LOSS COMPENSATION METHODS FOR STONEJOHN GRISWOLD, & SARI URICHECK
ABSTRACT—This article presents a compilation of considerations and approaches for loss compensation in stone, with emphasis on structural fills. A structural fill is one that imparts more than mere cosmetic reintegration of a damaged surface by contributing to the structural integrity of the whole, or part of the whole. Two primary bodies of literature, architectural preservation and art conservation, serve as the basic foundation for this review, supplemented by numerous citations of personal discussions with conservators.Treatments of sculptural and architectural stonework from both interior and exterior environments are discussed, following an outline of criteria to be considered when choosing a fill material. An overview of current practice is divided into two main approaches: replacement with a stone or stone substitute and plastic repairs. A number of different materials and techniques for stone filling are then reviewed, organized into two main categories of organic and inorganic binding materials. TITRE—Methodes de remplacement de la pierre. R�SUM�—Cet article constitute la compilation de diverses consid�rations et approches concernant le remplacement de la pierre et, en particulier, les r�parations structurelles de ce mat�riau. Ces derni�res, outre qu'elles redonnent un aspect esth�tique � une surface endommag�e, ont pour but de contribuer � un ensemble, ou � une partie d'un ensemble, son int�grit� structurelle.Cette compilation, fond�e, pour l'essentiel, sur des documents concernant, d'une part, la conservation des monuments et, d'autre part, celle des objets d'art, se compl�te de nobmreux t�moignages obtenus directement aupr�s de restaurateurs. Les auteurs d�crivent d'abord les traitments de la pierre, tant dans ses emplois en sculpture qu'en artchitecture, et tant dans un environnement ext�rieur qu'int�rieur. Ils �voquent ensuite les crit�res � prendre en compte lors du choix du'un mat�riau de remplacement ou de rebouchage. Leur �tude des m�thodes actuellement pratique�s se divise en deux approches principales: d'une part, le remplacement, par de la pierre ou par un mat�riau de substitution et, d'autre part, les r�parations � l'aide de mat�riaux de rebouchage. Les techniques de rebouchage sont ensuite r�pertori�es, selon que les mat�riaux employ�s comportent un liant organique ou inorganique. TITULO—M�todos para compensacion de faltantes en piedra. RESUMEN—El prop�sito de esta revisi�n es presentar una recopilaci�n de las consideraciones y enfoques para realizar compensaci�n de faltantes en piedra, con �nfasis en rellenos estructurales. Un relleno estructura es aqu�l que imparte m�s que una mera reintegraci�n cosm�tica a una superf�eie da�ada, contribuyendo con la integridad estructural del todo, o parte del todo. Sin embargo, se pueden utilizar los mismos materiales y t�cnicas, ya sea que se requiera o no de resistencia estructural o de soporte para los elementos circundantes. Un relleno puramente cosm�tico puede tener propiedades estructurales parecidas a la piedra original, adem�s de propiedades visuales similares.En esta revisi�n se discutir�n tratamientos a esculturas y obras arquitect�nicas de piedra en ambientes interiores y exteriores. Esta revisi�n empieza con un bosquejo de los criterios a considerar cuando se escoge un material de relleno. �ste es seguido de una discusi�n de las pr�cticas actuales, las cuales son divididas en dos enfoques principales: reemplazo de faltantes con una piedra o sustituto de piedra; y reparaciones modeladas 1 INTRODUCTIONStone restoration is an ancient and vast discipline that encompasses the treatment of individual stone artifacts, outdoor monuments, and architecture. While the deterioration mechanisms, cleaning, and consolidation of stone have received much coverage in the literature, the practical aspects of loss compensation have received little in comparison. The vital role of appropriate compensation methods in the overall longevity of an object, and the potential risks and damage from faulty ones, are important to recognize. The purpose of this article is to present a compilation of the considerations and approaches for loss compensation in stone. Treatments of sculptural and architectural stonework from both interior and exterior environments will be examined. The focus of the discussion is treatment methods that impart more than mere cosmetic reintegration of a damaged surface by offering not only visual but also structural unity. For instance, treatments such as masking a loss with a piece of Japanese tissue will not be described (Hatchfield and Marincola 1994). The review begins with an outline of criteria to be considered when choosing a compensation method. An overview of the two main approaches—replacement with a stone or stone substitute, and plastic repairs which harden in place—follows. Different materials and techniques for stone fill are then described. Two main bodies of literature: writings on architectural preservation and art conservation, serve as the basic foundation for this review. A general survey of the literature reveals that relevant articles in journals devoted primarily to architectural works significantly outnumber those found in the art conservation journals. Differences in terminology exist, for example the term “fill” used in the fine arts conservation literature, where the phrases “mortar repair” and “composite patch” communicate the same concept in architectural terms. If the literature is approached with a broad focus, a look at fill materials in glass, porcelain, ceramic, and wood conservation reveals methods and philosophies also directly applicable to structural fills for stone. The selection of materials and methods for structural fills for stone must be tailored to the specifics of each treatment. Stone type, environmental concerns, and cultural context will influence the conservator's choices. Mineralogical variations of types of stone may require different approaches, and often the same stone in different environments may receive different treatments. Cultural biases and expectations inform treatment decisions as well: for example, works of contemporary art may require a different degree of visual reintegration of losses (Lowinger and Williams 1994) than that of neoclassical, classical, or ancient works, while the treatment of Asian works (Scheifler Marks 1994) may follow a different philosophy of compensation than Western works. Archaeological stone objects may require reassembly for interpretation or stabilization, or they may not as the case may be. In short, no one treatment is applicable in all cases. In many instances, multiple approaches are used in a single project, a fact that emphasizes the importance of context in the selection of appropriate methods and materials (Bongirno 1977). 2 CRITERIAThe following criteria may be considered when choosing a compensation method for stone. The ideal compensation method:
While this generic list applies to any stone compensation, those exposed to outdoors are subject to additional criteria, such as:
An outdoor fill must possess similar physical properties to the original stone in order to allow equivalent exchange of water across the stone-compensation interface and to react to the environment in a compatible manner. If a fill is less porous than the stone, water and soluble salts can accumulate around it in the stone, leading to damage. Further damage can result along this interface if the stone and the fill have different dimensional responses to moisture and temperature. If the fill is harder than the original stone, the stone will be preferentially eroded. Since an exact match to the substrate is difficult to achieve, the fill should be somewhat more porous, more permeable, and slightly weaker than the original stone. Such a fill is “sacrificial,” attracting moisture and salts and thus causing disintegration of the fill instead of the stone. This condition is especially important when the stone is already extensively degraded. Discussion of these properties may be found in several sources (Peroni et al. 1981; Weber and Zinsmeister 1988; de Masy 1992). It should be noted that in architectural contexts, certain specifications based on governmental legislation and other established standards must also be obeyed in conservation treatments. These include local building codes, historic preservation guidelines, and standards from organizations including the British Standards Institution and the American Society for Testing and Materials (ASTM 1980). The above list is probably incomplete, and it is presented only as an ideal. For the purposes of this article, it may be used as a gauge by which to judge the acceptability of treatments in particular contexts. 3 TECHNIQUES OF LOSS COMPENSATIONTechniques of loss compensation for stone may be divided into two general classes: replacement and plastic repair.1 A replacement may fill the loss with a newly carved stone element or some other discrete modeled material that is adhered into place. Plastic repairs use a pliable material that hardens in place and thus adheres itself to the substrate while filling the void of the loss. The choice between the two classes is based on factors such as the size of the loss, resources available, and context. A large or protruding loss would be difficult to fill with anything other than a discrete replacement element, while local damage may require a plastic fill. Resources of time, money, and expertise must also be considered when choosing between repair and replacement. Carving a new stone element may cost more in time and materials than a budget allows. If a repeated element is to be compensated, the most practical way may utilize cast multiples. If a stone carver is not available, replacement stone pieces may be difficult to fashion. Historically, fitting a replacement stone piece into an area of loss was the primary mode of treatment. Today, plastic repairs are often the first choice as a compensation because their use allows preservation of all existing original material. Preparing the stone surface to receive a new addition traditionally requires planing of the break edges. While the planing of broken noses 4 STONE REPLACEMENTWhen part of a building stone is damaged to the extent that it cannot perform its function as a load-bearing element, stone replacement is often justified. Replacement is the system of compensation in which a piece or unit of stone (natural or imitation) is fitted to the area of loss in the original stone. Replacement may be “in kind,” with the unit made of the same exact stone, in “near kind” with a similar stone, or in an imitation stone. The process of using replacement stone pieces for localized damage is sometimes referred to as the dutchmen technique.2 The use of a dutchmen is practically suited to large-volume losses. It is often chosen in the case of marble and granite compensations, even indoors, because it is otherwise difficult to produce the appearance of these stone (Sourlis 1988). A dutchmen repair is a labor-intensive process. Standard procedure for filling a loss with a replacement is first to identify the substrate through petrographic analysis or archival information and to match this original with the chosen replacement stone. The damage must be assessed and the original profile determined with or without a previous model (photographic or otherwise). When required, the damaged or deteriorated part of the stone will be excavated and in many cases the break surfaces planed to allow a good fit for the replacement. A model of the form is made to fit the loss, usually out of clay or plaster. Based on this model, a copy is roughed out in the stone using pointing techniques. Sometimes the stone source for a dutchmen may be salvaged from an inconspicuous place on the monument, ensuring a proper match. With exacting measure the new piece must be tightly fitted into place. Supporting rods, usually stainless steel but sometimes titanium (Zambas et al. 1986) or polymeric composites, are used to dowel pieces together. The stone is adhered with epoxy, polyester, or mortar and clamped so the join may set. At this point, the roughed-out form is carved and refined to match the adjacent surface of the original. The process traditionally entails resurfacing the insert and the adjacent original stone to the same profile and finish. This last step is what often allows a virtually invisible replacement repair but is what commonly precludes the method's use in fine arts conservation. The use of dutchmen is often a more effective and appropriate solution in architecture than sculpture because the forms being replaced are usually rectilinear. Also, the presence of mortar joints between the stones allows the replacements to be integrated as an original element of the building design. On sculpture, a replacement seam is an interruption of the form that must be hidden, best accomplished by following a natural contour. The adhesive join between the replacement and the original is one of the most troublesome aspects of the dutchmen technique. This layer of epoxy or other adhesive could act as a moisture barrier, and its excessive strength may cause extraneous stress and possible delamination at the surface. Epoxy and polyester adhesives also weather poorly, darkening due to oxidation upon light exposure, which accentuates the seams of the compensations. To alleviate these problems, precautions such as bulking and pigmenting the adhesive, adding antioxidants, or “spot” adhering with adhesive and patching the rest with a cementitious grout may be employed. Despite their failings, these resins have not been superseded by other adhesives because they allow the thinnest possible seam to be made while successfully securing the replacement. Fresh replacement repairs of losses made with natural, newly carved stone are often aesthetically unpleasing. When first applied, the freshly finished surface may not match the surrounding original stone, although after exposure the repair surface may gradually become better integrated through soiling and weathering. In some cases a Another method of matching the stone more effectively is to use precast “artificial stone” that is manufactured to mimic the weathered surface in color and texture. Artificial stone can be cast in a mold to match the original, as utilized at the Metropolitan Club in Manhattan (Matero and Tagle 1995), or carved after casting to fit a repair and to match a surface, as at the reconstruction of the Basilica of St. Arbogast. This work is often performed off-site, made to order from plaster models by specialists (Murphy and Ottavino 1986). The creation of artificial stone for original building elements, sculpture, and structural repairs is an ancient practice. Old inserts of artificial stone were found in a sandstone during restoration (Kralova et al. 1986). “Roman stone” is the Renaissance term used by Vasari for artificial stone made using natural hydraulic mortar and aggregate (Fitchie 1978). An 1850 treatise on artificial stone describes recipes for the latest modern stone substitutes (Fowler 1850). The development of glazed terracotta and other materials that imitate stone, such as Coade stone (Kelly 1978) was spurred by a growing market for elaborate building facades. Replacement pieces can be made from a variety of other cast or pre-formed materials. These include modeled putties, laminated fiberglass shells (Fidler 1982; Rawson 1994), and other systems with underlying supports such as stainless steel armatures. Some complex treatments entail the use of fitted reversible or adjustable machined parts for reconstruction of monumental sculpture (Podany 1987; Garland and Rogers 1995). Painted wood has been used to reconstruct or fill large losses where stone replacement was not considered economically or structurally feasible (Phillips 1982). Care must be taken, however, that such nonstone replacement pieces do not merely cover the problem but eliminate sources of further deterioration. Such methods often involve the risk of exacerbating underlying problems, which can go undetected for long periods (Nelson 1992). 5 PLASTIC REPAIRWhile replacement repairs are sometimes required by the size or context of a loss, most conservation treatments now employ plastic repairs because they preserve all original material. The term “plastic repair” defines a moldable fill applied directly to the loss and set into place by its own adhesion to the substrate. It includes mixtures such as mortars and putties. Mortar is a lime-based or cementitious mixture traditionally used to join masonry units. Putty (as described in this article) refers to materials that contain an organic binder and have the working consistency of a dough3. The texture of each type of mix is dictated by the ratio of components and may be fashioned to fit the appropriate scale of a loss. Many larger-scale plastic repairs, including inorganic mortars and organic systems, often require internal reinforcements, such as wire mesh, epoxy splints, metal pins, or dowels. Often Plastic repairs are composed of a binder and filler (sometimes called matrix and aggregate), color components, and special additives. Constituents may be either inorganic or organic. The main classes of inorganic binders—plasters and natural and modern cements—as well as the various thermoplastic, reaction-cured, and solvent-cured organic binders will be discussed further. Fillers are inert materials such as glass microballoons, crushed stone, bits of solid resins, and other fine- to coarsely-ground materials. Color may be created by the chosen filler or added to a plastic repair by the inclusion of pigments, colored stone flour or sand, soil, dyes, or crushed enamel glass (Griswold 1990b). Additives modify the working or cured properties of a repair and can include light stabilizers, among other things. Fillers serve several important functions in plastic repairs. As their name implies, they act to bulk up the binder and in effect temper the qualities of the binder and create specific desirable properties. By the addition of a filler, adhesives may be made weaker (or stronger), dense materials may be rendered less dense, and the autoxidation process of organic resins may be retarded. The addition of inert inorganic fillers can reduce thermal stresses by lowering the coefficient of thermal expansion (CTE) of organic binders (Barov and Lambert 1984). Filler materials, ranging in particle size from fine powders to large aggregates, help create the desired density, texture, porosity, color, degree of translucency, and gloss. Careful selection and mixing of matrix and aggregate can result in a compatible match with a porous stone. In some cases, fillers reduce the necessary quantity of expensive adhesive binders and other components. Some additives do not just fill up space, but modify the properties of the binder. One of the most common additives is colloidal fumed silica. This material is a thixotropic agent that stiffens a mixture by forming a weakly bonded network of silicon dioxide in suspension. When agitated with a tool, the mixture becomes more fluid (Moll and Schirripa 1980), creating a thick but spreadable putty mixture. Additives of another nature are light stabilizers for polymers, such as UV absorbers. These are commonly added to commercial resins (such as Akemi Marmorkit 1000 polyester resin) and can be included in homemade mixes as well (G�nsicke and Hirx 1997). When choosing a binder-filler system, the compatibility between properties of filler and binder must be considered. Inappropriate combinations (such as stone chips of a high CTE within a resin of low CTE) may result in differential reactions to changes in moisture and thermal conditions. These reactions can result in internal stress and loss of adhesion. In addition to the proper selection of binder and filler types, the ratio of binder to filler and the relative percentage of aggregate particles with different sizes can be critical to achieving the desired properties of the fill. The shape and size of the aggregate determine the nature of the intergranular porosity, which may or may not be filled with the adhesive binder. 6 INORGANIC BINDERSThere are three general classifications of inorganic systems: plasters, naturally occurring cements, and modern cementitious mortars. Despite the advent of modern polymeric materials, which offer a greater range of visual effects, inorganics are still chosen for stone fills mainly because of their strength, stability, durability, and availability. In these fills, both binder and filler are composed of carbonate and silicate networks. The Disadvantages include excessive hardness; introduction of soluble salts; poor permeability by salts and moisture; shrinkage; introduction of large amounts of moisture to the substrate during use; and a generally cool, opaque appearance. 6.1 PLASTERThere are two main types of plaster: gypsum (calcium sulfate) and lime (calcium carbonate). Both have been used since antiquity for restoration of sculptural and architectural work. They are often the only component of a repair mixture, but they may be bulked with fill materials such as crushed stone and microballoons to enhance textural qualities. The color of a plaster repair may be integral (included in the curing mix) or added later as a painted surface finish. Gypsum plaster, e.g., plaster of paris, has traditionally been used for restoration in ceramics and stone, and much has been published on its properties. Its basic chemistry is calcium sulfate hemihydrate, which takes on additional water on mixing and loses it as the gypsum network recrystallizes on drying. Industrial grades are commonly superseded by dental plasters; these plasters are in a refined state and are quick-setting due to added fumed silica, which allows application of the fresh mix on vertical surfaces. Patent plasters such as Keene's Cement, invented in England in the late 19th century, are modified gypsum plasters. The manufacture of such plasters involves repeated heating above 170�C to form the anhydrite of calcium sulfate. A seed catalyst such as alum is added during heating to enable the plaster to set (Ashurst 1979). Because it forms a tightly intertwined network of needlelike crystals on curing, it becomes harder and is less soluble than plaster of paris. It found widespread use in the manufacture of scagliola (imitation marble) and for architectural casts. Unlike plaster of paris, which cures quickly, Keene's Cement can be kneaded into a doughlike loaf, which remains pliable for hours if kept moist. Introducing pigment on the surface of the loaf, folding it, and slicing thin layers with silk thread or dental floss creates a three-dimensional facsimile of veined marble. Many commercial plasters like Polyfilla (in the United Kingdom) and its counterpart Permafill (in North America) are also modified gypsum plasters. In their mixes are cellulose-based additives, which strengthen the mix and make it pliable for sculptural modeling, and whiting (calcium carbonate), which retards setting and increases the hardness of the gypsum plaster. A number of other additives have been used to modify the properties of gypsum plaster. To slow the setting time, glue, starch, and vinegar have been included in traditional recipes. Carbohydrates such as sugar or beer also slowed the setting rate and produced a harder plaster (Thornton 1992). Glues and polyvinyl acetate emulsion are also used to increase strength and reduce porosity of plaster fills. Common practice now includes wholesale consolidation of plaster fills with Paraloid B-72 (Koob 1987). It has been argued that gypsum plaster introduces soluble salts to a substrate (Ashurst and Dimes 1990), but others feel that the use of refined chemical-grade plaster eliminates this concern (Soultanian 1996). Acrylic resin barrier coats on break edges or the attachment of cured plaster fills with an adhesive are used to reduce salt migration. Despite these concerns, as well as slight solubility after curing, plaster repairs are often used on stone sculpture in indoor contexts. They are one of the simplest and quickest types to fashion and finish. Coloring of plaster fills may be accomplished by painting after cure, by introducing pigments in the dry plaster powder, or by adding pigments, paints, or dyes to the wet mixed plaster. Reasonably acceptable faux finishes are achievable by widely practiced decorative painting techniques. However, the durability of such painted surface finishes on plaster, outdoors or indoors, is often unsatisfactory in the long term. Lime plasters share a similar binding chemistry to gypsum plasters in that their curing is based on the mixture and subsequent loss of water, which returns the original compound before water was introduced. Lime technology is based on the burning of limestone (primarily calcium carbonate) to form quicklime (calcium oxide), which is then “slaked” by adding water to form the hydrated form, calcium hydroxide, Ca(OH)2. The mix sets on drying, and as carbon dioxide is absorbed the lime slowly hardens by reverting to calcium carbonate. For a concise review of chemical terminology and mechanisms pertaining to its manufacture, application, and curing, see Ashurt and Dimes (1990). Lime putty, the product of mixing water and quicklime together, is often used as a binder. However, it is not a strong binder by itself and requires a filler such as sand or stone powder (Szczerba and Jedrzejewska 1988). It shrinks on setting and remains slightly soluble in water over time. Being weaker than cements, these lime plasters are compatible with ancient mortars and traditional stonework, whose strengths have diminished over time. While lime mixes may lose structural integrity due to the water solubility of calcium carbonate, this quality is preferred over the use of a repair that is too strong and poses the risk of damage to the substrate (Sass 1996). Custom-made lime mortar mixes are often chosen over commercial mortar mixes because soluble-salt content is easier to control, since the choice of raw materials may be monitored by the conservator. Acid dissolution, difficult working properties, and the need for a dry-set sometimes preclude the use of lime plasters as fill materials. Monuments with moisture problems due to rising damp are especially poor sites for their use (Peroni et al. 1981). The dissolution products of lime plaster, including alkaline-soluble salts, can damage adjacent areas. However, because of their efficacy in other contexts, specialized techniques are constantly being developed for working with lime mortars, such as hammering components to induce microporosity and adding organic components such as straw (Ma 1995). 6.2 NATURAL CEMENTLime plaster is the main component in cements. A cement is a mortar that sets in the presence of water and is classified as a hydraulic mortar. Hydraulic mortars cure by the reaction of hydrated lime with silica and alumina components, which may be deliberately added or simply present in naturally occurring clays that contaminate the lime source. Limes that naturally contain a high amount of clay are called hydraulic limes and create a high-strength natural cement (Wisser et al. 1988). Added alumino-silicate sources may be termed pozzolanic additives. These are named after pozzolana, the high-silica volcanic ash from the Pozzuoli mountains, which so enriched the lime resources of Italy. Examples of pozzolanic additives include brick dust or fragments, charcoal ash, volcanic stone and ash, river sand, glass, and certain types of crushed stone. All impart greater strength and insolubility to the cured mortar. Since ancient times, repair of buildings has been accomplished by the use of natural cements otherwise known as Roman mortars. Vitruvius describes them in De Architectura (VII.II.2) (Morgan 1960), and since the first century A.D. they have not changed drastically. The same hydraulic lime and pozzolana-based binder is utilized (though more refined than ancient compositions) with the addition of aggregates of crushed stone or glass and pigments. Because cementitious fills shrink upon curing, a thickly applied layer will dry with extensive cracking. To compensate for this cracking in large fills, a two-layer system is used, with a coarse ground-primer layer (coccioposto), and a fine surface layer (stucco) above (Demitry 1988). A typical composition, as used for restoration of the Arch of Septimus Severus, is “for the coccioposto: 1 part slaked lime, 1.5 parts pozzolana, and 1 part brick fragments; and for the stucco: 1 part lime to 2 parts of fill which is: 4 parts marble Much research has been devoted to determining and repeating the compositions of ancient mortars (Penkala and Zasun 1988; Ma 1995) with pozzolanic additives (Wisser et al. 1988; Penelis et al. 1989) in building and ancient restoration campaigns. Their success and durability over the centuries gives natural aging data by which to judge them. Studies of their properties and how to manipulate them through compositional changes have been pursued (Holmstrom 1981; Peroni et al. 1981; Szczerba and Jedrzejewska 1988; Ma 1995), though the applied results are not generally found in the literature. However, the ancient and natural chemistries of pozzolanic additives have been adopted through the introduction of modern hydraulic additives like siliceous earth, condensed silica fume, fly ash, and phonolite dust (Wisser et al. 1988). 6.3 MODERN CEMENTSIn the 19th century, Portland cement was designed to mimic naturally hydraulic cements. By firing clay and lime at high temperatures, an “artificial” hydraulic lime is synthesized. Typical mortar mixes are made of white and/or gray cements, hydrated lime, sand, and stone flour with alkali-stable pigments. Variations of the ratios (Weiss 1989) and choice of additional aggregate can modulate the texture and color of the fills. Addition of crushed stone or soil for color is considered to be better than using pigments, as the migration of unreacted pigments to the perimeters of patches is a common problem. For conservation purposes, Portland cement's strength, low porosity, high alkalinity, and soluble salt content often limit its use because they contribute to accelerated deterioration of the adjacent stone (Cassar 1988). Changes in the cement/water ratio can aid in controlling the porosity and to some extent the strength of a mortar by dictating the free water in the mix. As the mix cures and this water evaporates, its volume becomes free space; hence, the greater the free water, the greater the porosity. However, this effect may be counteracted by the formation of a “float” of finer particles at the surface of the fill because of excess water, resulting in reduced porosity and other inhomogeneous properties. Resin and natural additives have traditionally been incorporated in cements to enhance the adhesion to the substrate along the “bond line.” Organic polymers in the form of aqueous dispersions or emulsions are added to cement in commercial formulations to improve adhesion, modify water absorption properties, shorten or lengthen setting time, increase strength, and reduce shrinkage on drying. Other organic additives to concrete, such as fibrous elements (e.g., jute), microfoaming agents, and surfactants, induce porosity but do not ultimately contribute to the strength or appearance of the fill on curing. The addition of acrylic latex or PVA emulsion additives can increase strength and water repellency in grouting mortars and has frequently been adopted in patching formulations. Ancient recipes cite the inclusion of “blood, egg, sugar, cheese and dung” (Sickels 1981) as additives in mortars. The incorporation of alkoxysilane stone consolidants in the patching mix has been reported as well (Hempel and Moncrieff 1977; Andersson 1986). The inclusion of microfoaming agents, which encourage air-entrainment, is shown to modify porosity in natural cement mixes, but this approach does not necessarily create increased air or water permeability because an interconnected network of pores is not guaranteed. The addition of natural or synthetic microfibers may help provide such a network (Ma 1995). The flexibility of these recipes is often cited as an advantage by those who utilize them. For some conservators, however, the versatility of such mixes is viewed as a flaw in the method. Accurately replicating a recipe consisting of many components is difficult, and for a large-scale 6.4 COMMERCIAL MORTARSCommercial products have been developed to make consistent results between batches easier to achieve. Some possess superior working properties compared to “homemade” recipes. The major disadvantage of choosing a commercial product is uncertainty about the ingredients due to the protections afforded proprietary companies, which allow the details of product compositions to remain secret. Despite this drawback, commercial mixes, with appropriate analysis, should not be ignored in cases where the consistency of results in a cementitious repair is vital. A number of commercial products are available, though the selection of conservation-quality materials is more limited. Products are often favored based on regional biases of individual manufacturing centers and advertising focus. For example, Ledan and Mapei products are primarily manufactured and used in Italy. Keim is a German company whose sales efforts focus on Germany, Scandinavia, and Great Britain, but these products are also used in North America to a lesser extent. Jahn Mortars is a Dutch company that has an international clientele with a growing base in America through its sole distributor, Cathedral Stoneworks. Edison Coatings is an American product line whose market includes Mexico, Canada, and England. What distinguishes these products, in general, from other nonrestoration quality materials is lower alkalinity, lower soluble salt content, and lower strength. Some products are also custom matched by the manufacturer to visual and physical properties of specific stones. This discussion is restricted to two commercial products available in the United States, Jahn mortars and Edison patching systems. Both of these product lines are in common use by conservators, but few references to their use have been made in the conservation literature (e.g., Wheeler and Newman 1994). In essence the basic cementitious lime-silicate chemistry is the bonding mechanism for both product lines, although Jahn Mortars include natural pozzolanas. Additives like microfoaming agents, which create porosity, may also be included in each, but this is not confirmed. As for all cementitious mixes, the curing schedule is a crucial part of a system's inherent properties, and Jahn and Edison provide specific mixing and curing instructions and suggestions. The properties for Jahn are stated in the product literature, and preparation and application parameters are aggressively encouraged through written instructions and mandatory training courses. Each mortar mix is composed of sand, lime, and mineral additives, with the addition of crushed stone in the Edison line. A look at the raw components of each dry mix demonstrates this marked difference in the textures. Jahn Mortars appear as a fine grained powder, while Edison's Customs System 45 shows a texture with a greater variety of aggregate size. Discrete stone flakes can be easily discerned in Edison, while not in Jahn. This variance in raw material translates directly into different cured textures as well. Admixtures of other fillers can be incorporated, but the guarantee of certain physical properties will be lost. In addition, there are limits to the amount of bulking of a mix that can be tolerated in the network strength, and these limits need to be determined. Both product lines offer the service of custom matching to a provided substrate sample. Jahn products are completely inorganic and, as such, circumvent issues of light stability of resin components. The products are dry powders to which is added a relatively small amount of water. Edison's products include an acrylic emulsion added to a dry powder and water mix. Although the acrylic is not part of the curing mechanism, it augments the strength of the bond between the patch and the substrate. Therefore, a thinner, more feathered application is reportedly possible For Jahn M70 and M120 products and the Edison Custom System 45, standard batches for limestone, marble, sandstone, granite, and other materials are modified to match the specific stone sample submitted. Matching is offered in color, texture, and physical properties, such as compressive and tensile strength, permeability, modulus of elasticity, and coefficient of thermal expansion (see product literature). Multiple mixes of hues may be ordered to accommodate the natural color variations in a given stone. Individual modification of the batches by the addition of pigments is feasible but not recommended by the manufacturers for most uses. Consultation with technical support staff has proven fruitful in addressing specific conservation treatment problems for objects. According to the practical observations of several conservators, the strengths of the Jahn line include carvability and ability to build up large complex shapes in a single application. Reported shortcomings include their fine texture, which does not accurately match all stone, and their opacity, which means they cannot closely match a translucent stone. Since there is no added acrylic polymer, a minimum depth and keyed edges are needed for a Jahn fill to adhere effectively; feathering out the edges of the fill is not advisable. Edison products reportedly can have a slight sheen, which can be favorable or unfavorable depending on what type of surface is to be repaired. Diluting the additive or redressing the patch surface can reduce this sheen and is usually considered a minor concern (Champe 1996). In spite of strict recommendations for the preparation and application of these commercial products, conservators have found ways to modify their properties to suit particular requirements of deteriorated stone. For example, different ratios of mix to water will yield varied properties (Williams 1996). For degraded local areas of a particular stone, the commercial patching mixes made specifically for that stone can still be too strong. Conservators use several ways to reduce the strength of the mortar in the field. Adding crushed or powdered stone to the batch, adding excess water in the mix, adding water to reconstitute a partially cured mixture in the bucket, or skipping the recommended regimen of misting the patch with water after application can all result in a weaker fill. The results, however, are not easy to predict or control. The inorganic mortars described above are almost exclusively used on stone treatments in outdoor environments. Rarely are they used as a repair method for museum objects, where such strength and weather resistance are generally not required. Ashurst and Dimes (1990) note that because hydraulic limes are too high strength and impermeable, and transfer soluble sodium salts, cements are excluded for stone sculpture. In architecture, appearance is often sacrificed for durability, while in sculpture, appearance is often paramount. Inorganic mixes are sometimes difficult to incorporate aesthetically with a substrate. Emulating smooth and translucent stone is always a problem. Even if the initial match is successful, it may become differentially soiled in comparison to the original stone, particularly around the edges. The wet curing conditions required for these mortars contribute to their exclusion from museum conservation because the wetting of the treated object is often not advisable. Other disadvantages are the soluble-salt content, shrinkage, incompatibility due to differential thermal expansion, and the need for aggressively interventive surface preparation. Obviously, the relative risk that these factors pose, weighed against the benefits of their usage, must be assessed before deciding to use an inorganic repair mortar. 7 ORGANIC BINDERSOften, a repair system that incorporates an organic binder with transparent or translucent fill materials is found to be the most successful Organic systems can cure either by solvent evaporation (for example, a methacrylate in solution in acetone), phase transition (the cooling of a melted wax), or chemical reaction (the cross-linking of an epoxy). Each type of system has its advantages and disadvantages. Solvent-based systems are often stringy, difficult to work, and exhibit high shrinkage upon evaporation of the solvent. Proper hardness and texture can be difficult to achieve with thermoplastic systems and organic reaction-cured systems. Reaction-cured resins are often excessively strong, toxic, have high shrinkage, are difficult to reverse, and unstable to environmental exposure. Many combinations of organic resins in organic solvents have been attempted. Shellac diluted in alcohol was popular before the advent of modern materials. Plenderleith and Werner (1988) describe a putty made with nitrocellulose in acetone and amyl acetate, plus added white sand. This recipe was recommended where an anhydrous, nonshrinking fill was needed. “AJK Dough” is a traditional putty made with polyvinyl acetate resin as a binder (Cornwall 1965) used by archaeologists on a variety of artifacts where nonaqueous fills are required. Today, thermoplastic resin and solvent-based fills often use a more stable acrylic resin such as Paraloid B-72 (an ethyl methacrylate/methylacrylate copolymer) or polymethyl methacrylate (PMMA) as a binder. Variation of the solvent composition can tailor the evaporation and thus the working time of the mix. Unless extensively bulked with an appropriate filler, shrinkage is still a problem. Mixing dry resin powder with the aggregate, then adding an appropriate solvent is one means of achieving minimal shrinkage (Domaslowski and Strzelczyk 1993). Experimentation with other resins continues among conservators. Cyanoacrylate mixed with granulated methacrylates and stone flour was reported as having “the density closest to the stone”(Yakhont 1991) of many adhesives mixtures tested. Aqueous organic binders have also been used for fill materials on stone. “Gesso” coatings and putties made of glue and added chalk or stone dust have been found on polychrome stone sculpture from a number of periods and cultures. Traditional scagliola recipes are based on a glue binder (Ashurst 1979). Modern acrylic emulsions are used in commercially made artists' modeling pastes such as Liquitex (Pocobene 1994). The slight resiliency afforded such a fill by the acrylic emulsion binder makes it useful for specific applications, but shrinkage and introduction of water to the substrate can be problematic. An alternative to the aqueous or solvent-based organic mixes is the use of a solvent-free organic binder applied thermoplastically. With no solvent present, shrinkage upon evaporation of a carrier is not a concern. A traditional example of this type of fill is the use of tinted shellac sticks, applied with a torch to preheated, dark-colored stones where the yellow-brown color is not a distraction (Kibby 1996). Hempel (1968) introduced the use of a polyvinyl acetate melted directly onto a stone object for fills, a method which has been modified (Burke 1996; Colton 1996) and published since (G�nsicke and Hirx 1997). The revised method consists of a mix of ethylene acrylic acid copolymers (Allied Signal AC-540 and AC-580) with the PVA AYAC. To impart light-stability to the mix, an antioxidant (Irganox) is added. The mix produces a transparent and colored fill material, which can be manipulated by the addition of other fillers like pigments and marble dust. This mix has been used on marble and alabaster objects in indoor and outdoor contexts over the last 15 years Epoxy systems may also be utilized when a strong fill is required for a translucent stone. Epoxies are a class of synthetic resins characterized by a molecular structure with a highly reactive oxirane ring. The oxirane ring acts as the mechanism for cross-linking the polymer chains when catalyzed by an amine hardener. Epoxy resins are generally more expensive than other thermosetting resins. They resist common solvents, oils, and chemicals, are inert, have high mechanical strength, exhibit negligible shrinkage, and can be formulated to have a wide variety of properties, such as resiliency and heat resistance (Brady 1991). Much has been written regarding their use in conservation (Selwitz 1993; Kotlik 1983). The most light-stable epoxies must be used for stone fills, even if the stone is dark. There is a great risk of excess epoxy staining the stone and discoloring, and invisible residue may darken with time. HXTAL NYL-1 has been shown to be the least likely to yellow (Down 1986), but its extremely slow curing time (48 to 72 hours) makes it difficult to work with. In spite of this fact, it may be the most widely used among conservators for strong, translucent fills. Other epoxies have also been used extensively because of their reasonably good resistance to yellowing and their faster cure times. These epoxies include some of the Araldite AY series and Epotek (Down 1986). The use of epoxy in plastic repairs has also been favored because of the potential range of its optical properties when modified with various fillers. Repairs that incorporate microcrystalline wax along with fumed silica as a filler for epoxy can effectively emulate large-crystal translucent marbles (Craft 1996). The technique of first casting the fill in place with the use of a polyethylene film barrier and then adhering the cast fill with a reversible adhesive improves reversibility and greatly reduces the problem of migration of the hardener or other components into the substrate. Epoxy fills are sometimes colored with organic dyes because their color disperses more readily than pigments. Epoxy solutions in alcohol show superior reticulation capacity (the ability to form a strong network on dilution) compared with solutions in aromatic hydrocarbons (Domaslowski 1990). If a highstrength epoxy binder can be used in dilute form with a carrier solvent evaporating out of the intergranular spaces, a high degree of porosity can be maintained in the fill, and concerns about yellowing are significantly reduced. One serious problem encountered in “homemade” formulations of fill materials based on epoxy resin is migration of resin or hardener out of the bulked fill and into the surrounding substrate during curing. An isolating coating of a stable resin such as Paraloid B-72 is generally used to mitigate this problem. However, several conservators report they found this barrier layer to be insufficient to prevent staining of the substrate (Burke 1996). This disadvantage of the epoxies leads some to use polyester resins, the most popular being the Akemi Marmorkit 1000, a particularly light-stable polyester resin. It has a faster setting time and greater resistance to penetration into the substrate due to its viscosity (Burke 1996). Polyester resins have traditionally been used by stonemasons for repairs. They became commonly used for restoration after their adoption by the marble industry after World War II (Brady 1991). Polyesters include a large group of synthetic resins made by the condensation of maleic, phthalic, or other acids with an alcohol or glycol to form an unsaturated polyester. The resin mix is composed of this polymerized polyester, which is copolymerized by an unsaturated hydrocarbon such as styrene monomer. The reaction is catalyzed by other additives such as benzoyl peroxide. The styrene hardener is added in small amounts to catalyze the copolymerization and cross-linking of the resin (Werner 1959; Brady 1991). Thixotropic additives are used to make specific formulations with “knife Polyesters are subject to deterioration on exposure to weathering, with resulting embrittlement, shrinkage, yellowing, crazing, and failure of adhesion. These problems have been ameliorated by the addition of light stabilizers to compositions such as Akemi's Marmorkit 1000. Their common usage warrants the continual comparative studies of their properties and degradation (Shashoua 1992). 7.1 FILLERS FOR ORGANIC SYSTEMSThere are numerous organic materials commonly used as fillers for organic binders. Broken chunks of precast tinted epoxy or polyester, Plexiglas crumbs, wax, PVAs, and methacrylates can be mixed with adhesive binders and cut to resemble a composite stone, breccia stone, granite, or crystalline marble. Hollow phenolic microballoons can be added to epoxy or polyesters to lower the overall strength of the fill or to induce a degree of porosity (Brady 1991). Epoxy with an organic “blowing agent” additive to induce foaming can be used for lightweight filling of voids (Blackshaw and Cheetham 1982; Sturge 1987). Paper pulp has recently been used successfully by several conservators with a number of different binders including Polyfilla (a cellulose-based plaster), PVA emulsion, or methyl cellulose (Podany et al. 1995). The high strength achievable with variations of this technique make it valuable for some structural applications. The most common combination of binder and filler types used by sculpture conservators is an organic binder with inorganic fillers. The use of colloidal fumed silica, alumina, or titania preserves translucency while increasing viscosity and can reduce or increase the overall strength. Fumed silica also lowers the weight of fill material per unit volume (Berrett 1996; Byrne 1996; Vine 1996). Fumed titanium oxide has also been used to great advantage by conservators in achieving a white, translucent effect while thickening the mixture (Berrett 1996). Super-loading epoxy with fumed silica (e.g., 10:1 v:v) creates a pliable, marblelike dough (Barenne-Jones 1989). In addition to fumed silica, numerous inorganic additives are commonly used. Stone flour, sand (e.g., washed silver sand), crushed stone, calcium carbonate, aluminum oxide, and pigments are some of the more common additives. Silica beads have been used in architectural fills, and glass microspheres from 3M Corporation are used in a range of fills in conservation (Hatchfield 1986). Successful results have been achieved with ceramic microspheres (Maltby 1996), powdered fired clay with glass microballoons (Higuchi and Setsuo 1984), and white glass enamel powder (Griswold 1990b) as fillers for epoxies. The glass enamel allows the epoxy to achieve a warm white, highly translucent appearance for outdoors use (Griswold 1990a). Stone aggregate and sand are commonly added to epoxy or polyester for cast replacements and plastic repair in architectural and monumental contexts, particularly in Europe. This type of polyester cast replacement was used for the restoration of Michelangelo's vandalized Pieta(Wihr 1986), with the original crushed bits of stone as the filler. Fills on monumental sandstone sculpture in the Czech Republic have been made with epoxies after consolidation with dilute epoxy under vacuum (Selwitz 1993). Recipes for filling losses in marble and alabaster based on polyester resin with marble and alabaster powder have proved effective over time (Larson 1978). In an interesting note, Larson recommends heating water-soaked alabaster to increase whiteness before crushing it into powder. For a stronger fill, commercial epoxy putties may be used. These include Pliacre, Milliput, and Martin Carbone AB123, which are based on an epoxy resin with alumino-silicate ceramic fillers, titanium dioxide, and other inorganic pigments. 8 FUTURE RESEARCHAccording to the recent survey of members of the American Institute for the Conservation of Historic and Artistic Works (AIC) by the National Center for Preservation Technology and Training on funding priorities in materials conservation, fill materials are among the top 10 priorities in conservation research (Derrick 1996). Much of what is known about the successes and failures of various structural fill materials for stone is anecdotal. Nonetheless, there are well-documented treatments that are 10 to 25 years old, and some very useful observations may be made regarding their successes and failures. More and more the challenges of re-treatment face the conservator, where previous efforts have permanently changed the stone into a composite material. Despite the numerous methods that have been described, gaps in the present repertoire of appropriate structural fills still exist. Research focused on finding methods for stable compensations of translucent stone in the outdoors and for patching mortars for thin cracks and shallow spalls would be invaluable additions. Present methods for structural fills of stone could be improved by learning how to modulate and control the porosity of cementitious and polymer patching mortars (Ma 1995). Methods like the inclusion of microfoaming agents and other materials (straw, polyfilaments) could be further investigated. Research that adapts industrial methods like those used for producing multidensity foams could be utilized. Further study of existing and new commercial products for dental molding materials could prove invaluable. Increasing the UV exposure stability, and therefore the lifetime of polymers in mixes, is another important step for fills used in both indoor and outdoor environments. The addition of polymer stabilizers to synthetic resins used as binders may be appropriate for many treatments (de la Rie 1988). Stabilized polymer and UV-absorbing coatings for fill materials may also be found useful in prolonging their effectiveness. Understanding the quantitative effects that bulking agents have on impeding the yellowing of polymers and decreasing or increasing their strength could also have practical application in the field. 9 CONCLUSIONThere are myriad solutions to the development of a structural fill for losses in stone. Approaches vary from traditional methods to experimental use of new materials. The variations found in recipes for fills express the constant search for a perfect technique. Matching elusive optical properties and meeting the specifications for “ideal” fills are the factors that make each treatment a unique challenge. While searching for solutions, the fundamental criteria should be remembered, and cross-disciplinary communication among related practitioners (conservators, stonemasons, chemists, engineers, and others) encouraged and exercised. ACKNOWLEDGEMENTSThe authors are indebted to numerous conservators, many of whom could not be cited in the final version of the paper. We wish to thank the members of the Objects Specialty Group Publications Committee, particularly Ellen Pearlstein, Leslie Ransick Gat, and Jane Williams, for improving the quality of the initial draft, as well as Carol Grissom for acting as designated facilitator for this paper. Thanks are also due Julie Unruh and Glenn Wharton for their editing and proofreading. NOTES1. “Plastic” defines a physical property of the 2. “Dutchmen” is a slang term first used by English stonemasons. 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Podany, J., and P.Leavengood. 1992. Chinese monumental sculpture: Treatment and seismic stabilization. In Proceedings of the 7th International Congress on Deterioration and Conservation of Stone, vol. 3. Lisbon: Laboratorio Nacional de Engenharia Civil. 1517–526. Weil, P. D.1967. Contributions towards a history of sculpture techniques: T. Orfeo Bosselli on the restoration of antique sculpture. Studies in Conservation12:81–101. SOURCES OF MATERIALSAkemi Marmorkit 1000Wood and Stone, Inc., 10115 Residency Rd., Manassas, Va. 22110 Araldite AY EpoxiesCiba-Geigy Corporation, Seven Skyline Dr., Hawthorne, N.Y. 10532-2188 Carbone #AB-123 PuttyMartin R. Carbone, Inc., 2519 Bath St., Santa Barbara, Calif. 93105 Edison CoatingsEdison Coatings, Inc., Waterbury, Conn. Epotek epoxiesEpoxy Technology, Inc., 14 Fortune Dr., Billerica, Mass. 01821 Ethylene acrylic acid copolymers A-C 540 and 580Allied Signal Corp., A-C Performance Additives, P.O. Box 2332, Morristown, N.J. 07962-2332 Glass microspheres from 3M CorporationConservation Support Systems, 924 W. Pedregosa St., Santa Barbara, Calif. 93101 HXTAL NYL-1Conservation Materials, Ltd., P.O. Box 2884, Sparks, Nev. 89431 Irganox, antioxidan T and thermal stabilizerCiba-Geigy Corp., Seven Skyline Dr., Hawthorne, N.Y. 10532-2188 Jahn MortarsCathedral Stoneworks, 8332 Bristol Ct. #107, Jessup, Md. 20794 Liquitex modeling pastesBinney & Smith, Inc., Easton, Pa. 18044-0431 MilliputMilliput Corp., Unit 5, The Marion, Dolgellau, Mid Wales LL40 1UU, U.K. PermafillBondfast Co., Bridgewater, N.J. 08807 PliacrePhiladelphia Resins Corp., 20 Commerce Dr., Montgomeryville, Pa. 18936 PolyfillaConservation Materials, Ltd., P.O. Box 2884, Sparks, Nev. 89431 Polyvinyl acetate resins PVAC AYAA and AYACUnion Carbide Corp., Specialty Chemicals and Plastics, Old Ridgebury Rd., Danbury, Conn. 06817 PolyfixLepage's Ltd., Bramalea, Ontario L6T 2J4, Canada Thompson Enamel, a division of Ceramic Coating Co., 650 Colfax Ave., Bellevue, Ky. 41073 AUTHOR INFORMATIONJOHN GRISWOLD received his B.A. in art history from the University of California at Irvine and a master's of art conservation (research) from Queen's University, Ontario, Canada. He is a partner in Wharton & Griswold Associates, a private conservation firm based in Los Angeles and Santa Barbara, California, specializing in the conservation of objects and architectural elements. He is a Professional Associate of AIC, and a past chair of the AIC Objects Specialty Group. He is currently president of the Western Association for Art Conservation (WAAC). Address: Wharton & Griswold Associates, 549 Hot Springs Rd., Santa Barbara, Calif. 93108. SARI URICHECK received her bachelor's degree in chemistry from Johns Hopkins University and is currently studying conservation at the Conservation Center, Institute of Fine Arts, New York University. Address: New York University, Institute of Fine Arts, 14 E. 78th St., New York, N.Y. 10021.
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