, the paint can be secured, according to circumstances, with a dilute protein adhesive such as gelatin or sturgeon glue, a synthetic polymer, or a wax adhesive. The paint is usually coaxed into place with an electrically heated spatula or a micro hot-air tool.

      As painting materials became more readily available in commercial preparations in the 18th and 19th centuries, systematic methods of painting that were once passed from master to apprentice were replaced by greater individual experimentation, which in some cases led to faulty technique. Artists sometimes used too much oil, leading to ineradicable wrinkling, or they superimposed layers that dried at different rates, producing a wide craquelure as a result of unequal shrinkage, a phenomenon that occurred increasingly as the 19th century progressed because of the use of a brown pigment called “ bitumen.” Bituminous paints never dry completely, producing a surface effect resembling crocodile skin. These defects cannot be cured and can be visually ameliorated only by judicious retouching.

      A notable defect arising from aging is the fading or changing of the original pigments (pigment) by excessive light. Although this is more evident with thin-layer paintings, such as watercolours, it is also visible in oil paintings (oil painting). The palette (colour) of the earlier painters was, in general, stable to light; however, some of the pigments used, notably the “lakes (lake),” which consisted of vegetable dyestuffs (dye) mordanted onto translucent inert materials, often faded easily. Copper resinate, a transparent green much used from the 15th to the 18th century, became a deep chocolate brown after prolonged exposure to light. After the discovery of synthetic dyestuffs in 1856, a further series of pigments were created, some of which were later discovered to fade rapidly. Unfortunately, it is impossible to restore the original colour, and in this case conservation, in its true sense of arresting decay, is important; i.e., to limit the light to the lowest possible level consistent with adequate viewing—in practice about 15 lumens per square foot (15 foot-candles; 150 lux). Ultraviolet (ultraviolet radiation) light, the most damaging kind of light, which comes from daylight and fluorescent fixtures, can and must be filtered out in order to avoid damage.

      Almost every painting of any degree of antiquity will have losses and damages, and a painting of earlier than the 19th century in perfect condition will usually be an object of special interest. Before a more conscientious approach to restoration became general in the mid-20th century, areas of paintings that had a number of small losses were often—indeed, generally—entirely repainted. It was considered normal in any case to repaint not only losses or gravely damaged areas but also a wide area of surrounding original paint, often with materials that would visibly darken or fade with time. Large areas with significant detail missing were often repainted inventively in what was supposed to be the style of the original artist. It is customary nowadays to inpaint only the actual missing areas, matching carefully the artist's technique and paint texture. Some restorers adopt various methods of inpainting in which the surrounding original paint is not imitated completely. The inpainting is done in a colour or with a texture intended to eliminate the shock of seeing a completely lost area without actually deceiving the observer. The aim in inpainting is always to use pigments and mediums that do not change with time and might be easily removed in any future treatment. Various stable, modern resins are employed in place of oil paint to ease reversibility and to avoid discoloration. Exact imitation of the original entails close study of the painter's technique, especially the multilayer methods, since the successive layers, being partly translucent, contribute to the final visual effect. Minute details of texture, brushstrokes, and craquelure must also be simulated.

      A variety of natural resins, sometimes mixed with drying oil or other constituents, have been used to varnish paintings. Although the traditional use of varnish was partly to protect the paint from accidental damage and abrasion, its main purpose was aesthetic: to saturate and intensify the colours and to give the surface a unified appearance. mastic and damar (dammar), the most commonly used natural resins, are subject to deterioration. Their chief limitations are that they become brittle, yellow, and less soluble with age. In most cases a discoloured varnish may be safely removed by using organic solvent mixtures or other cleaning agents, but the process is very delicate and may cause significant physical and aesthetic harm to the painting when it is done improperly. Some paintings exhibit a greater sensitivity to cleaning than others, and some varnishes may be unusually intractable owing to their formulation. In addition, many organic solvents are known to leach components of the medium from oil paint. For these reasons, cleaning should be carried out only by an experienced professional, and the frequency of the procedure should be kept to an absolute minimum.

      When the varnish is in good condition but covered with grime, the conservator may, after close inspection, clean the surface with aqueous solutions of nonionic detergents or mild solvents. Choice of solvent mixture and mode of application has always depended on the skill and experience of the conservator, but modern scientific theory has clarified the procedures. Synthetic resins (resin) have been widely adopted for use as picture varnishes. They are chosen for chemical stability with regard to light and atmosphere so that they can eventually be removed by safe solvents and will not rapidly discolour or physically deteriorate. Acrylic copolymers and polycyclohexanones have been the most commonly used since the 1960s. The synthetic varnish resins may be broadly divided into two classes of high-molecular-weight and low-molecular-weight resins. The high-molecular-weight resins are judged by many conservators to lack the desirable aesthetic and handling characteristics that are found in natural resins. Low-molecular-weight resins approach the appearance and behaviour of natural resins more closely and are currently receiving more attention. Recently introduced varnishes based on hydrogenated hydrocarbon styrene and methyl styrene resin hold promise as substitutes for natural resins. Research continues, however, in order to find the “ideal” varnish, combining ease of application, chemical stability, and an acceptable aesthetic quality. Paintings that are varnished, contrary to the intention of the artist, can become permanently altered in appearance over time and become diminished in value. In the last quarter of the 19th century, certain artists, most notably the Impressionists (Impressionism) and Post-Impressionists (Post-Impressionism), began to eschew the use of varnish.

Norman Spencer Brommelle Frank Zuccari

Wall paintings (mural)
      Wall paintings are the oldest known form of painting, dating back to the prehistoric paintings in the Altamira cave in Spain and the Lascaux Grotto in France. In the last decades of the 20th century, the conservation and restoration treatment of two Renaissance masterpieces of wall painting, Michelangelo's frescoes in the Sistine Chapel in Vatican City and Leonardo da Vinci's Last Supper (1495–98) in Milan, drew the world's attention to the environmental and structural vulnerabilities of these treasures.

      Commonly, large paintings placed into architectural niches are considered “mural paintings,” even those stretched over stationary or expandable wooden bars in the manner of easel paintings. Strictly speaking, however, “wall paintings” are distinguished from other murals by virtue of being executed directly onto primary wall supports, which are typically plaster, concrete, masonry, or stone. Wall paintings are integral to architecture, in both a material and aesthetic sense. The conservation of wall paintings inevitably concerns not only the paintings themselves but also the larger context of adjacent building materials, building maintenance, use, and preservation. Depending upon their construction and the degree of involvement of the wall support, wall paintings' conservation and restoration needs may be closely allied to those typical of easel painting or to those of porous stone (see Paintings on canvas (art conservation and restoration), above, and Stone sculpture (art conservation and restoration), below).

      From the point of view of conservation, different types of wall paintings have features in common, though the techniques of restoration required for each can differ greatly in detail. In buon (“true”) fresco, pigments mixed only in water are painted directly onto a freshly prepared layer of damp lime plaster. Pigments are permanently bound to the plaster as a result of a chemical change, as the fresh lime becomes calcium carbonate upon drying. In fresco secco (“dry”), the artist applies paints to already dried plaster. The stability of these paintings depends upon the presence of a binding medium—such as egg, oil, gum, or glue—mixed with the pigments to adhere them adequately to the wall surface. This type of painting is found in the wall paintings of ancient Egypt. In marouflage, a more modern variety of wall painting, paintings on canvas are mounted to the wall using an adhesive.

      Chief among the hazards to all these types of wall paintings is excessive moisture (water). Damp may rise through the walls, originating at the level of ground contact and spreading upward. Prevention of rising damp is sometimes achieved by cutting into the wall beneath the mural and inserting a “damp course” of water-impermeable material or a high capillary tube that draws and deflects the harmful accumulation (see Architecture (art conservation and restoration), above). These avenues of intervention are, however, often prohibitively expensive due to the complex engineering they require. If these approaches are not possible, amelioration of the problems may be achieved by reconfiguring drainage at the exterior of the building, and thereby reducing the overall quantity of available moisture. Damp may also come from the outside wall, where direct infiltration of rainwater may penetrate through the substrate to the face of the painting, evaporating at the paint surface. In this instance, localized building repairs or efforts to shield the exterior wall may attenuate the problem. Moisture may also result from condensation on a cold mural surface, a phenomenon common in churches, tombs, or buildings that are heated only intermittently or that are subject to excess ambient moisture generated by the respiration of crowds of visitors. More continuous and uniform heating of the wall may adjust this situation, provided that ambient air is not dried so rapidly that “efflorescence” (the formation of salts) occurs. Lastly, water damage caused by leaking roofs, clogged drainpipes, and faulty plumbing is easily stopped by repairing these systems. Conscientious maintenance is the best preventative treatment.

      Damages to wall paintings due to moisture may include blanching, drip staining, and delamination of paint layers due to efflorescence. Crystallized salts may form above, below, or within the painted image, resulting in disintegration or obfuscation of the image and creating a salty “veil.” The conservator must avoid coating the painting with a water-impermeable material, such as wax or resinous products, so that the damp can penetrate freely without meeting a barrier at the inner surface; when evaporative sites are blocked, moisture will move laterally, expanding areas of damage. Problems such as mold growth and mildew are secondary results from overly damp environments.

      Another enemy of wall paintings is more insidious and also more pervasive. Due to the worldwide use of fossil fuels and automobile emissions, concentrations of sulfur dioxide in the atmosphere have markedly increased. In the presence of moisture, pollutants forming sulfuric acid can quickly erode the calcium-carbonate component of most cement- and lime-based wall paintings. This “acid-rain” effect converts calcium carbonate to calcium sulfate. The volume of the sulfate crystal is almost twice that of the original carbonate of the mural, which causes internal pressure within the pores of wall fabric that can lead to fracturing. Further, the sulfate has a greater capacity to absorb moisture, thus perpetuating and exacerbating the cyclic wet-dry process of decay. Polluted environments can bring about the blackened, sooty surfaces associated with fossil-fuel particulates to a wall painting and can also discolour certain pigments traditionally found in Renaissance paintings, such as white or red lead, malachite, and azurite.

      In the face of such damage from moisture and pollution, the conservator works to halt the causative agents of deterioration and then proceeds to stabilize insecurities such as spalling plaster or flaking paint. Many new conservation treatments were developed in the second half of the 20th century: chemical poultices, gel technology, and ion-exchange resins have allowed advances in cleaning methods, reduction of salt deposits, and consolidation techniques. Natural or synthetic adhesives and inorganic consolidants are now utilized, but they must be chosen for compatibility with the paint medium and used with discretion to avoid film-forming blockages. Hypodermic injection of adhesives followed by light pressure while drying has also become an effective way to mitigate many problems of detached paint or wall support.

      Conservators often develop solutions in the face of a specific problem. For example, after the flood of the Arno River in Florence in 1966, Italian conservators developed drastic but necessary and highly expert methods to transfer frescoes from decayed walls. These range from the strappo technique to the stacco a massello. While in practice these methods are not always clearly distinguishable, strappo, the more radical procedure, consists of gluing canvas firmly to the surface of the fresco and then pulling and easing away a thin layer of the plaster containing the pigment particles of the fresco. The bond between the facing and the fresco must be stronger than the internal cohesion of the plaster. Excess plaster is removed from the back, revealing the thinned fresco in reverse. This thinned pictorial layer is then fixed to a rigid support after recoating the reverse with materials optically simulating the original underlying plaster. Unfortunately, much of the original surface character of the wall and density of the pigment layer is sometimes irreversibly altered by this technique, so the method is now seldom used. Less intrusive is the stacco method; a thicker layer of plaster is retained along with the fresco and is smoothed flat on its back surface before the composite rigid layer is mounted to a prepared support. Lastly, in the procedure called stacco a massello, the least intrusive to the fresco but more challenging transfer procedure due to mass and weight, the wall painting is removed with its entire original substrate. This feat requires bracing the wall with counter-forms to avoid damages due to torque, vibration, and other mechanical strains. Selecting the method of transfer depends greatly on the stability of the painting, the type of deterioration encountered, and the limitations of size, weight, and practicality.

      Whenever possible, transfer techniques are abandoned in favour of conservation and restoration treatments carried out in situ, with the conservator working from the surface and preserving as much original building fabric, character of surface, and contextual meaning as possible. The art conservation community, including art historians and preservation specialists, generally hold that murals and wall paintings are physically and aesthetically dependent upon their architectural context. The so-called “site-specific” nature of the paintings is valued, and the character of the original site is maintained as nearly as possible; relocation may cause diminishment of meaning or appreciation. The disciplines of wall and mural painting conservation, engineering, and architectural conservation are symbiotic, and each specialty is increasingly called upon to contribute to a holistic preservation plan.

Paintings on ivory (ivory carving)
      Ivory came into popular use as a painting support in the 18th and 19th centuries as part of miniature-painting (miniature painting) traditions based largely in Europe and the United States. The naturally translucent material was well suited to the luminous techniques of portrait painting. Derived from the tusks and large teeth of elephants, walruses, and whales, ivory is composed of both organic and inorganic constituents of dentine. However, its porous and hygroscopic qualities render it vulnerable to many agents of deterioration. Ivory, especially in thin layers, responds with dimensional changes to fluctuations in the moisture content of the air. Miniature paintings on ivory are particularly vulnerable, expanding and contracting across the grain in a manner similar to paintings on wood (see Paintings on wood (art conservation and restoration), above). The conservation of ivory paintings depends to a large extent upon maintaining stable environmental controls, in the optimum region of 50–60 percent relative humidity, with the temperature not exceeding 70 °F (21 °C). At lower levels of relative humidity, paintings on ivory desiccate, shrink, and crack, especially if constrained. Relative humidity above about 68 percent promotes expansion and warping; cyclical fluctuations also place severe stresses on the paint media.

       light is another damaging agent and can be responsible for bleaching ivory surfaces. watercolour and gouache, the most common painting media used in ivory miniatures, are sensitive to light and particularly subject to fading. Ideally, lighting of these objects should not exceed 5–10 lumens per square foot (5–10 foot-candles; 50–100 lux), and daylight should be avoided as much as possible. Since it is a porous substance, ivory is susceptible to staining and to retaining unwanted oils; the use of cleaning agents, especially aqueous solutions, can result in damage and removal of patina. It is therefore advisable to wear gloves while handling these objects, and while in storage, ivory paintings should come in contact with only neutral pH materials such as soft cotton, linen, or unbuffered acid-free tissue.

Paintings on paper
      Whereas paintings on parchment, vellum, papyrus, and bark in various forms date back to ancient times, it was not until after the invention of paper by the Chinese in the 2nd century AD that thin, felted cellulose sheets of true paper were available, mostly for calligraphic or printing uses. After the very slow progression of papermaking technology to the West, the first papers introduced for drawing or painting during Renaissance times were used as working sketches for paintings, sculptures, or architecture, rather than as completed artworks in themselves. During the Industrial Revolution, manufacturers created heavy paper sheets and laminated paperboards specifically for painting; having the mechanical properties requisite for full colour illustration with pastel, gouache, oil, acrylic, or other paint media, these papers can clearly be considered apart from papers traditionally used for prints and drawings.

      Depending upon the method of fabrication of the painting on paper, the design layer itself or the support may be the feature most responsible for an object's condition and for the response of the artwork to its environment. For instance, a thin sketch on lightweight paper, which has a great quantity of exposed paper as part of the image, is quite different in appearance and behaviour than a finished, opaque oil painting on heavy laminated paperboard, which may reveal none of the paper surface in its design. In the former case, the object may respond to its environment more characteristically like a print or drawing, whereas, in the latter case, the object may respond characteristically like a painting on wood or canvas. In this respect, paintings on paper may vary widely, and conservation requirements may fall generally into the category of either paper or painting. Conservation treatment is usually best addressed by the professional whose expertise is in the field of the dominant material.

      Paintings on paper require the same environmental protections of most organic materials, namely, stable temperature and relative humidity within the limited range recommended (50–55 percent RH, 60–68 °F [16–20 °C]). Prolonged light exposure inevitably causes loss of colour and distortion of the artist's original concept or intention. A darkened paper support, combined with fading colorants, may seriously skew intentional contrasts and result in much loss of depth or detail. The conservator should avoid daylight exposure as much as possible, because the ultraviolet (ultraviolet radiation) light component is particularly damaging. Ultraviolet filters over windows or incorporated into Plexiglas framing may be of some benefit in controlling light exposure, but these efforts are not a panacea and should not be used in place of prudent monitoring. The total quantity of light should be considered, based upon time and intensity, because damage due to light is both cumulative and irreversible. Storage or framing should always require use of 100 percent acid-free (preferably all rag) matte boards and storage folders in order to limit the chance of acidic transfer to the artwork. The humidity and ventilation of storage facilities should also be carefully monitored so as to avoid mold growth on sizing (the glue used in the binding process) and paint media, as well as to prevent secondary infestation by silverfish or book lice.

Norman Spencer Brommelle Anne Lee Rosenthal

Prints (printmaking) and drawings on paper
      Prints, drawings, and manuscripts have been created in many cultures over the centuries, with prints often tied to traditions of book illustration. Despite variables of media and forms of printing, a defining characteristic of prints and drawings is the way in which colorants such as inks, washes, pencils, and pastels become incorporated into the absorbent, fibrous texture of paper. Unlike paintings on canvas, which are laminated structures with distinct layers, even hard-pressed and heavily sized papers seize ink and colour; art on paper is a kind of amalgam, in which paper and pigment become inseparable. The permanence of prints and drawings is thereby greatly influenced by the quality of the paper support and by the environmental circumstances under which the artworks are housed. Despite being considered a fragile or ephemeral material, good-quality paper that receives proper handling and environmental stability has been known to survive for over one thousand years. Countless modern masterpieces have been made with inferior papers containing wood pulp, fugitive media, or poor technique, however. These qualities identify works with “inherent vice,” and there is little the art conservator can do but provide the best possible environment to slow the inevitable deterioration of such works.

      Most conservation treatments of prints and drawings or archives on paper aim to reduce the discolorations and acidity brought about by unfavourable climatic and storage conditions. These are commonly caused by contact with poor-quality acidic framing materials, matte-burn due to proximity to acidic window or back mattes, darkening due to light exposure and chemical deterioration, and brown spots known as “foxing,” which may result from the combined influence of metallic particles in paper and mold. Additionally, attack on the cellulose and sizing of paper and paint media by biological pests such as silverfish, book lice, beetle larvae, mold, or fungus can result in very destructive and unsightly damages. The absorbent nature of paper renders it especially vulnerable to chemical transfer or image offset during storage, and so storage and framing with only acid-free archival papers (preferably 100 percent rag content) is generally the museum standard. Careful human handling, including prudent policy management for exhibition, ranks high among the factors influencing length of preservation of artworks on paper.

      In terms of remedial treatment for deteriorated art on paper, there are numerous techniques and specialized equipment available to the paper conservator, including vacuum-suction tables, humidity chambers and platens, semipermeable plastic sheeting, steam and hot-air pencils, and leaf-casting apparatus. The conservator limits the use of moisture in procedures such as washing and stain reduction based on the degree of tolerance of the individual drawing media and on the subtle qualities of the paper. Immersion in water baths is limited to the most stable situations. Prudent use of bleaching, deacidification, and other reagents depend upon myriad circumstances, including the long-term aging characteristics after treatment and the possible consequences of residues left in the paper.

      Repairs of mechanical damages to prints and drawings such as tears, thinning, or losses can be remedied by applying reinforcements, new paper inserts, or pulp into damaged areas. Additional overall support can be provided by adhering new paper (or backing sheet) to the reverse of the original. Typically, Japanese tissues, pure paper pulps, archival papers, and stable antique papers, used in combination with wheat- and rice-starch pastes, can be used for this purpose.

Anne Lee Rosenthal

      Throughout history, artists and craftsmen have created sculpture by using virtually every material imaginable. Stone has been chiseled, metal hammered or cast, wood carved, and clay molded. Bone, ivory, and resins have been shaped with knives. Reeds have been bundled, and skins have been stretched to shape. At the turn of the 21st century, modern industrial and space-age materials such as plastics, composites, and exotic alloys have been added to the sculptor's ever-widening resources.

      Although some prove more durable and resistant than others, all sculptural materials are susceptible to environmental agents that initiate deterioration, decay, and destruction. The approaches taken by the conservator to slow this deterioration are guided by a large number of complex considerations. The inherent nature of the material itself comes into play, as does the environment in which the sculpture has existed or will exist. The degree to which the sculpture has already deteriorated before conservation or restoration is also considered important. The original or intended purpose of the sculpture may have significant implications for its condition and for its survival, and various values (aesthetic, historic, cultural, religious, and monetary) may dramatically influence the conservator's course of action.

Stone sculpture
      With examples dating back to the enormous prehistoric statues of Easter Island, many types of stone have been employed over the centuries in sculpture. Some of these stones yield more readily to the sculptor's chisel (such as limestone, marble, and soapstone), while others, such as granite, are more difficult to carve but have proved more durable over time. All of these are susceptible to the deterioration caused by water. Water can either directly dissolve stone or wear it away by carrying abrasive particles over its surface. Water can also deteriorate stone when it freezes and turns to ice. Ice crystals have greater volume than liquid water, and when water is contained in the porous structure of stone and then freezes, the resulting ice crystals place enormous stress on the pore walls. This stress leads to microfractures in the structure of the stone. If the ice then melts, migrates to another location in the porous stone, and freezes again (as will happen with the changing of seasons in temperate climates), it begins what is called a “freeze-thaw cycle,” in which repeated migration and freezing of the water causes the stone to lose cohesive strength, particularly near the surface. Freeze-thaw cycles can result in spalling, or delamination of the stone surface, eventually leaving no more than a shapeless mass in a relatively short amount of time.

      Water can also carry soluble salts (salt)—such as the sodium chloride present in seawater or the nitrates found in groundwater polluted by fertilizers—into the porous structure of stone. These salts stay in solution and travel through the pores of the stone until the water begins to evaporate at the surface of the sculpture. Upon losing water, the salt will effloresce. Salt crystals, like ice crystals, have greater volume and place greater stresses on the pore walls, which leads to the same flaking or spalling caused by the freeze-thaw cycle. When the majority of the soluble salt crystallizes at the surface of the stone sculpture and forms a white powdery deposit, the process is defined as “surface-efflorescence.” Although this process is unsightly and can cause damage, it is not as destructive as “subefflorescence,” which occurs when the salt crystallizes in the pores of the stone below the actual surface. In the process of subefflorescence, the salt crystals are contained within the pores and hence place enormous pressure on the pore walls. Some types of stone contain large amounts of salt as part of their natural composition and as such are highly susceptible to damage under the right conditions. Other stones acquire salts from the environment, such as during burial, when they are exposed to groundwater laden with salt, or when they are exposed to water that has peculated through natural or man-made material (such as gypsum or cement) that also contains large amounts of salt.

       water also plays a role in the aggressive attack on stone by industrial air pollutants (pollution). Since the 19th century and to a limited degree well before that, the destructive properties of sulfur (released when fossil fuels are burned) have been well documented. Sulfur reacts in the atmosphere to form sulfur dioxide (sulfuric acid), which in turn combines with available moisture to form sulfuric acid. When in contact with marble or limestone (both of which are calcium carbonates), the sulfuric acid transforms the surface of the stone to gypsum (calcium sulfate). This transformation has several unfortunate results. First, the gypsum has greater volume and greater porosity, so it will hold more acidic water at the surface, continuing the acid attack on the underlying stone and encouraging other destructive processes such as biological activity (for example, the growth of mold). The gypsum crust formed often incorporates dark particulate matter from the polluted atmosphere, such as carbon, resulting in the unsightly black crusts seen on many urban buildings and monuments. This crust has a very different response to changes in temperature and will often crack or peel away, leaving fresh stone exposed to the same destructive cycle.

      Biological deterioration of stone is also a concern. root or vine growth can physically fracture marble, for example, if the root finds its way into a crack or fissure, similar to the way tree roots or weeds can fracture sidewalks or roads. Direct dissolution of stone by lichens and ivy is also possible, and the presence of such plants leads to the retention of water, which, as aforementioned, accelerates other destructive processes.

      In the past, restoration of stone sculpture involved many aggressive methods aimed at erasing or disguising any damage or loss due to age, weathering, or accident. These techniques extended to the recutting of the sculpture or the reduction of the sculpted surface by means of abrasives or acids to remove damage or to “improve” the aesthetic appearance of the sculpture. Aesthetic dictates and fashion of the particular time in which the restoration was undertaken greatly influenced these choices, and often the sculpture became more of a product of the restorer's hand and time, rather than a work reflecting the intent of the original artist.

      Today sculpture restoration (normally limited to the cleaning and repair of major damage) is guided by the various professional codes of ethics (such as the Code of Ethics and Guidelines for Practice of the American Institute for Conservation of Historic and Artistic Works [AIC]) followed by professional conservators. Original material and surface are carefully guarded, and the conservator takes great care not to alter the intent or “spirit” of the object or influence the way in which it may be interpreted. Missing areas are often left missing, and damage is often repaired only if doing so does not require unacceptably invasive treatments that are extensive in nature or that may not be reversible. Nonetheless, when replacing missing segments is acceptable or necessary, the conservator does this in such a manner as to make the replacements or additions apparent under close inspection or through using easily available inspection techniques.

      Because soluble salts are so aggressively destructive to stone sculpture, their removal is of paramount importance if they are present in sufficient quantities. Traditionally, if the object is small enough to be submerged in water that is regularly refreshed, salts are soaked out until they are completely removed from the stone. However, when the sculpture is too large to submerge, too fragile to soak, or secured to a site, other methods must be employed. Also, some stones are composed of minerals that themselves will readily dissolve after prolonged contact with water; in such instances, poulticing is an optional method that avoids prolonged submersion of the stone in water and yet maximizes desalination. Poulticing involves wetting the sculpture with water and then placing a clay or paper pulp-based material mixed with water on the surface. As the water is drawn to the surface of the poultice by evaporation, the salts dissolved in the water are carried along and deposited in the poultice material. The poultice is then removed from the stone surface and the process repeated until all, or an acceptable amount, of the salts present are removed.

      Stone can lose its cohesive strength when the material that binds the grains together becomes disrupted or lost through dissolution. In such a situation, the stone is described as “sugary,” because the individual grains or crystals become easily dislodged and have the appearance of loose sugar granules. The stone may begin to delaminate in flakelike sections. In such cases, the cohesive and structural strength of the stone must be reinstated by the introduction of a consolidant. The characteristics of good stone consolidants include long-term stability and strength under adverse conditions (outdoors), the ability to penetrate deeply into the stone and provide even distribution of the final consolidating product throughout the stone, and a minimal effect upon the appearance of the stone once it is introduced (i.e., it should not change the colour or other characteristics such as translucency or opacity of the stone).

      Consolidants can be divided into two major categories: mineral and synthetic consolidants. Among the mineral consolidants are “lime water,” which is the introduction of a saturated water solution of calcium hydroxide into the matrix of a calcium-based stone (such as limestone or marble). Once the calcium hydroxide is deposited, its eventual interaction with atmospheric carbon dioxide forms a network of calcium carbonate, similar to that which makes up the stone itself. In a similar manner, the application of alkoxy silanes in recent decades offers the conservator a method by which amorphous silica can be introduced as a binder and strengthener for deteriorated sandstone. Some silanes will also impart water repellency to the stone. Synthetic polymer-based consolidants include acrylic polymers, epoxies, and polyesters. Although these are a considerable improvement over past materials such as wax and natural resins, some have proved unsuitable in certain environments and over long periods of exposure. Some epoxies have altered over a relatively short period of time and dramatically changed the appearance of the sculpture, while other synthetic consolidants have proved unable to penetrate deeply enough into the stone, and their application has resulted in a thin, dense, and impermeable crust that falls away owing to the buildup of salts or water vapour behind it.

      A variety of coatings ranging from natural resins to waxes have been used for the protection of stone sculpture from either the outdoor elements or the deposition of dust and grime within the indoor environment. Acrylic polymers are now more commonly used for the less-demanding environments, whereas surface consolidants and water repellents based on silicone materials or hydrophobic silanes are often used for sculptures placed outdoors. Surface coatings can function to repel unwanted deposits or to serve as sacrificial layers that, when removed during regular maintenance, carry the deposits with them.

      Cleaning was once undertaken with relatively aggressive methods such as abrasives, acids, and even chisels to remove the offending deposits or stains. More often than not, these approaches resulted in considerable damage to the original sculpted surface. At the turn of the 21st century, the professional conservator aggressively guards against any loss of original surface, even to the point of accepting the presence of a deposit or stain rather than endangering the original material of the sculpture. In some cases, the deposits that are obscuring the detail or subtle carving on the surface are in themselves informative and important and must be preserved rather than removed. In the case of many archaeological artifacts or ethnographic objects, for example, minute amounts of preserved material such as traces of pigment or deposits from original use can shed a great deal of light on the original appearance of the sculpture: its history, function, method of manufacture, and, to a degree, the artist's intent.

      Contemporary techniques of cleaning may range from simple mechanical removal of the deposit with a common soft eraser to the use of surgical scalpels, often with the aid of a binocular microscope for more cautious and delicate cleaning. Small-scale power tools are commonly used when the deposit is extremely hard—for example, dental ultrasonic descalers can be used to remove hard calcite- or silica-based deposits or residues of modern cement and grout. The conservator sometimes employs microair abrasive equipment that uses fine particulate powders such as walnut shell or talc. The technique requires that the operator have considerable experience and skill so that the stone surface itself is not abraded. Chemical agents such as surfactants (agents that reduce surface tension between a liquid and a solid), chelates (agents that form compounds with metal ions, making them more easy to remove), or solvents can also be used either in local application using a small cotton swab or mixed in a poultice. Just as poulticing works as a means of desalination, it also can be used to eliminate deposits and stains. Poulticing material may include clays (such as sepiolite, a magnesium trisilicate clay), paper pulp, or gel materials such as carboxymethylcellulose. steam cleaning and water misting (sometimes called “nebulization”) are also often employed in the cleaning process, though like all the techniques already mentioned, they must be cautiously applied to ensure that only the desired deposit or grime is removed, without damaging the stone surface or other decorative elements.

      First used in the 1970s to clean the black pollution crusts from stone architectural sculpture, laser technology has rapidly developed as a promising method for cleaning stone surfaces. Laser energy dislodges or vaporizes the offending material that is normally of a darker colour than the stone. The laser has become one of the most promising tools for future use in conservation due to the advancement of more commonly available units, a relative drop in cost of the equipment, and a greater familiarity with laser technology in the field of conservation.

Metal (metalwork) sculpture
      Metal sculpture ranges from solid-cast statuettes of the ancient Near East to the massive steel public monuments of the late 20th century. In most instances, the deterioration of metal sculpture is due to the reversion of the metal to a more stable mineral state. In the case of iron, the process is most commonly known as “rusting” and results in a red-brown, powdery mineral iron oxide. Copper (copper work) and its alloys most commonly alter to the green or blue carbonates of copper, malachite, or azurite or to the red-oxide mineral cuprite. Copper and its alloys may also quickly corrode in the presence of chloride by the cyclic process called “bronze disease,” during which copper is altered to copper chloride, a powdery white-blue product. Silver (silverwork) tarnishes rapidly even in the presence of minute amounts of sulfur, and lead will quickly corrode in the presence of acetic acid. Common to all of the processes is the presence of water, which is needed to initiate and complete the corrosion of the base metal to a more voluminous and less cohesive mineral product.

      In the past, the treatment of metal sculptures often involved completely stripping the surface until it was free of all corrosion product or alteration. Abrasive techniques such as sandblasting or microbead blasting were regularly used, as was chemical stripping (which dissolved the mineral alteration products) and electrochemical reduction, which also stripped the surface of any corrosion products and of “patina,” the term usually given to corrosion products that are either naturally occurring or artificially formed on the metal surface. Patinas are valued for aesthetic beauty and for the authenticity that they lend the object. Today treatment of metal sculptures is far more conservative than in the past. Although sculpture may be polished (as in the case of silver sculpture that has been tarnished) or stripped of its alteration patina (as in the case of some monumental outdoor sculptures), alteration products are carefully evaluated for their importance and authenticity before their removal is considered, and patinas are far more often protected than removed. Any treatment that results in the reshaping of the metal or in any irreversible addition, such as soldering or welding to secure broken segments, is now considered with great caution.

      At the turn of the 21st century, the conservator's main intervention in the process of corrosion involved providing a more benign environment (usually meaning as dry as possible and as free of harmful pollutants as possible) and maintaining the sculpture's stability through a series of preventive maintenance procedures, such as regular cleaning and the application of protective coatings. Regular maintenance has proved to be highly cost-effective and successful in the preservation of outdoor sculpture over the long term. Regular cleaning and coating (with waxes or synthetic polymers or both, which sometimes contain corrosion inhibitors) have kept corrosion processes in check, even in aggressive and polluted urban environments. In some cases, however, the conservator's only option is to recommend that the sculptures be removed from the outdoor environment, placed in a protected area, and replaced by a replica made of a more-resistant material.

      Although cleaning of metal sculpture can include the total removal of all corrosion products, including those termed and valued as patina, a more conservative approach continues to develop within the field, which recognizes the value of naturally occurring change to the metal surface. In the case of archaeological material and ethnographic sculpture, the corrosion products may hold remnants of original surface treatments or remains of associated materials or evidence of use. This evidence must be carefully studied, and a full understanding of the sculpture's importance (now and in the future) must be weighed against its loss by cleaning.

wood sculpture
      Although relatively little wood sculpture survives from prehistorical and early historical periods, an enormous amount of sculpture was produced in the last millennium, particularly the polychrome sculptures of western European religious devotion and those of India, China, Japan, and other Asian nations. Wood is a very open and porous structure, the bulk of which is water, absorbed or chemically bound to its thin-walled structural cells. Like many plant materials, wood responds to changes in the humidity of its surrounding environment, taking up available water to reach equilibrium with the environment or, conversely, giving up water if the surrounding air is dryer. Dimensional changes to the wood occur when this exchange takes place. As wood takes up water, it will swell. As it loses water, it will shrink, sometimes dramatically. Both actions induce considerable stresses on the structure of the wood, resulting in irreversible warping or complete splitting of the wood section. Additionally, the physical strain placed on the structure by continual expansion and contraction weakens the wood or may cause further serious damage to wood already weakened by insect attack or age. When decorated with paint, wood will respond to heat and moisture with greater movement, destroying the bond between the wood and the less elastic paint and ground preparation, resulting in the painted decoration's flaking away from the surface.

      Wood can also be a food source or a nesting place for a variety of insects such as wood-boring beetles, termites, and grubs. Infestation can be so severe that the sculpture loses all of its structural strength and collapses. Wood can also be damaged by a variety of fungi and bacteria with similar results.

      The predominant concern regarding the preservation of wood is the control of the environment. Exposure to light, particularly the ultraviolet (ultraviolet radiation) and shorter wavelengths of the visible spectrum, results in both the chemical and physical alteration of all organic material, including wood. Wood can become darker or lighter or lose its structural integrity through the action of light energy acting as a catalyst for other chemical reactions.

      Appropriate and stable temperature and humidity levels and an environment low in ultraviolet radiation, illumination, and pollutants can ensure the slowing of any deterioration. Regular dusting and general maintenance of the sculpture, as well as vigilant actions to keep damaging insects away, are also paramount. When intervention is necessary with wood sculpture, it normally involves some form of consolidation, either of the wood sculpture's structure or of its decorative surface. The range of consolidants for each of these actions is broad, including synthetic acrylic polymers, organic-based natural resins, and animal glues.

Jerry C. Podany J.H. Larson

Decorative arts (decorative art)

      A small amount of furniture from ancient civilizations has been preserved in extreme environments, such as the dry desert of Egypt or the water-logged soils of England (United Kingdom). These surviving pieces have proved that the craft of furniture making has remained relatively consistent for centuries. If a piece of furniture is equilibrated to a moist environment and then put in a dryer one, as in the case of centrally heated (heating) homes of modern times, it will lose moisture and shrink. Boards and segments will warp, and those restricted from movement will crack and even split. Veneers (thin wood panels placed over the surface of the structure of the furniture) may lift, crack, and separate from the underlying structure. High humidity will result in many of the same problems and also encourage mold and the decomposition of fabric, leather, or other skins. The various metal fittings, particularly iron, may also corrode. High humidity will also encourage the decomposition of the glue (in most traditional furniture this would be animal- or fish-based glues) through the action of microorganisms.

      Light is also a problem in that the visible spectrum, especially the ultraviolet aspect, will bleach, fade, or discolour the wood. Light may also alter any additional decorative elements. For example, marquetry on 17th- and 18th-century furniture is often stained with plant dyes that rapidly deteriorate (fade) when exposed to light. Exposure to light will also cause leather, skins, and most upholstery to deteriorate more rapidly. Indoor and outdoor urban and industrial pollution will deteriorate metal fittings, wood, and upholstery fabric.

      If the furniture is made of wood, the finish of the wood can function as both a decorative and a protective layer. The coating may also act as a barrier to either retain moisture in the wood or prevent the wood from absorbing additional moisture. Finishes are often natural resins, such as sandarac or mastic, that are contained in waxes or solvent. Waxes or drying oils such as linseed oil may also be used alone. Furniture can be painted with a wide variety of pigments bound in wax (encaustic) or in organic binders of oils or gums. Lacquer, which was first developed in China and then imported to Europe, is manufactured by the application of specially treated tree sap.

      The conservation treatment and restoration of furniture today is a mixture of traditional crafts and modern scientific investigation. On a basic level, the conservator tries to keep objects at optimum and stable values of relative humidity—between 55 and 65 percent—year-round, with little change, avoiding any abrupt or extreme changes in humidity or temperature. The conservator will avoid exposing the object to strong light or light sources with high ultraviolet content, such as many types of fluorescent tubes, and will avoid localized heating through radiators or strong lights in close proximity to the object. A regular inspection for insect attack is of paramount importance. Repair and stabilization of furniture may range from the simple repair of a small crack or lost area of gilding to stabilization of a major joint, complete reupholstering, refinishing, or the design and manufacture of alternate structural supports. Whatever the degree of intervention, the conservator increasingly seeks out materials and methodologies that both respect the original material and condition of the piece of furniture and ensure reversibility of the added materials.

glass and other vitreous materials
      Since ancient times, glass has been used for both decorative and everyday use. Glass, glaze, enamel, and faience—the four vitreous products—are manufactured from three basic components: silica, alkali, and small amounts of calcium. Glass, glazes, and enamel (but not faience) contain high amounts of alkali, such as sodium oxide (soda glass) or potassium oxide (potash glass).

      Generally speaking, the mechanisms that are involved in the deterioration of glass are identical for all vitreous materials. Although a wide variety of agents are involved, inherent susceptibility to deterioration plays an important role. Composition is crucial, because it will determine how susceptible the glass is to various agents and processes of deterioration; for example, Roman period soda-lime-silica glass is quite durable, while medieval window glass is highly unstable due to its large content of potash (from beech wood ash). Lime is also unstable. Glass deteriorates quickly in an alkaline environment due to the breakdown of the silica network.

      Of all the agents in the environment that are aggressively damaging to glass, none is more directly or indirectly destructive than water. Water, especially when it mixes with pollution products, becomes acidic (has a low pH) and extracts the alkali from the silica network of the glass. The alkali modifiers are leached from the glass structure and brought to the surface, where they attract and absorb more water. This buildup of moisture on the surface can often be seen as small droplets. The glass can also have a slippery feel. In either case the glass is then said to be “weeping.” The loss of the alkali from the silica structure leaves the structure under stress, resulting in numerous microfractures and a cloudy appearance. This is termed “crizzled” glass. The formation of soluble alkalies at the surface can cause a flaking of thin layers there, resulting in layers that become detached and reflect and refract light differently from that of the glass body. The result is often an opalescent and pearlescent surface with multiple colours. This particular phenomenon is often seen on ancient and archaeological glass, but it was chemically reproduced and intentionally caused for decorative purposes in the Art Nouveau glass of Louis Comfort Tiffany (Tiffany, Louis Comfort).

      Vitreous materials are by nature brittle and fail catastrophically under loads that exceed their strength. Glass vessels and windows shatter under impact, and glazes can crack from thermal shock or pressure from salts crystallizing between the glaze and the underlying ceramic body. An unusual deterioration is a process called “solarization,” which is a change in the colour of the glass due to a reaction between the iron and manganese oxides in the glass initiated by light. The result—an irreversible alteration—can be a deep purple colour but is more often a subtle change in hue.

      Glass can become so weak or its surface so delaminated that it is necessary to strengthen it. This is often done by the infusion of light-stable epoxy resin with an identical or similar refractive index to the glass itself. In recent years consolidation has also been carried out by using a variety of silane solutions as well as acrylic copolymers. Mending, meaning the rejoining of shards of glass, is carried out by using low-viscosity, light-stable epoxies with a refractive index similar to glass. Recently acrylic monomers and polymers, as well as some of the cyanoacrylate adhesives and acrylic copolymers, have been used as well. Infilling, or replacement, of missing segments is often accomplished with a synthetic resin of similar optical properties (refractive index and colour). Often the fills are made slightly different in colour, transparency, or thickness to clearly mark them as a restoration and not part of the original glass object.

      Coatings for glass are normally reserved for windows that must resist the aggression of the outdoor environment. A range of products based on epoxy resins, silanes, and silicones, as well as amorphous silica, are available. Double-glazing can be quite successful in some instances for protecting stained-glass (stained glass) windows from the damaging effects of exterior (and even interior) environments. The process involves placing a clear pane of glass over the original stained glass with a suitable space for air circulation to prevent condensation. The exterior modern glass is meant to act as a protective and sacrificial barrier. Regular light cleaning has proved to have an enormous advantage in long-term preservation. The provision for appropriate environmental storage or exhibition conditions has also been a major contribution to their preservation.

      There are a great variety of clays (pottery) in the world, used since prehistoric times to make everything from utilitarian and ceremonial objects to decorative friezes, small figurines, and large-scale sculpture. The actual chemical deterioration of clay and ceramic ware, though possible, is usually slow. Nonetheless, ceramic remains a brittle material and one that is susceptible to dramatic and catastrophic damage by impact or stress loading beyond the material's strength.

      Crystallization of soluble salts can result in serious damage to the ceramic structure and the decorative surface, especially if it is glazed. Soluble salts such as phosphates, nitrates (in soil and groundwater laden with fertilizer and industrial pollutants), and especially chlorides (such as those found in the sea and sometimes in the ground) will combine with water and migrate through the pore structure of the ceramic. When the water evaporates from the ceramic, the salt will effloresce. Since salt crystals have greater volume than salt in solution, they can impose impressively high stress loads in the pores of the ceramic structure, leading to microfracturing and damage. The process is especially damaging when the salts build up under the glaze surface, which is less permeable to the passage of water vapour and salt crystals. Because the salt cannot grow out from the surface, the crystals form below or at the body-glaze interface. The result is either a weakened ceramic structure just below the glaze or a separation fracture between the glaze and the ceramic body. In either case, the end result is that the ceramic becomes powdery and the glaze flakes away.

      When soluble salts are present within the ceramic structure at a percentage considered threatening, the conservator must remove them. The most common method of removal is by soaking the ceramic in deionized water for extended periods of time. The water dissolves the salt and draws it out of the ceramic. As the water is regularly refreshed, it is tested for salt content. The process is continued until the water no longer contains salt or includes a very low percentage that the conservator deems safe. Desalination can also be carried out through the application of water-based poulticing. Paper pulp is often used for this purpose.

      Salt-damaged ceramic ware must often be consolidated before mending. Acrylic copolymers in solution are the most common choice for this purpose. The copolymer is introduced into the ceramic body as a low-percentage solution in a solvent. The ceramic body is then slowly dried in an atmosphere containing the fumes of the solvent, in order to control the rate of drying and even the amount of deposition of the consolidant within the ceramic body. In some cases, alkoxy silanes are used for consolidation. These materials leave an amorphous silica network within the structure of the ceramic body, introducing greater strength.

      Adhering (adhesive) ceramic shards together has in the past been carried out with a wide variety of material ranging from natural resins such as shellac to plasters, grouts, and cements. Today the conservator has a variety of synthetic materials at hand that offer a degree of reversibility and long-term stability necessary to meet the ethical guidelines of modern practice. Acrylic copolymers have proved quite useful in mending ceramic fractures. However, larger vessels or sculptural forms often require stronger structural adhesives. In such cases, the conservator turns to polyesters and even epoxy adhesives. Whatever the choice of adhesive, the conservator will always make the choice on the basis of long-term stability and reversibility of the join.

      In modern conservation practice, the infill of a loss on a ceramic vessel is often painted a monochromatic colour sympathetic to the original material but not fully matching it. The fill might also be slightly depressed from the original surface, further indicating that this is a modern addition that does not attempt to complete complex drawing or decorative detail that may not be fully known or may be quite specific to an artist's style. Sometimes reconstruction is necessary when an original piece can be reconnected to the original sculpture or vase only by filling a gap caused by the loss of material between the two sections. Any fills, bridges, and reconstruction are often done in plaster, lime putty, or synthetic resins such as polyesters or epoxy. In the case of more “invisible” restoration—where the repair is not meant to be seen, thus giving the impression that damage never occurred—the restorer might use epoxy or polyester resins with clay or other mineral powders to mimic the colour and translucency of the clay or glaze. This is often the case in porcelain restorations. Although this is common, it is important that the conservator follow ethical guidelines by recording this repair fully so as to not mislead future observers or scholars as to the true condition of the object. Infill materials and paints or colorants used must be fully reversible, and, in most cases, it is not acceptable to overpaint an original surface in order to camouflage a repair.

      The approach taken to cleaning ceramic material is dependent not only on the deposit to be removed but also very much on the ceramic body itself. High-fired porcelain might withstand more aggressive actions than a delicate, low-fired coarseware. Approaches in either case range from light brushing to removal or reduction of hard encrustation by surgical scalpel. Ultrasonic descalers can be used, as can a variety of chemical agents, including solvents and chelates. The application of laser energy is a fairly new frontier in the cleaning of ceramic material and promises very exciting future possibilities.

Jerry C. Podany

Textiles
      Environmental requirements for textile preservation are similar to those for paintings on paper, but neglect of textiles can in general cause more damage. Fading is a serious problem, but light also weakens the fibres of the material, especially silk. Gaseous air pollution is harmful, and soiling from airborne grime leads to the need for washing, which is best avoided. Where washing is necessary, nonionic detergent formulations are used but never ordinary commercial detergents; dry cleaning with selected solvents may be substituted in particular cases. Handling and storage of fragile textiles require special care: loose wrapping with acid-free tissue paper; storage containers ventilated to avoid local humidity buildup; folding with sharp edges avoided; for tapestries (tapestry), rolling with weft (design weave) along the axis; and so forth. New acquisitions and stored material require inspection for insect infestation. The feasibility of insect poisons and repellents in textile preservation remains uncertain.

      Restoration of valuable textiles, generally by means of skilled needlework, does not normally involve the replacement of worn or decayed materials. When this has to be done for structural reasons, informed judgment is required. When a material is so decayed that it cannot be reinforced by stitching it to a backing material, it may require an adhesive bond. After decades of discussion over the use of synthetics, research now points to hydrolyzed starch (an old Japanese recipe) as a solution or, when the use of water is inadvisable, methylcellulose in an organic solvent.

Norman Spencer Brommelle

Additional Reading

General
Valuable general accounts of the field are contained in Art and Archaeology Technical Abstracts (semiannual), published by the International Institute for Conservation of Historic and Artistic Work, London. Other general examinations include Andrew Oddy (W.A. Oddy) (ed.), The Art of the Conservator (1992); James Beck and Michael Daley, Art Restoration: The Culture, the Business, and the Scandal (1993, reprinted 1996); and Nicholas Stanley Price, M. Kirby Talley, Jr., and Alesandra Melucco Vaccaro (eds.), Historical and Philosophical Issues in the Conservation of Cultural Heritage (1996). See also John M.A. Thompson et al. (eds.), The Manual of Curatorship: A Guide to Museum Practice, 2nd ed. (1992); and Garry Thomson, The Museum Environment, 2nd ed. (1986, reprinted 1998). Up-to-the-moment information on specific topics can be found at the Web site for the American Institute for Conservation of Historic and Artistic Works, http://aic.stanford.edu; and the Web site for the International Centre for the Study of the Preservation and Restoration of Cultural Property, http://www.iccrom.org.

Conservation and restoration of architecture
Important sources on architecture include Jane Jacobs, The Death and Life of Great American Cities (1961, reissued 1972); Orin M. Bullock, Jr., The Restoration Manual: An Illustrated Guide to the Preservation and Restoration of Old Buildings (1966, reissued 1983); Great Britain, Preservation Policy Group, Report to the Minister of Housing and Local Government (1970), a concerted attack upon the problems of historic city conservation in Britain; Donald W. Insall, The Care of Old Buildings Today: A Practical Guide (1972); Jane Fawcett (ed.), The Future of the Past: Attitudes to Conservation, 1174–1974 (1976); John F. Smith, A Critical Bibliography of Building Conservation: Historic Towns, Buildings, Their Furnishings and Fittings (1978); Jack Bowyer, Vernacular Building Conservation (1980), a technical guide to architectural restoration; James Marston Fitch, Historic Preservation: Curatorial Management of the Built World (1982); Bernard M. Feilden, Conservation of Historic Buildings (1982), a fully illustrated work on architectural preservation techniques, one in the authoritative series of Technical Studies in the Arts, Archaeology, and Architecture; David Pearce, Conservation Today (1989); Arthur Cotton Moore, The Powers of Preservation: New Life for Urban Historic Places (1998); and Jukka Jokilehto, A History of Architectural Conservation (1999).

Conservation and restoration of paintings
General studies of issues related to paintings can be found in Helmut Ruhemann, The Cleaning of Paintings, with a comprehensive bibliography by Joyce Plesters (1968, reissued 1982); Anthony E. Werner, The Conservation of Antiquities and Works of Art, 2nd ed. (1971, reissued 1976); Harold J. Plenderleith, Norman Brommelle, and Perry Smith (eds.), Conservation and Restoration of Pictorial Art (1976); David Bomford, Conservation of Paintings (1997); Knut Nicolaus, The Restauration of Paintings (1999), ed. by Christine Westphal, trans. from German; and Great Britain, National Gallery, National Gallery Technical Bulletin (annual). A study of painting on wood is Norman Brommelle, Anne Moncrieff, and Perry Smith (eds.), Conservation of Wood in Painting and the Decorative Arts (1978). A study of wall paintings is Paolo Mora, Laura Mora, and Paul Philippot, Conservation of Wall Paintings (1984; originally published in French, 1977). Studies of works on paper include Francis W. Dolloff and Roy L. Perkinson, How to Care for Works of Art on Paper, 4th ed. (1985); Anne F. Clapp, Curatorial Care of Works of Art on Paper: Basic Procedures for Paper Preservation, 3rd rev. ed. (1978, reissued 1987); and Chris Foster, Annette Manick, and Roy L. Perkinson, Matting and Framing Works of Art on Paper (1994).

Conservation and restoration of sculpture
General issues related to sculpture are addressed in Soprintendenza Alle Gallerie di Bologna, La conservazione delle sculture all'aperto (1971); Norman Brommelle, Garry Thomson, and Perry Smith (eds.), Conservation Within Historic Buildings (1980); Norman Brommelle and Garry Thomson (eds.), Science and Technology in the Service of Conservation: Preprints of the Contributions to the Washington Congress, 3–9 September 1982 (1982); and Norman Brommelle et al. (eds.), Adhesives and Consolidants: Preprints of the Contributions to the Paris Congress, 2–8 September 1984 (1984). Stone sculpture is examined in Deterioration and Preservation of Stones: Proceedings of the 3rd International Congress, 1979 (1979); Giovanni G. Amoroso and Vasco Fassina, Stone Decay and Conservation: Atmospheric Pollution, Cleaning, Consolidation, and Protection (1983); John Ashurst and Francis G. Dimes (eds.), Conservation of Building and Decorative Stone, 2 vol. (1990, reissued 2 vol. in 1, 1998); C.A. Price, Stone Conservation: An Overview of Current Research (1996); and Josef Riederer (ed.), Proceedings of the 8th International Congress on Deterioration and Conservation of Stone, 3 vol. (1996). Good sources on metal sculpture are David A. Scott, Jerry Podany, and Brian B. Considine (eds.), Ancient & Historic Metals: Conservation and Scientific Research (1994); Terry Drayman-Weisser (ed.), Gilded Metals: History, Technology and Conservation (2000); and E. Slater and N. Tennent (eds.), The Conservation and Restoration of Metals: Proceedings from the SSCR [Scottish Society for Conservation-Restoration] Symposium Held in Edinburgh, 30–31 March, 1979 (1979). Wood, as well as stone, is examined in Preprints of the Contributions to the New York Conference on Conservation of Stone and Wooden Objects, 1970, 2nd ed., 2 vol. (1971). See also Jackie Heuman (ed.), Material Matters: The Conservation of Modern Sculpture (1999).

Conservation and restoration of decorative arts
A good general source is Velson Horie (ed.), The Conservation of Decorative Arts (1999). The routine care of portable works of art is comprehensively treated in Hermione Sandwith and Sheila Stainton (comps.), The National Trust Manual of Housekeeping (1984), based on experience in maintaining the contents of England's historic houses. A good source on furniture is Robert F. McGiffin, Jr., Furniture Care and Conservation (1983). Ceramics are examined in W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed. (1976); Judith Larney, Restoring Ceramics, 2nd ed. (1978); W.D. Kingery and Pamela B. Vandiver, Ceramic Masterpieces: Art, Structure, and Technology (1986); Susan Buys and Victoria Oakley, The Conservation and Restoration of Ceramics (1993, reissued 1996); and Norman H. Tennent (ed.), The Conservation of Glass and Ceramics: Research, Practice, and Training (1999). See also Sandra Davidson and Roy Newton, Conservation and Restoration of Glass (2001). Textiles are treated in Sheila Landi, The Textile Conservator's Manual, rev. 2nd ed. (1998).

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Universalium. 2010.

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