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Stone-Consolidat~ng Mat-enals: A Status Report JAMES R. CLIFTON and GEOFFREY J. C. FROHNSDORFF Mechanisms by which stone consolidants funct~on~are outlined. Evaluation of stone consolidants usually requires both laboratory and field tests to de- termine their initial and long-term performances. ASTM Standard E 632, Rec- ommended Practice for Development of Accelerated Short-Term Tests for Prediction of the Service Life of Building Materials and Components, can be used to provide guidance on the test program. Materials that have been investigated as stone consolidants are reviewed. They fall into four main groups: inorganic materials, alkoxysilanes, synthetic organic polymers, and waxes. Epoxies, acrylics, and alkoxysilanes are the most commonly used consolidants, but no consolidant can be considered completely satisfactory and able to meet all the desired performance requirements. Building stones may lose their integrity {i.e., decay) as a result of weath- ering.i Loss of matenal fiom the exposed surfaces of stone masonry units and the reduction in compressive strength and other mechanical properties of the units usually proceed slowly. However, such changes James R. Clifton is Group Leader, Inorganic Materials, Building Materials Division, Center for Building Technology, National Bureau of Standards, Washington, D.C. Geoffrey J. C. Frohnsdorff is Chief, Building Materials Division, Center for Building Technology, National Bureau of Standards, Washington, D.C. The authors wish to acknowledge the encouragement of Hugh Miller, Chief Historical Architect of the National Park Service, and Henry Judd, former Chief Historical Architect of the National Park Service, both of whom provided information valuable to this review. 287

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288 cows ERVATION OF BIONIC STONE BUILDINCS me likely to be sift ~ Me case of species fat we hope to preserve ~r ma awe generations. PIoblems cased ~ loss Of ~- te=~ me well fog HI mason cats consisting ~ porous se~- ment~ Iocks sum as sandstone ad limestone. We deck is ~neIaDy bobeved to reset hom ~ss~ubon ~ We mutely cements We was gone together OI ~~ eta ~ We ~te~l~ bonds hom Accessed tensHe stresses caused by sum processes as sat ~st~li- zadon ad ~~ fusion. ted bonds ~ ~ ~ porous stone con be mpres~ted schematically as ~ Fife 1. We adds I~Ies~t ~ mated class ~- ~e od~ stone; ~temativel~ de- cayed st~e treated ~ ~ consoLd~t accumulates at the contact pouts to restore bonds between pa~ b. Stone treated aim ~ consort ~~ provides ~ chow tam costing on Me Ins Id bonds them at the contact ports. c Stone treated with ~ consolid~t cb almost hits the pores. ~ ; I ~ _ _ now 1 S^emadc As Resent mated cIoss~ecdons Cot ~ porous stone Seated Ail consoLd~ti

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Stone Consolidating Materials section through limestone or sandstone. (In reality, of course, a planar section through a stone would be unlikely to intersect so many contact points between adjacent grains.) If a sufficiently large proportion of the cemented bonds at the contact points is broken within any volume of the stone, the integrity of the stone in that volume will be lost. This usually happens close to the surface. If it is necessary to restore the integrity of decayed stone, the stone must be treated with a material that will effectively restore the bonds between adjacent grains.2 Ma- terials used for treating stone to restore integrity are termed stone consolidants. While there is evidence that decayed stones can be con- solidated, at least in the short term, knowledge of stone consolidation is not at a level where the performance of consolidants over many years can be confidently predicted and the treatments guaranteed not to harm the stone. To provide perspective on stone consolidants, reference will be made to Figure 1. Figure la represents the original stone with cemented bonds between the grains; it could also represent what might be achieved if, following the breaking of intergranular bonds, a stone consolidant could be made to accumulate in the contact areas to reestablish intergranular adhesion. Figure lb represents a decayed stone which has been treated so as to produce a thin coating of consolidant covering the surface of each grain and bonding the grains together at the contact points. Figure to represents a decayed stone which has been treated with a consol- i]ant that almost completely fills the pores of the stone, leaving only relatively small voids or pores within the consolidant. The diagrams show, in a simplistic way, how stone consolidants may affect the microstructure of a treated stone and influence the properties of the surfaces and interfaces within it. Treated stones are composite materials, and their properties reflect the properties of their individual ingredients, the interactions among them, and their spatial distributions. Since the properties and long- term performance of such composites cannot be satisfactorily pre- dicted, the selection of consolidating materials and treatments must usually be based on laboratory and field tests of treated stones. To aid the necessarily complicated development of durability tests, American Society for Testing and Materials (ASTM) Subcommittee E 6.22 recently established standard E 632, Recommended Practice for Development of Accelerated Short-Term Tests for Prediction of the Service Life of Building Materials and Components.3 4 Figure 2, which is taken from ASTM E 632, summarizes the recommended practice. It outlines an approach to follow if the durability of a treated stone is to be evaluated in a rational way. Stone consolidants are applied as liquids but, to be effective, they 289

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290 CONSERVATION OF HISTORIC STONE BUILDINGS PART 1 - PROBLEM DEFINITION r ~ Identify critical per- formance character] sties and properties that can serve as degradation i nd1 caters ! ~ 1 I I requirements and criteria | ~ , _ 2 Characterize the component or material 3 9 4 Identify the expected type and range of degradation factors including those related to weathering biological, stress, 1ncompat1b111ty and use factors _ __ I- Postulate ho, degradation characteristic of 1n- use performance-can be induced by accelerated aging tests I dent i fy pos s 1 bl e degra - da t i on mec ban i smut 5 _ __, 6 PART 2 - PRE-TESTING PART 3 - TESTING Def i ne performance I requ1 remeets for I predictive service 1 life tests I 8 Design and perform prel1m- 1nary accelerated aging tests to demonstrate rapid failures caused by indiv1d- ually applied extreme degra- dat10n factors and to con- firm degradation mechanisms . Design and perform predictive service life tests using the degradation factors of ~ importance to determine the dependence of the rate of degradation on exposure conditions PART 4 - INTERPRETATION AND REPORT I NG OF DATA 11 ~ ~ . . Compare types of degra- dat i on obta i ned by both in-service and predictive servi ce 11 fe tests QUESTION ~< induced by predictive Service life tests sentative of those observed in-service~ ~ Yes 13 Develop mathematical models of degradation and compare rates of change 1 n pred1 ct i ve service life tests with those from 1p-service tests Establ 1 sh performance cr1- 14 teria for predictive service life tests . Predict service life is under expected 1n- serv i ce condo t i ons. . . Design and perform long- lO term tests under service condi teens repro- No Report 16 the data FIGURE 2 Steps in the recorn~nended practice for developing predic- tive service life tests, ASTM Standard E 632. 1

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Stone Consolidating Materials must cause a solid material to be laid down in the pores of the stone. The initial properties of a stone consolidant depend on many factors. The penetration of the consolidant into the stone and its distribution within the stone depend on the structure of the stone, the viscosity and surface tension of the liquid, and the contact angle of the liquid against surfaces within the stone.5 6 Consolidation may result from solidification of the liquid within the pores, as by polymerization of a monomer or cooling of a molten solid, or from evaporation of a volatile solvent from a solution of a resin or other solid material. It may also result from nucleation and growth of relatively small quantities of solid from the liquid phase. Examples of consolidants depending on each of these mechanisms are given below under Stone Consolidants. The durability of a consolidated stone depends in part on the du- rability of the consolidant in the environment encountered in service. It may also reflect the fact that the distribution of stresses within a treated stone may differ from that in the untreated stone because of the changes in microstructure once the characteristics of the exposed surfaces. A list of degradative factors that should be considered in evaluating the durability of any material is given in Table 1, and a matrix to aid the identification of possible degradation mechanisms ~ individual phases and at interfaces between phases is given in Figure 3.3~4 With this general discussion as background, the range of stone con- solidants that have been used, or proposed for use, will now be re- viewed. Ike review is based on a previous paper by James R. Clifton, which gives a more extensive bibliography.2 291 STONE CONS-OLIDANTS In this review, stone consolidants are divided into four mam groups: inorganic materials, alkoxysilanes, synthetic organic polymers, and waxes. Considerations of their performance are based on generally applicable requirements, discussed in Clifton's paper. Inorganic Materials Inorganic stone consolidants were used extensively during the nine- teenth century and are still used occasionally. Most inorganic consol- idants produce an insoluble phase within the voids and pores of a stone, either by precipitation of a salt from the liquid or by chemical reaction of the liquid with the stone. It has been suggested that the development of a new phase similar in composition to the matrix of a stone wiD

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292 CONSERVATION OF HISTORIC STONE BUlEDINGS TABLE 1 Degradation Factors Affecting the Service Life of Building Components and Matenals Weathering Factors Radiation Solar Nuclear Thermal Temperature Elevated Depressed Cycles Water Solids Snow, ice) Liquid train, condensation, standing water) Vapor (such as high relative humidity) Normal air contaminants Oxygen and ozone Carbon dioxide Air contaminants Gases {such as oxides of nitrogen and sulfur) Mists (such as aerosols, salt, acids, and alkalies dissolved in water) Particulates {such as sand, dust, dirt) Freeze-thaw cycles Wind Biological Factors Microorganisms Fungi Bacteria Stress Factors Stress, sustained Stress, periodic Stress, random Physical action of water, as rain, hail, sleet, and snow Physical action of wind Combination of physical action of water and wind Movement due to other factors, such as settlement or vehicles Incompatability Factors Chemical Physical Use Factors Design of system Installation and maintenance procedures Normal wear and tear Abuse by the user

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Stone Consolidating Materials 293 be effective in binding together the grains of deteriorated stone. For example, consolidants that result in the formation of a siliceous phase should be used to consolidate sandstone, and calcium carbonate or barium carbonate should be used to consolidate calcareous stones such as limestone. In practice, however, little concern seems to be given to chemical compatibility between the consolidants and the stone. Little success has been achieved in consolidating stone with inor- ganic materials, and in some cases their use has greatly accelerated decay.7-9 Some of the reasons given for the poor performance of in- organic consolidants are their tendencies to produce shallow, hard cruets, 7 i0 ii the formation of soluble salts as reaction by-prod- ucts,7 i0 i2 i3 60 94 growth of precipitated crystals, 8 and the questionable ability of some of them to bind particles of stone together.~4 i5 Of these, the most difficult problem to overcome is the formation of shallow, hard surface layers by inorganic consolidants because of their poor penetration abilities. Precipitation processes are often so rapid that precipitates are formed before the inorganic chemicals can appreciably penetrate the stone. Precipitation from homogeneous solutions has been used to obtain deeper penetration of stone by some inorganic consolidants. This method is discussed below under Alkaline Earth Hydroxides. Siliceous Conso~iciants Siliceous consolidants are materials that have been used to consolidate sandstone and limestone through the formation of silica or insoluble . . S1. .lCateS. Alkali Silicates Both nonstoichiometric dispersions of silica in so- dium hydroxide and soluble alkali silicates have been used to preserve and consolidate stone. When dispersions of silica in sodium hydroxide solutions are applied to a stone, silica is deposited.7 i6 If sodium hy- droxide is not removed by washing, it can react with carbon dioxide or sulfur trioxide to form sodium carbonate or sodium sulfate, respec- tively. These salts may cause unsightly efflorescence and salt crystal- lization damage. In addition, it seems that sodium hydroxide can react with the constituents of some stones, thereby accelerating deteriora- tion.7 Silica can be precipitated by the reaction between sodium silicate or potassium silicate and acids such as hydrochloric, arsenic, and car- bonic acids.7 i6 i7 However, these reactions result in the formation of soluble salts such as sodium chloride and sodium arsenate. If the so-

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294 \\\\ CD AS \) A A _ : _ / ~ m US ~ I .~ | ~ | m I con Cal 4= V) o Cal - C~ ._ a) 4= Cal en in 4 - o o v he ._ =:s ._ o On v - r9 CO o do ._ ._ 4~ ._ o x ._ en Cal o - ~4 x - C' CL) E E' - Q a, _ C C $ 2 ~ O ~ D _ O ~ ' & ~ .' ~ _ 013 ._ . C) m ~ m Or _

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Stone Consolidating Materials 295 dium sflicate-arsenic acid mixture is used to consolidate limestone, crystalline calcium arsenate can be produced by a reaction between calcium carbonate and arsenic acid. The crystalline calcium arsenate appears to damage limestone by anisotropic crystal growth.7 Insoluble silicates have been precipitated in stone by alternate treat- ments of sodium silicate and salts such as calcium chioride7 i5 i6 i~ and zinc carbonate.~997 The colloidal silicates that are first produced even- tually crystallize, while soluble salts are produced as by-products.7 Also produced are impervious surface layers that trap underlying water.20 Apparently the silicates precipitate relatively rapidly and are deposited near the surfaces of the treated stones. Even with all the problems associated with the use of alkali silicates, they are still occasionally applied.2i Recently, the successful use of soluble silicates was reported.22 However, present evidence suggests that alkali silicates should not be used as stone consolidants. Silicofluorides Both hydrofluosflicic acid and soluble sflicofluorides have been used to preserve and consolidate stone. Hydrofluosilicic acid should not be used on limestone because it reacts vigorously with calcium carbonate to form crystalline calcium silicofluoride, carbonic acid, and carbonate salts. The reaction occurs upon contact of the acid with the limestone, producing a shallow crust with little consol- idating value. Hydrofluosilicic acid reacts more slowly with sfliceous- based sandstones to form a cementitious material, but again only the surface is hardened. Hydrofluosilicic acid has a tendency to discolor both limestones and sandstones, especially if they contain iron.7 Many soluble silicofluorides, such as those of magnesium, zinc, and alun~i- num, have been applied to limestone. Resulting products are silica, insoluble fluoride salts, and carbon dioxide, which are formed near the surface of the limestone. Therefore, only the surface is hardened, and eventually it exfoliates.~2324 Soluble silicofluorides also react with calcareous sandstones, and again only a hardened surface is obtained. Further, soluble salts are formed when both limestone and calcareous sandstone are treated with silicofluorides.~ These soluble salts have caused damage through salt recrystallization processes. Penkala re- cently carried out a systematic study of several stone treatments and also found that fluorosilicates were not effective consolidants.25 Alkaline Earth Hydroxides Calcium Hydroxide Aqueous solutions of calcium hydroxide (its sat- urated solution is often called limewater) have been used for many

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296 CONSERVATION OF HISTORIC STONE BUILDINGS centuries to protect and consolidate limestone.26 Calcium hydroxide itself does not appear to consolidate stone, but in solution or in a wet state it reacts with atmospheric carbon dioxide to form insoluble cal- cium carbonate, which may bind particles of calcareous stones to- gether. The solublility of calcium hydroxide is only about 1 g per liter at room temperature;27 therefore, repeated applications are necessary to produce sufficient calcium carbonate to consolidate stone. Further- more, unless very dilute solutions are used, only the calcium hydroxide deposited near-the surface of a stone is carbonated. This happens if the dense calcium carbonate formed at the surface fills the pores and voids in the stone and severely impedes the migration of carbon dioxide through the treated surface to the interior. The newly produced cal- cium carbonate is susceptible to the same deterioration processes as the calcareous stone. For example, it can react with sulfur trioxide to form calcium sulfate, which is relatively soluble compared to calcium carbonate. Therefore, the treated stone may not be protected against further weathering. However, it may eventually gain the authentic appearance of the weathering stone. Conflicting opinions have been given of the effectiveness of the calcium hydroxide process. Some conservators have felt that while treatment with calcium hydroxide causes no harm, little permanent consolidation is obtained, 7 i0i while others have recommended the use of limewater to protect limestones from weathering and to consolidate them.8 26 28 29 The effectiveness of freshly prepared slaked lime (calcium oxide mixed with water) in consolidating statues at the Wells Cathedral in England is being investigated by Baker.30 He applies 38 mm thick layers of slaked lime to statues and removes the layers several weeks later. Some consolidation appears to occur. Apparently, repeated treat- ment with limewater and staked lime can gradually consolidate lime- stone, but such processes are economically feasible only for small objects.49 Strontium an c] Barium Hydroxicles Like calcium hyroxide, stron- tium and barium hydroxides will react with carbon dioxide to forth insoluble carbonates, but again, only the hydroxide near the surface of a stone is carbonated. The carbonate may subsequently be converted to sulfate by interaction with atmospheric sulfur oxides. However, unlike calcium sulfate, the strontium and barium sulfates that may be formed are insoluble. Thus the application of strontium and barium hydroxides may reduce the weathering of stone exposed to environ- ments polluted by sulfur oxides. The early work on the use of barium hydroxide to preserve stone

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Stone Consolidating Materials 297 was performed by Church.3~3233 Initially, excellent results appeared to be obtained. However, only surface hardening occurred, and the barium carbonate or barium sulfate layer eventually exfoliated.7 ~ ii 20 The exfoliation problem has been attributed not only to the formation of a dense, impervious surface layer, but also to anisotropic crystal growth of barium carbonate and barium sulfate.7 ~ Lewin and Sayre have developed methods intended to precipitate barium carbonate and barium sulfate deeply within a stone.34 35 These methods are based on precipitation from homogeneous solution.36 In this process, the material to be precipitated and the precipitating chem- icals are present in the same solution. For example, barium carbonate is precipitated from an aqueous solution of barium hydroxide and urea.3437 The urea hydrolyzes slowly, at a rate dependent on the pH, to produce ammonium carbonate (or ammonia and carbon dioxide) in the solution. This causes the pH to rise and the hydrolysis to accelerate. At the same time, the carbonate reacts with the barium ions in solution to precipitate barium carbonate. The reaction rate can be controlled so the precipitate forms days after a stone is treated. The slow for- mation of barium carbonate is reported to give a crystalline-solid so- Jution with the calcite crystals of calcareous stone. Barium sulfate can be precipitated in a stone by an analogous method. An aqueous solution of a barium monoester of sulfuric acid hydrolyzes slowly when a base is added, releasing barium and sulfate ions.36 The precipitation of barium carbonate and barium sulfate from ho- mogeneous solution is a promising approach. To date, however, only experimental testing has been carried out, and little is known of the long-term consolidating effectiveness of this approach. Warnes and Marsh have both suggested that crystalline inorganic precipitates, such as barium carbonate and sulfate, do not have long-term consolidating value.78 They have also commented that the precipitates of barium carbonate and barium sulfate have larger volumes than calcite and appear to exhibit anisotropic crystal growth. It should not be assumed that deteriorated stone will have sufficient empty volume to accom- modate these precipitates. Therefore, until more is known of the Tong- term effects of barium carbonate and barium sulfate on the durability of stone, they should be regarded as experimental materials and should not be applied to important historic structures. Other Inorganic Consonants Many other inorganic materials have been used in attempts to preserve or consolidate stone. They include zinc and aluIIiinum stearates,7 ~ 2538

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Stone Consolidating Materials 301 The use of synthetic organic polymer systems to consolidate stone is a recent development, dating back to the early 1960s. Therefore, little is known of the long-term performance of these materials. Some organic consolidants have been found to improve the mechanical prop- erties of deteriorated stone significantly. Many organic polymers are susceptible to degradation by oxygen and ultraviolet radiation, but this should only affect the materials on the surface of a treated stone.60 Riederer reported that the surfaces of some structures in Germany that had been consolidated with organic polymers in 1965 had exhibited deep channel erosion by 1975.2i Apparently water gradually eroded the consolidated surface and, once the protective surface layer was pierced, the untreated stone was eroded rapidly. Acrylic Polymers Methyl methacrylate and, to a lesser extent, butyl methacrylate have been used to consolidate concreted 62 and stone.50 These monomers can be applied solvent-free to porous solids and can be polymerized in situ. An excellent source of information on their polymerization, as well as on polymer-impregnated concrete, is the report by Kukacka et al.6i Methyl methacrylate has been polymerized into poly~methyl methacrylate) by heating with an initiator, by gamma radiation, and at ambient temperature by a combination of promoters and initia- tors.6i 63 For thermal polymerization, the chemical initiator Catalyst) 2,2'-azobistisobutyronitrile) has been found to be effective.64 Heating blankets could be used to polymerize thermally methyl methacrylate or other monomers applied to a stone structure. Polymerization by radiation must usually be carried out in special chambers because of the radiation hazards. Chemical promoters convert initiators into free radicals at ambient temperatures, and the free radicals induce the po- lymerization of methyl methacrylate. Munnikendam used N,N-di- methyI-p-toluidine to decompose benzoyl peroxide into free radicals.59 He foment, however, that oxygen inhibited the subsequent polymeri- zation reaction of methyl methacrylate. Better success probably could be achieved by using 2,2'-azobislisobutyronitrile) as the initiator.64 Where deep impregnation and complete polymerization was achieved, methyl methacrylate and other acrylates have been shown to improve substantially the mechanical properties and durability of porous ma- terials such as concrete. However, incomplete impregnation with acrylates may result in the fo~ation of a distinct, probably undesir- able, interface between treated and untreated stone.54 As shown by their stress-strain curves, concretes impregnated with

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302 CONSERVATION OF HISTORIC STONE BUILDINGS acrylic-based polymers are classified as brittle materials.6~6465 Stone consolidated with methyl methacrylate and other acrylics can be ex- pected to exhibit similar brittle behavior. Methyl methacrylate can harden the surface of a stone once effec- tively consolidate the stone if both deep penetration and complete polymerization are achieved. However, as is the case with alkoxysi- lanes, stone impregnated with methyl methacrylate will probably weather differently from untreated stone. In addition, erosion through the treated stone could contribute to the development of an unsightly appearance.2i Acrylic Copolymers Copolymers are produced by joining two or more different monomers in a polymer chain.66 A commercially available acrylic copolymer used for stone consolidation is produced Tom ethyl methacrylate and methyl acrylate.55 67 Other acrylic copolymers that have been studied for stone conservation include copolymers of acrylics and fluorocarbons 68~69~99 and of acrylics and silicon esters.4~ 55 The acrylic copolymers are dis- solved in organic solvents and then applied to stone. As discussed earlier, unless very dilute solutions are applied, solvent evaporation will tend to draw the acrylic copolymers back to the surface of a stone. Then, even if diluted to the lowest concentration that wit! give some consolidation, their solutions may still have high viscosities, which will impede their penetration. Viny] Polymers Several viny] polymers have been studied or used for preservation and consolidation of stone. They include polylviny! chloridel,70 7~ 96 chlor- inated polylvinyl chloridel,7i and polylviny! acetatel.67707~72 These polymers are dissolved in organic solvents and then applied to stone. Photochemical processes could release chlorine from these chIorine- containing polymers, which could damage stoned Polyvinyl acetate) has been found to produce a glossy stone surface.7i If vinyl polymers are not sufficiently diluted and carefully applied, their use undoubtedly will result in the formation of impervious layers which could entrap moisture and salts within the stone.67

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Stone Consolidating Materials Epoxles 303 An epoxy consists of an epoxy resin and a curing agent, which is actually a polymerization agent. Mixing the epoxy resin with the cur- ing agent converts it into a hard, therrnosetting, cross-linked polymer. The most cornrnonly used epoxy resins are derived from diphenyI- olpropane {bisphenol A) and epichIorohydrin. Resins produced from these reactants are liquids that are too viscous to penetrate stone deeply. Therefore, they are diluted with organic solvents. These epoxy resins are often cured using an amine curing agent. Their cure time can be adjusted by selecting a slowly or rapidly reacting curing agent and by controlling the curing temperature. The resulting cross-linked poly- mers have excellent adhesion to stone and concrete and excellent chemical resistance. Lee and Neville, and Gauri, are recornrnended sources for information on the chemistry, curing, and applications of epoxies.73 75 Gauri developed a way to achieve deep penetration with viscous epoxy resins and at the same time to avoid the formation of a sharp interface between the consolidated and untreated stone.75 76 i00 Spec- imens were soaked in acetone, then in a dilute solution of epoxy resin in acetone, Then in increasingly concentrated solutions. This method is feasible for tombstones and statues, but-probably would be too time- consurning and expensive for large structures. Less viscous epoxy resins are available, including diepoxybutane diglycidy] ether and butanedio! diglycidyl ether.50 Munnikendarn cured butanediol diglycidy! ether with alicyclic polyarnines such as men- thane diarnine. However, the viscosity was still too high, and he diluted the mixture with tetraethoxysilane and tetramethoxysilane. A reaction involving the epoxy resin, curing agent, and solvent took place to produce a tough, glassy material. A white efflorescence also developed from a reaction between the polyamine and carbon dioxide to form aminecarbonates.S9 77 Formation of the aminecarbonates can be avoided by preventing carbon dioxide from coming in contact with the solution before the desired reaction is complete. Gauri observed that when low- viscosity aliphatic epoxy resins were applied to calcareous stones, the rates of the reactions between the stones and carbon dioxide and super dioxide-were faster then the rates with entreated stones.68 78 He sug- gested that the increased reactivity could be caused by absorption of the gases by the epoxy polymer or by the polymer acting as a semi- perrneable film to the gases. In contrast epoxy polymers based on bisphenol A were found to protect the stone from both carbon dioxide and sulfur dioxide.

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304 CONSERVATION OF HISTORIC STONE BUILDINGS The use of epoxies has been suggested for consolidating lime- stone,~6869 marble, 75-80 and sandstone,5659 as well as for readhering large stone fragments to mass stone.60 Hempe! and Moncrieff found that certain epoxies could encapsulate salts in marble, thereby pre- venting them from recrystallizing.~ A large restoration project using epoxies for masonry consolidation is that at the Santa Maria Maggiore Church in Venice. Like poly~methy! methacrylate), epoxies have produced brittle epoxy- impregnated concretes with high mechanical properties.62 82 83 Me long- term effect of incorporating a brittle material in stone is not known, but such a material could render a structure more vulnerable to seismic shock, vibrations, and effects of thermal expansion. Many types of epoxies have a tendency to chalk (i.e., to form a white powdery surface) when exposed to sunlight.73 Therefore, epoxy should be removed from the surface of a treated stone before it cures. Other Synthetic -Organic Polymers Other synthetic organic polymers studied as possible stone consoli- dants include polyester,67 84 polyurethane,55 and nylon.85 Polyester has been shown to decrease the porosity of stone substantially84 and, there- fore, may form an impervious layer that prevents the passage of en- trapped moisture or salts.67 Manaresi and Steen observed that poly- urethanes were poor cementing agents.5686 Steen also foment that a polyurethane film gradually became brittle when exposed to sunlight.87 Similarly, DeWitte found that nylon can produce a brittle film on the surface of stone.85 Waxes Waxes have been applied to stone for more than 2,000 years. Vitruvius described the impregnation of stone with wax in the first century s.c.88 A wax dissolved in turpentine was one of several materials applied to the decaying stone of Westminster Abbey between 1857 and 1859.89 Cleopatra's Needle in London was first treated with wax in 1879 and has been treated several times since.90 Kessler found that paraffin waxes were effective in increasing the water repellency of stone.9~ Waxes have also been found to be effective consolidants.40 50 7092 For example, a paraffin wax increased the tensile strength of a porous stone from 1.06 MN/m2 {153 psi) to 4.12 MN/m2 t594 psil, while triethoxy- methylsilane only increased it to 1.88 MN/m2 {271 psi).40 92 In addition,

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Stone Consolidating Materials 305 paraffin waxes are among the most durable stone conservation materials7 70 and can immobilize soluble salts.50 Waxes have been applied to stone in solution in organic solvents, 7 9 90 by immersing a stone object in molten wax,50 and by applying molten wax to preheated stone.93 If deep penetration is not achieved, a non- porous surface layer may be formed, causing the eventual spelling of the treated surfaced Major problems encountered in using waxes to conserve stone in- clude their tendency to soften at high ambient temperaturesii and to entrap dust and grime.50 70 i02 Wax applied to Cleopatra's Needle has gradually converted to a tarry substance which cannot be removed by ordinary washing. A mixture of carbon tetrachIoride, benzene, and detergent was needed in 1947 to clean the Nee~e.90 COMMENTS ON STONE CONSOLIDANTS Although stone consolidants have been used for more than a century, their selection is still based largely 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 it is compatible with them. Some of the factors affecting the per- formances of consolidants are known, such as depth of penetration and moisture transfer through consolidated stone. However, insuffi- cient consideration has been given to equally important factors such as their consolidating abilities and the compatibility of their thermal expansion properties with those of stone. Finally, the long-term per- formances of consolidated stones in historic structures are rarely doc- umented. These considerations point to the inadequacy of the present state of stone consolidation and conservation technology. For example, stone consolidants should be selected on the basis of an understanding of the deterioration processes of stone and treated stone, of the factors affecting the performances of consolidants, and of the compatibility of consolidants with specific stones. Currently, such information often is not available. Further, standard test methods and performance cri- teria should be developed as a basis for selecting promising consoli- dants. Documentation of the performances of stone consolidants should be an integral part of each preservation or restoration program. Doc- umenting unsuccessful consolidation work is just as important as doc- umenting successful work in that it enables other stone conservators to reject ineffective materials and methods. This review clearly indicates that a perfect stone consolidant has

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306 CONSERVATION OF HISTORIC STONE BUILDINGS l not been developed and that many of the proposed treatments can harm stone. Therefore, the general use of stone consolidants is open to-question. In fact the British Commonwealth War Graves Co~nmis- sion, which is responsible for more then 1 million headstones ill: Eu- rope,- has concluded that no consolidant should be applied to head- stones.~02 This commission has more than 50 years of experience with the chemical treatment of stone. There are cases, however, in which the use of stone consolidants can be beneficial. The work by Hempel anC Moncrieff has shown that decaying stone statues can be preserved by deep impregnation with certain stone consoli`dants.48 49 72 79 80 Stat- ues and smaller objects can be removed to laboratories, thoroughly cleaned, freed from soluble salts, and treated on all sides withy con- solidant, but such processes are not possible with masssive stone struc- tures. The risks involved in treating massive structures, therefore, are greater. Consolidants might be used on structures of little historical or intrinsic value and in other cases where the benefits outweigh the risks involved.~02 For example, consolidants could be applied to de- teriorated stone to delay the need to replace it with new stone. Any permanent consolidation effort involving important historic stone structures, however, should be carefully planned once carried out to minimize the risks. This includes making certain that moisture and soluble salts are not trapped behind the layer of treated stone. In ad- dition, the compatibility of a consolidant with a specific stone should be determined with separate test specimens rather then by using an important historic structure as an experiment. There is an obvious need for caution, even in the use of materials that have shown promise in accelerated laboratory tests. While accel- erated tests designed in accord with ASTM Standard E 632 should be useful in the evaluation of stone consolidants, there will always be assumptions to be made about factors affecting performance. These assumptions will leave a measure of uncertainty about the reliability of predictions based on the test results, but the tests will minimize the risks in selecting a stone consolidant. SUMMARY AND CONCLUSIONS The main function of a stone consolidant is to reestablish the integrity of deteriorated stone by restoring intergranular bonds. In addition to consolidation, a stone consolidant should meet performance require- ments concerning depth of penetration, compatibility with stone, ef- fect on permeability and moisture transfer, effect on appearance, and durability. These may be termed "primary performance requirements"

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Stone Consolidating Materials 307 because they are applicable to all stone consolidants regardless of the specific use. Secondary performance requirements may sometimes have to be imposed because of specific problems encountered with certain structures. An example would be to require a consolidant to immo- bilize soluble salts in a stone. In the selection of a consolidant many factors must be considered. These include the type of stone to be consolidated, the processes re- sponsible for the deterioration of stone, the degree of deterioration, the environment, the amount of stone to be consolidated, and the impor- tance of the structure. A universal consolidant does not exist because many of these factors will vary from structure to structure. Therefore, the preservation of each stone structure should be considered a unique problem. Few cases of long-term success with consolidating stone struc- tures were disclosed in this review. Some apparent success has been achieved in consolidating small stone objects, such as statues, which can be treated in a laboratory. Consolidants should be used on his- toric stone building or structure only after a careful appraisal has been made of the risks involved, the benefits to be realized, and the probability of success. ASTM Standard E 632 is a useful guide to considerations that should govern the development and use of ac- celerated tests for evaluating stone consolidants and building ma- terials in general. REFERENCES 1. E.M. Winkler, Stone: Propernes, Durability in Man's Environn~ent, 2nd edition (Springer-Verlag, New York, 1975~. 2. J.R. Clifton, Stone Consolidating Materials: A Status Report, NBS Technical Note 1118 {National Bureau of Standards, Washington, D.C., 1980~. 3. ASTM Standard E 632, Recommended Practice for Development of Short-Term Accelerated Tests for Prediction of Service Life of Building Materials and Components {American Society for Testing and Materials, Philadelphia, 1980~. 4. G. Frohnsdorff and L.W. Masters, The Meaning of Durability and Durability Prediction, pp. 17-35 in Durability of Building Materials and Components, ASTM STP 691, P.J. Sereda and G.G. Litvan, eds., {American Society of Testing and Materials, Philadelphia, 1980~. 5. A.M. Schwartz, Capillarity: Theory and Practice, pp. 2-13 in Chemistry and Physics of Interfaces, II, D.E. Gushee, ea., "American Chemical Society, Washington, D.C., 1971~. 6. W.A. Zisman, ea., Contact Angle, Wettability and Adhesion: Advances ire Chemistry Series No. 43 {American Chemical Society, Washington, D.C., 1964~. 7. A.R. Warnes, Building Stones: Their Properties, Decay' and Preservation keenest Benn, London, 1926~. 8. I.E. Marsh, Stone Decay and Its Prevention {Basil Blackwell, Oxford, 1926~.

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308 CONSERVATION OF HISTORIC STONE BUILDINGS 9. A.P. Laurie and C. Ranken, The Preservation of Decaying Stone, Journal of the Society of the Chemical Industry, 37, pp. 137T-147T 11918~. 10. G. Torraea, Brick, Adobe, Stone and Architectural Cerarnies: Deterioration Proe- esses and Conservation Practices, pp. 143-165, in reference 14. 11. N. Heaton, The Preservation of Stone, foumal of the Royal Society of Arts, 70, pp. 129-139 (1921J. 12. J. Lehmann, Damage by Accumulation of Soluble Salts in Stonework, pp. 35- 46, in reference 94. 13. D.S. Knopman, Conservation of Stone Artworks: Barely a Role for Science, Science, 190, pp. 1187-1188 ~ 1975~. 14. Preservation and Conservation: Principles and Practices (The Preservation Press, Washington, D.C., 1976~. 15. A.P. Laurie, Building Matenals {Oliver and Boyd, Edinburgh, 1922~. 16. J.W. Mellor, A Comprehensive Treatise ore Inorganic and Theoretical Chemistry, Vol. VI: Silicates {Longmans, Green and Co., London, 1925~. 17. Encyclopedia of Chemical Reactions, Vol. VI {Reinhold, New York, 1956~. 18. L. Kessler, A Process for Hardening Soft Limestone by Means of the Fluosilieates of Insoluble Oxides, Comptes Rendus, 96, pp. 1317-1319 {1883~. 19. F.S. Barff, Stone, Artificial Stone, Preserving Stone, Colouring, British Patent 2608 {1860~; from reference 97. 20. The Artificial Hardening of Soft Stone, Stone, 34, pp. 365-366 {1913~. 21. J. Riederer, Further Progress in German Stone Conservation, pp. 369-385, in reference 60. 22. R. Wihr and G. Steenken, On the Preservation of Monuments and Works of Art with Silicates, pp. 71-75, in reference 94. 23. T. Stambolov, Conservation of Stone, pp. 119-124, in reference 94. 24. B.G. Shore, Stones of Britain (Leonard Itill Books, London, 1957~. 25. B. Penkala, The Influence of Surface Protecting Agents on the Technical Prop- erties of Stone, Ochrona Zabytrow, 17 {No. 1), pp. 37-43 1964. 26. A.R. Powys, Repair of Ancient Buildings {J.M. Dent, London, 1929~. 27. W. Linke, Solubilities of Inorganic and Metal Organic Compounds, Vol. I (Van Nostrand, Princeton, 1958~. 28. H.G. Lloyd, Hardening Stone and Earth, British Patent 441,568 (1934~. 29. V. Mankowsky, The Weathering of Our Large Monuments, Die Denkmalpflege, 12 {No. 1 i, pp. 51-54 ~ 1910~; from reference 97. 30. Breathing New Life into the Statues of Wells, New Scientist, pp. 754-756, (De- eember 1977~. 31. A.H. Church, Stone, Preserving and Colouring; Cements, British Patent 220 {1862~; from reference 97. 32. A.H. Church, Treatment of Decayed Stone-Work in the Chapter House, West- minster Abbey, lournal of the Society of Chemical Industry, 23, p. 824 {1904J. 33. A.H. Church, Conservation of Historic Buildings and Frescoes, Proceedings of the Meetings of the Members' Royal Institution of Great Britain, 18, pp. 597-608 ~ 1907~. 34. S.Z. Lewin, The Conservation of Limestone Objects and Structures, in Study of Weathenng of Stone, Vol. 1 {Intemational Council of Monuments and Sites, Paris, 1968~. 35. E.V. Sayre, Direct Deposition of Barium Sulfate from Homogeneous Solution Within Porous Stone, pp. 115-118, in reference 94. 36. G.G. Scott, Process as Applied to Rapidly-Deeayed Stone in Westminster Abbey, The Builder {London), 19, p. 105 t 1861 I; from reference 97.

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Stone Consolidating Materials 309 37. S.Z. Lewis and N.S. Baer, Rationale of the Barium Hydroxide-Urea Treatment of Decayed Stone, Studies in Conservation, 19, pp. 2~35 (1974J. 38. J.M. Garrido, The Portal of the Monastery of Santa Maria de Ripoll, Monun~en- tum, 1, pp. 79-98 ~ 1967J. 39. G. Zava, B. Badan, and L. Marchesini, Structural Regeneration by Induced Mi- neralization of the Stone of Eraclea (AgrigentoJ Theatre, pp. 387-399, in reference 95. 40. L. Arnold, D.B. Honeyborne, and C.A. Price, Conservation of Natural Stone, Chemistry and Industry, pp. 345-347 (April 1976J. 41. R.A. Munnikendam, Acrylic Monomer Systems for Stone Impregnation, pp. 15- 18, in reference 94. 42. C.A. Price, Research on Natural Stone at the Building Research Establishment, Natural Stone Directory ~1977J. 43. Bosch, Use of Silicones in Conservation of Monuments, pp. 21-26, in First International Symposium on the Deterioration of Building Stones t1972J. 44. H. Weber, Stone Renovation and Consolidation Using Silicones and Silicic Esters, pp. 375-385, in reference 95. 45. Preserving Building Stone, BRE News, 42; pp. 1~11 "Winter, 1977J. 46. J. Taralon, C. Jaton, and G. Orial, Etat des Recherches Effectuees en France sur les Hydrofuges, Studies in Conservation, 20, pp. 455~76 {1975J. 47. J. Riederer, Die Erhaltung Aegyptischer Baundenkmaeler, Maltechnik Restauro, 74 (No. 1J, pp. 43-52 ~1974J. 48. A. Moncrieff, The Treatment of Deteriorating Stone with Silicone Resins: In- terim Report, Studies in Conservation, Vol. 21, pp. 179-191 (1976J. 49. K. Hempel and A. Moncrieff, Report on Work Since Last Meeting in Bologna, October 1971, pp. 319-339, in reference 95. 50. C.A. Price, Stone Decay and Preservation, Chemistry in Britain, 11 {No. 9J, pp. 35(~353 {19751.- 51. A.P. Laurie, Preservation of Stone, U.S. Patent 1,607,762 t1926J. 52. H.D. Cogan and C.A. Setterstrom, Ethyl Silicates, Industnal Engineering Chem- istry, 39, pp. 136~1368 ~ 1947J. 53. H.D. Cogan and C.A. Setterstrom, Chemical Engineering News, 24, pp. 2499- 2501 1946. 54. H. Marschner, Application of Salt Crystallization Test to Impregnated Stones, paper 3.4, in reference 98. 55. R. Snethlage and D.D. Klemm, Scanning Electron Microscope Investigations on Impregnated Sandstones, paper 5.7, in reference 98. 56. R. Rossi-Manaresi, Treatments for Sandstone Consolidation, pp. 547-571, in reference 95. 57. M.B. Caroe, Wells Cathedral, The West Front Conservation Programme: Interim Report on Aims and Techniques {June, 1977J. 58. R.A. Munnikendam, Preliminary Notes on the Consolidation of Porous Building Stones by Impregnation with Monomers, Studies in Conservation, 12 ~No. 4J, pp. 158- 162 ~1967~. 59. R.A. Munnikendam, A New System for the Consolidation of Fragile Stone, Studies in Conservation, 18, pp. 95-97 {1973J. 60. G. Torraca, Treatment of Stone in Monuments. A Review of Principles and Processes, pp.297-315, in The Conservation of Stone, I: Proceedings of the International Symposium, Bologna, June 1975 {Centro per la Conservazione delle Sculture all'Aperto, Bologna, Italy, 1976J. 61. L.E. Kukacka, A. Auskern, P. Colombo, J. Fontana, and M. Steinberg, Introduc-

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310 CONSERVATION OF HISTORIC STONE BUILDINGS tion to Concrete-Polymer Materials, Brookhaven National Laboratory. Available from National Technical Formation Service, No. PB 241-691. 62. J. Clifton and G. Frohnsdorif, Polymer-Impregnated Concretes, pp. 174-196, in Cements Research Progress 1975 {American Ceramic Society, Columbus, Ohio, 1976~. 63. M. Steinberg et al., Concrete-Polymer Materials, First Topical Report, Brook- haven National Laboratory Report No. BNU50134 {1968~. 64. L.E. Kukacka et al., Concrete-Polymer Materials, Fifth Topical Report, Brook- haven National Laboratory Report No. BNL~50390 {1973~. 65. E. Dahl-Jorgensen and W.F. Chen, Stress-Strain Properties of Polymer Modified Concrete, Fritz Engineering Laboratory Report No. FEL 390.1 Lehigh University, Penn- sylvania, 1973~. 66. R.B. Seymour, Introduction to Polymer Chemistry {McGraw-Hill, New York, 1971~. 67. H.A. LaFleur, The Conservation of Stone. A Report on the Practical Aspects of a UNESCO Course, October and November 1976, Venice, Italy (National Park Service, Washington, D.C., 1977). 68. KL. Gauri, P. Tanjaruphan, M.A. Rao, and T. Lipscomb, Reactivity of Treated and Untreated Marble in Carbon Dioxide Atmospheres, Transactions of the Kentucky Academy of Science, 38 No. 1-2), pp. 38 4~1 jl977~. 69. K.L. Gauri and M.V.A. Rao, Certain Epoxies, Fluorocarbon-Acrylics and Silicones as Stone Preservatives, pp. 73-80, in reference 99. 70. K.L. Gauri, J.A. Gwinn, and R.K. Popli, Performance Criteria for Stone Treat- ment, pp. 143-152, in reference 96. 71. Usefulness of Some Vinyl Polymers in the Conservation of Monuments, Och- rona Zabythow, 14 {No. 3-4), pp. 81-92 {1961~; from reference 97. 72. K.F.B. Hempel, Notes on the Conservation of Sculpture, Stone, Marble, and Terra-cotta, Studies in Conservation, 13, pp. 34 44 { 1968~. 73. H. Lee and K. Neville, Handbook of Epoxy Resins {McGraw-Hill, New York, 1967~. 74. W.G. Potter, Uses of Epoxy Resins {Chemical Publishing Company, New York, 1975J. 75. K.L. Gauri, Cleaning and Impregnation of Marble, in reference 100. 76. K.L. Gaun, Improved Impregnation Technique for the Preservation of Stone Statuary, Nature, 228 {No. 5274i, p. 882 (1970~. 77. G. Marinelli, Use of an Epoxy Aliphatic Resin in the Consolidation of Porous Building Materials Having Poor Mechnical Properties, pp. 573-591, in reference 95. 78. K.L. Gauri, Efficiency of Epoxy Resins as Stone Preservatives, Studies in Con- servation, 19, pp. 10~101 { 1974~. 79. A. Moncrieff, Work on the Degeneration of Sculptural Stone, pp. 103-114, in reference 94. 80. K. Hempel and A. Moncrieff, Summary of Work on Marble Conservation at the Victoria and Albert Museum Conservation Department up to August 1971, in reference 100. 81. Epoxy Resin Saves Venice Church, Corrosion Prevention and Control, pp. 12- 13 "October, 1974~. 82. D.A. Whiting, P.R. Blankenhom, and D.E. Kline, Effect of Hydration on the Mechanical Properties of Epoxy Impregnated Concrete, Cement and Concrete Research, 4 (No. 3i, pp. 467-476 {1974~. 83. D.A. Whiting, P.R. Blankenhom, and D.E. Kline, Mechanical Properties of Epoxy Modified Concrete, journal of Testing and Evaluation, 2 {No. 1), pp. '14~9 (1974~.

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Stone Consolidating Matenals 311 84. M. Kranz, l 'evaluation de l'Etat de Conservation de la Pierre et de l'Efficacite des Traitements, pp. 443~53, in reference 95. 85. E. DeWitte, Soluble Nylon as Consolidation Agent for Stone, Studies in Con- servation, 20 {No. 1J, pp. 33-34 {1975J. 86. C.R. Steen, Some Recent Experiments in Stabilizing Adobe and Stone, pp. 59- 64, in reference 94. 87. C. Steen, An Archaeologist's Summary of Adobe, El Palacio, 77 iNo. 4J, pp. 29- 38 {1971J. 88. Vitruvius, De Architectura, I.X.; from reference 50. 89. G.G. Scott, Process as Applied to Rapidly-Decayed Stones in Westminster Abbey, The Builder, 19 No. 941J, pp. 105 t1861J; from reference 97. 90. S.G. Burgess and R.J. Schaffer, Cleopatra's Needle, Chemistry and Industry, pp. 1026-1029 11952J. 91. D.W. Kessler, Exposure Tests on Waterproofing Colourless Materials, Techno- logical Papers of the Bureau of Standards, No. 248, {192~25J. 92. L. Arnold and C.A. Pnce, The Laboratory Assessment of Stone, pp. 695-704, in reference 95. 93. The Preservation of Ruins, l~terary Digest, pp. 9~95 {July, l910J. 94. Conservation of New Stone and Wooden Objects, New York Conference, June 1970 {The International Institute for Conservation of Historic and Artistic Works, Lon- donJ. 95. The Conservation of Stone, I, Proceedings of the Intemational Symposium, Bologna, fun e 1975 (Centro per la Conservazione delle Sculture all'Aperto, Bologna, Italy, 1976J. 96. Y. Efes and S. Luckat, Relations Between Corrosion of Sandstones and Uptake Rates of Air Pollutants at the Cologne Cathedral, pp. 193-200, Intemational Symposium on the Deterioration of Building Materials {Athens, Greece, 1976J. 97. S.Z. Lewin, The Preservation of Natural Stone, 1839-1965, An Annotated Bib- liography, Art and Archaeology Technical Abstracts, 6 iNo. li, pp. 185-277 {1966J. 98. Deterioration and Protection of Stone Monuments, International Symposium {Paris, June 1978J. 99. Decay and Preservation of Stone, Engineering Geology Case Histories No. 11, E.M. Winkler, ed. (The Geological Society of America, Boulder, 1978J. 100. The Treatment of Stone, Proceedings of the Meeting of the foint Committee for the Conservation of Stone, Bologna, October, 1971 {Centro per la Conservazione delle Sculture all'Aperto, Bologna, Italy, 1972J. 101. Stone Preservatives, Digest First Series No. 128, Building Research Station (Gar- ston, England, 1963J. 102. W.H. Dukes, Conservation of Stone: Chemical Treatments, The Architects' [ournal Information L~brary, pp. 433~38 (August 23, 1972J.