brittle materials based on their stress-strain curves [64, 125, 126]. Stone consolidated with methylmethacrylate and other acrylics can be expected to exhibit a similar brittle behavior. Methylmethacrylate, no doubt, can harden the surface of a stone and effectively consolidate the stone if both deep penetration and complete polymerization are achieved. Similar to the case with alkoxysilanes, however, stone impregnated with methylmethacrylate will probably weather differently than untreated stone. In addition, erosion through the treated stone [5] could contribute to the development of an unsightly appearance. 4.3.2 Acrylic Copolymers Copolymers are produced by the joining of two or more different monomers in a polymer chain [127]. A commercially available acrylic copolymer used for stone consolidation is copolymerized from ethylmethacrylate and methylacrylate [38, 121]. Other acrylic copolymers which have been studied for stone conservation include copolymers between acrylics and fluorocarbons [128, 129] and between acrylics and silicon esters [59, 121]. The acrylic copolymers are dissolved in organic solvents then applied to stone. As discussed earlier, unless very dilute solutions are applied to a stone solvent evaporation will tend to draw the acrylic copolymers back to the surface. Then, even if diluted to the lowest concentration that will give some consolidation, their solutions still may have viscosities which impede their penetration into stone. 4.3.3 Vinyl Polymers Several vinyl polymers have been studied or used for conservation and consolidating of stone including poly(vinylchloride) [54, 130], 131]. These polymers are dissolved in organic solvents and then applied to stone. Photochemical processes could release chlorine from the chloride polymers, which could damage stone [130]. Poly(vinylacetate) has been found to produce a glossy stone surface [130]. If not carefully applied and if not sufficiently diluted, use of the vinyl polymers undoubtedly will result in the formation of impervious layers which entrap moisture and salts underneath [38]. 4.3.4 Epoxies The feasibility of using epoxies to consolidate stone is addressed The epoxy by first briefly discussing their chemistry and then applications. reactants are liquids, but are too viscous to penetrate stone deeply. Gauri [134-135] developed a method to achieve deep penetration with viscous epoxy resins and at the same time avoid the formation of a sharp interface between the consolidated and untreated stone. First, specimens followed by soaking in increasingly concentrated solutions. This method is feasible for small stone objects such as tombstones and statues, but would be too time consuming and expensive for stone structures. Less viscous epoxy resins are available including diepoxybutane diglycidyl ether and butanediol diglycidyl ether [58]. Munnikendam [61] cured butanediol diglycidyl ether with alicyclic polyamines such as menthane diamine. However, the viscosity was still too high and he diluted the mixture with tetraethoxysilane and tetramethoxysilane. A complex reaction took place involving the epoxy resin, curing agent and solvent to produce a tough, glassy material. A white efflorescence also developed due to a reaction between the polyamine and carbon dioxide to form aminecarbonates [61, 137]. Formation of the amine carbonates can be avoided by preventing carbon dioxide from coming in contact with the solution. Gauri [128, 136] observed that when low viscosity aliphatic epoxy resins were applied to calcareous stones, the reaction rates between the stones and carbon dioxide and sulfur dioxide were increased compared to the rates with untreated stones. He suggested that the increased reactivity could be caused by absorption of the gases by the epoxy polymer or by the polymer acting as a semipermeable film to the gases. In contrast, bisphenol Abased epoxy polymers were found to protect the stone from both carbon dioxide and sulfur dioxide. The use of epoxies has been suggested for consolidating limestone [14, 128, 129], marble [134-139], and sandstone [61, 121] as well as for re-adhering large stone fragments to mass stone [1]. Moncrieff and Hempel [138] found that certain epoxies could encapsulate salts in marble, using epoxies for masonry consolidation is that of the Santa Maria Maggiore Church in Venice [140]. Similar to poly(methylmethacrylate), epoxies have produced brittle epoxy-impregnated concretes with high mechanical properties [65, 141, The long-term effect on 142]. incorporating a brittle material in stone is not known, but could render a structure vulnerable to seismic shock, vibrations and thermal-dimensional effects. Many types of epoxies have a tendency to chalk, i.e., to form a white powdery surface, when exposed to sunlight [132]. Therefore, epoxy should be removed from the surface of a treated stone before it cures. 4.3.5 Other Synthetic Organic Polymers Other synthetic organic polymers studied as possible stone consolidants include polyester [38, 143], polyurethane [121], and nylon [77]. Polyester has been shown to decrease the porosity of stone substantially [143] and, therefore, may form an impervious layer which prevents the passage of entrapped moisture or salts [38]. Manaresi [121] and Steen [144] observed that polyurethanes were poor cementing agents. Steen [145] also found that polyurethane film gradually became brittle when exposed to sunlight. Similarly, DeWhite (77) found that nylon can produce a brittle film on the surface of stone. 4.4 Waxes Vitruvius Waxes have been applied to stone for over 2,000 years. [146] described the impregnation of stone with wax in the first century B.C. A wax dissolved in turpentine was one of several materials applied to the decaying stone of Westminster Abbey between 1857 to 1859 [147]. 1879 and several times since [148]. Kessler [149] found that paraffin waxes were effective in increasing the water repellency of stone. Waxes have also been found to be effective consolidants [25, 53, 54, 58]. For example, a paraffin wax increased the tensile strength of a porous stone from 1.06 MN/m2 to 4.12 MN/m2, while triethoxymethylsilane only. increased it to 1.88 MN/m2 [25, 53]. In addition, paraffin waxes are among the most durable stone conservation materials [16, 54] and can immobilize soluble salts [58]. Waxes have been applied to stone by applying the wax dissolved in organic solvents [16, 78, 148], by immersing a stone object in molten wax [58], or by applying molten wax to preheated stone [150]. If deep penetration is not achieved a nonporous surface layer may be formed causing the eventual spalling of the treated stone surface [78]. Major problems encountered in using waxes to conserve stone include their tendency to soften at high ambient temperatures [76], and to entrap dust and grime [2, 54, 58]. Wax applied to Cleopatra's Needle has gradually converted to a tarry substance which cannot be removed by ordinary washing methods. For example, a mixture of carbon tetrachloride, benzene and detergent was needed in 1947 to clean the Needle [148]. Although stone consolidants have been extensively used for over a century, their selection is still largely based on empirical considerations. If a consolidant appears to give acceptable results with one type of stone, it is often applied to other types of stone, without properly determining if the consolidant is compatible with them. Some of the factors |