any attempt to conserve and repair works of architecture, painting, sculpture, and the decorative arts (furniture, glassware, metalware, textiles, ceramics, and so on) from the effects of negligence, willful damage, or, more usually, inevitable decay caused by the effects of time and human use on the materials of which they are made. Ceramics, glassware, and gold are exceptionally durable, but artifacts in other materials are more susceptible to dampness, corrosion, and changes in atmosphere that cause warping and cracking; attacks by insects, fungi, and frost; tearing and staining; and the effects of simple aging or, if they are kept out of doors, weathering, effects that are likely to be compounded by the industrial atmosphere. 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 their original undamaged condition. 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 crude and unscientific methods of art restoration used in previous times yielded in the 20th century to dramatic technical advances that have made possible much safer methods of cleaning and repairing objects. Art restoration has also become an important tool of research, and it frequently enables the viewer to appreciate the original intention of the artist. any attempt to repair and preserve works of sculpture, painting, drawing, architecture, and the decorative arts (furniture, glassware, metalware, textiles, ceramics, and so on) from the effects of negligence, willful damage, or, more usually, inevitable decay caused by the effects of time and human use on the materials of which they are made. Ceramics, glassware, and gold are exceptionally durable, but artifacts in other materials are more susceptible to dampness, corrosion, and changes in atmosphere that cause warping and cracking; attacks by insects, fungi, and frost; tearing and staining; and the effects of simple aging or, if they are kept out of doors, weathering, effects that are likely to be compounded by the industrial atmosphere. Paintings are the most frequently and widelyrestored of all art objects. Paintings on wood panels may require straightening, because the panel may have warped and become bent convexly at the front (painted) surface. Such panels are reinforced at the back, or the paint surface is removed from the panel and transferred to a new canvas backing. In the case of paintings on canvas that has deteriorated, the old canvas is attached at its back surface to a new canvas in a process that is known as relining; the two canvases are sealed together with a thermoplastic cement. Areas of paint that have cracked, flaked, or lifted off the backing are fixed to the canvas with special adhesives or, alternatively, by relining the entire canvas. Areas in which the paint has been severely damaged or is entirely gone can be restored by inpaintingi.e., by applying new paint to those areas while carefully matching the technique and paint texture employed by the original artist. Darkened and discoloured varnishes are removed from a painting's surface with the use of alcohol solvents; this painstaking process constitutes the single most frequently practiced type of painting restoration. Sculpture restoration is a burgeoning field of activity, mostly because the moisture-bound pollutants in the air of urban or industrialized areas tend to attack and dissolve the surfaces of stone sculptures. Restoration and conservation involve first cleaning the stone and then applying certain chemicals to it that waterproof and strengthen the stone to a depth of several inches, rendering it resistant to further decay. The restoration of furniture, stained glass, textiles, and ceramics involves such techniques as replacing lost or broken elements from chairs and tables and substituting lost pieces of stained glass in a window; gluing broken pots or other ceramics together; impregnating the wood in a piece of furniture with special chemicals that protect it from the atmosphere and from fungi and insects; and washing and cleaning valuable textiles. Architectural restoration can involve stabilizing a building's foundation or structural elements through the use of concrete and of metal rods; repairing and waterproofing sections of a building that have been damaged by water or dampness; reinforcing a building's walls or roof; and cleaning and protecting a building's stonework, brickwork, or other forms of exterior decoration. The field of art restoration has always been marked by controversies, many of which have arisen when art restorers try to restore a damaged object to its original condition when that condition is unknown or uncertain to begin with. Nowhere did such controversies rage more fiercely than in the 19th century in the field of architectural restoration. In the late 18th and early 19th centuries, its purpose was to impose unity of stylethe style, in general, being Gothic. Where more than one medieval style was present, the architect would make a choice (commonly in favour of Decorated). This practice of restoring buildings to the style of periods especially favoured by the restorer came to be regarded by the middle of the 19th century as a falsification of history. Gradually the creed conserve, but not rebuild prevailed in Europe; and by 1900 it had started to be enforced by government legislation. Laws enacted in the 20th century and the success of foundations such as the national trust of England to preserve historic buildings reflect not only an increasing concern but an ever-widening view of the kind of building coming within the scope of such protection. In the United States both the National Trust for Historic Preservation and the National Park Service have somewhat similar functions. Additional reading Conservation of buildings 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; James Marston Fitch, Historic Preservation: Curatorial Management of the Built World (1982); 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; Jane Fawcett (ed.), The Future of the Past: Attitudes to Conservation, 11741974 (1976); Donald W. Insall, The Care of Old Buildings Today: A Practical Guide (1972); 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; Orin M. Bullock, Jr., The Restoration Manual: An Illustrated Guide to the Preservation and Restoration of Old Buildings (1966, reissued 1983); and Jane Jacobs, The Death and Life of Great American Cities (1961, reissued 1972). Conservation of paintings Helmut Ruhemann, The Cleaning of Paintings, with a comprehensive bibliography by Joyce Plesters (1968, reissued 1982); Harold J. Plenderleith and Anthony E. Werner, The Conservation of Antiquities and Works of Art, 2nd ed. (1971, reissued 1976); Norman Brommelle and Perry Smith (eds.), Conservation and Restoration of Pictorial Art (1976); Norman Brommelle, Anne Moncrieff, and Perry Smith (eds.), Conservation of Wood in Painting and the Decorative Arts (1978); Paolo Mora, Laura Mora, and Paul Philippot, Conservation of Wall Paintings (1984; originally published in French, 1977); and Great Britain, National Gallery, National Gallery Technical Bulletin (annual). Conservation of sculpture Soprintendenza Alle Gallerie di Bologna, La conservazione delle sculture all'aperto (1971); Preprints of the Contributions to the New York Conference on Conservation of Stone and Wooden Objects, 1970, 2nd ed., 2 vol. (1971); Deterioration and Preservation of Stones: Proceedings of the 3rd International Congress, 1979 (1979); Norman Brommelle, Garry Thomson, and Perry Smith (eds.), Conservation Within Historic Buildings (1980); Adhesives and Consolidants: Preprints of the Contributions to the Paris Congress, 1984 (1984); and Science and Technology in the Service of Conservation: Preprints of the Contributions to the Washington Congress, 1982 (1982). Conservation of other works of art The most valuable accounts are contained in the quarterly Studies in Conservation, published by the International Institute for Conservation of Historic and Artistic Work, London. See also Robert F. McGiffin, Jr., Furniture Care and Conservation (1983); S. Landi, The Textile Conservator's Manual (1985); Judith Larney, Restoring Ceramics, 2nd ed. (1978); John M.A. Thomson et al. (eds.), The Manual of Curatorship (1984); and Garry Thomson, The Museum Environment (1978). 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. Donald W. Insall Norman Spencer Brommelle J.H. Larson Decorative arts Furniture Apart from physical damage, effects of woodworm, and failure of glued joints, deterioration of furniture today is mainly attributable to the dry air (i.e., low relative humidity) of centrally heated rooms in winter. In the past, the air in houses was not much different from that of the cabinetmaker's workshop. Dry conditions, however, cause the carcass wood to warp and split, joints to loosen, and surface decorations such as veneer to detach. Further damage can be eliminated by humidity control (within 5060 percent relative humidity) the year round; a simpler alternative is the use of humidifiers in winter. Furniture should not be situated close to either humidifiers or sources of heat. Restoration of fine furniture requires a range of skills similar to those of the original cabinetmaker, coupled with restraint and judgment with regard to the integrity of the object. Replacements required for mechanical or aesthetic reasons are preferably of matching material finished to simulate the original patina. Replacement of worm-eaten wood can now often be avoided by impregnation of the damaged original with resin, usually an epoxy. For live woodworms there are proprietary liquid pesticides, although care must be exercised during application to avoid damaging original finishes. A gas chamber using, for example, methyl bromide or a mixture containing ethylene oxide is more effective. Repair requiring specialized skills may be needed for a range of materials. These include tortoiseshell, ivory, pewter, mother-of-pearl, and the materials of upholstery. Oriental lacquer presents unsolved problems that are still the subject of international discussion. Stained glass Stained glass windows may include pieces of coloured glass, uncoloured glass to which a thin coloured layer has been applied (flashed glass), and, in examples from the 15th century and later, uncoloured glass with a painted and fired design of coloured enamel. Deterioration other than physical damage results mainly from rain and condensation and consists of overall corrosion on both sides and pitting, generally on the outside only. Susceptibility to corrosion varies with glass composition. The corrosion layer darkens the glass, but its removal is a matter of some controversy. A variety of methods of local abrasion may be employed. Missing pieces can be replaced with new coloured glass and glass enamelled and fired in accord with the design. Original glass from which enamel has flaked owing to imperfect firing cannot be refired and has to be retouched with a cold-setting paint. The tendency of the enamel to flake makes the simplest operationgrime removal by washinga matter for particular care. Breaks, formerly mended with so-called strap leads, are now fixed with epoxy resins. Sometimes badly broken pieces are dry-plated with cover glasses, the pieces being inserted together into the leading. At the Cologne cathedral workshop, all the pieces are plated, using a clear resin adhesive. Deterioration of leading makes releading necessary every century or so. Organic surface coatings have been used for protection from the weather, but opinion is turning toward isothermal glazing, in which the window is replaced with plain glass, and the stained glass panel is hung inside with a narrow gap between the two, open to the interior atmosphere. Paintings 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. 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) painting on paper and ivory. More or less portable paintings on canvas or panel are called easel paintings. 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 layer itself, which may be complex in structure; 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 Wood-panel supports were used almost universally in European art before about 1450, when canvas began to gain ground. Wood has the disadvantage of swelling and shrinking across the grain with 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 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 to a higher humidity may suffer considerably. In both Europe and the United States, the effect of an unsuitable environment of low or changing relative humidity and 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 across the back of the panel as constraints. This solution, however, often led to severe distortion of the front surface and cracking of the whole panel in lines along the wood grain. Extensive damage to the paint sometimes occurs, and drastic restoration is needed. In terms of preservation, the ideal solution is a form of air conditioning in which the relative humidity is maintained as nearly constant as possible at what is generally agreed to be the most reasonable level; i.e., about 55 percent. 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 gentler constraint over the whole surface. It is normal in the 20th century to accept as inevitable some permanent convex curvature. The adhesives used and the composition of the new secondary support take many forms. One consists in backing the panel with strips of a very light, open-textured wood (balsa), using as a cement a mixture of beeswax, a natural resin, such as dammar, and an inert filler. This thermoplastic cement, which is applied as a hot, creamy liquid, solidifies without contraction. The epoxy resins, which also harden without contraction, have been used as well and have the additional advantage of not requiring heat. The material and cement used are chosen according to the nature of the original panel. Some restorers reduce the strength of the original panel, before applying the secondary support, by reducing its thickness. This practice is not universally approved. Occasionally, when the panel is badly worm-eaten or severely cracked, it has to be removed from the paint and ground altogether in the process known as transfer. This is accomplished by pasting a substantial support of paper and, possibly, canvas to the front surface and then gently gouging away the wood on the back. An entirely new, inert support of balsa wood or compressed board is then cemented to the back and the facing removed. Sculpture Until the mid-1960s, painting conservators led the whole field of conservation with their technical expertise and experience. Today that imbalance has shifted, and it is in the field of sculpture conservation that many of the most complex and exciting technical developments are taking place. This change can be attributed partly to the growing international concern with the problems of stone decay and partly to the rapid development of more sophisticated synthetic resins for use in repair. The realization that the heritage of stone sculpture and buildings might not remain in a recognizable form for more than another 50 years has stimulated an interest in the conservation of sculpture that would not have occurred were the problems merely confined to museum collections. New pressures also are being exerted on museums. As the main centres of conservation expertise, they are called upon to lend their aid to the organizations dealing with these problems in cathedrals, churches, and historic houses. They also act as places of refuge for endangered sculptures. The scale of the problem (the rapid increase in air pollution worldwide and the vast number of sculptures involved) has caused the profession of sculpture conservation to develop rapidly. Although it has been possible to borrow from painting conservation many surface cleaning techniques for use on terra-cotta, plaster, and polychrome wood sculpture, very few of these techniques are suitable for marble and limestone, with their particular susceptibility to damage from water and soluble salt migration. The main problems facing the stone conservator are stabilization, consolidation, and further protection against pollutant gases and soluble salts. Stone is extraordinarily unstable in the modern environment. Once it has been attacked by pollutant gases, such as sulfur dioxide, or migrating salts, such as nitrates or chlorides, it is difficult to return the stone to a stable condition, even when it is placed in a museum environment. Although some temporary stability may be achieved by putting a damaged sculpture in a temperature- and humidity-controlled glass case, it is commonly found that degradation will continue and, in certain instances, even accelerate. The mechanism by which soluble salts cause damage in stone and marble is complex, but the broad effect can be simply characterized. These salts, which can be derived from pollutant gases, a marine climate, or natural ground salts, dissolve in water and seep through the pores of the stone. The water may derive from the ground, from the atmosphere, or from oversaturation of the stone by inappropriate water-washing. As the salts move to the drier air at the surface of the stone, they begin to crystallize either on the surface (efflorescence) or beneath the surface (subflorescence). It is this crystallization and resultant expansion of the salts that breaks open the pores of the stone and creates the damage. To have any hope of halting this activity, the stone conservator must interfere with the deep structure of the stone, sealing it against moisture movement and strengthening it against salt damage to the pore structure. Probably the most popular means of stabilizing stone is the introduction of a consolidant. In the past, consolidants such as wax and shellac have been tried. These do not penetrate deeply into the stone and often aggravate the problem. Various synthetic resins, such as acrylics, epoxies, polyesters, and silicones, have been used with greater success. By far the most successful, however, have been the alkoxy-silanes. These have several distinct advantages over other consolidants. They penetrate deeply into the stone (two to three inches in some limestones), and they deposit a hard, almost indestructible network of silica in the pore structure of the stone, which waterproofs and strengthens it. There are many forms of alkoxy-silanes in use and many ways of applying them. The commonest methods are simple brushing, spraying, and vacuum impregnation. Of these, the first is the most controllable and delicate approach, while the last is the least controllable and most potentially dangerous. The full treatment of a sculpture must, of course, include cleaning. Any consolidation treatment or attempt to remove salts from stone must be carefully integrated with an appropriate cleaning system. In the past, the most common way to clean and desalinate stone was to immerse it in a tank of water for a period of weeks or months. This process can cause considerable damage because it loosens friable stone and pigment from the stone surface. A better method was developed in the 1960s, by which a clay poultice (magnesium silicate and deionized water) is used to suspend a thin layer of water over the surface of the sculpture, like a cosmetic mudpack, sucking out both dirt and salts. This treatment minimizes the contact with water and also does less harm to the fragile surface of the sculpture. The use of sophisticated tools and techniques such as ultrasonic dental descalers and abrasion by air-blasted microscopic glass beads helps to give the conservator much greater control over the cleaning process. Lasers were first used for removing pollution deposits from stone in 1970. Improved laser technology, decreasing cost, and the concurrent development of fibre optics suggests that it may soon be possible to produce a flexible precision tool that is capable of removing dirt and other encrustations from the surface of sculpture by vaporization, without harming the stone itself. The other main advance in sculpture conservation has been the growing use of the binocular microscope. When the surface of a sculpture is examined under a surgical microscope of from 10 to 40 magnification, the immense damage that can be inflicted on sculpture by indiscriminate cleaning becomes strikingly apparent. Tiny fragments of original pigment often remain on sculptures that to the naked eye appear completely unpainted. From these pinpricks of paint, technicians can determine the exact composition of ancient sculptural pigments and their binding mediums. J.H. Larson

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