ANALYTICAL APPROACH
HERITAGE IN STONE George Segan Wheeler Objects Conservation Department The Metropolitan Museum of Art 1000 5th Avenue New York. N Y 10028 Alan Schein Science and Technology Communications 1701 North Point San Francisco. CA 94123 Gretchen Shearer Department 01 Biochemistry University 01 Iowa Bowen Science Building Iowa City, IA 52242 S. H. Su and C. Scott Blackwell Union Carbide Corp. Tarrytown Technical Center Old Saw Mill River Rd. Tarrytown. NY 10591 0003 2700920364-347A802500 1992 Amer can Cnem ca Soc.ely
The use of stone as a medium of expression greatly antedates recorded history. Stone artifacts from every culture document the experiences of artists, toolmakers, hunters, architects, rulers, priests, and philosophers. Each work, from tiny ancient cylinder seals to the great Buddhist temples of Cambodia and the massive granite carving of Mt. Rushmore, is a unique treasure of human heritage worthy of preservation for generations to come. The diversity of materials from which stone objects were created pre-
sents a daunting challenge to the art conservation community. In today's industrial society, acid rain generated hy the burning of fossil fuels has complicated the task of preservation by accelerating the deterioration of buildings, monuments, and sculptures. Particularly hard hit a r e works of limestone and marble, composed primarily of the acid-soluble minerals calcite or dolomite. Longterm burial in marine or land environments also can prove highly destructive to stone objects. Over time, large quantities of salts may be deposited within the pore structure of the stone. When these objects are re-
LQJ?:impression ofa steatite ancient cyiinder seal from Mesopotamia (diameter of the seal is 2 cm); top lep: a 3-m sandstone figure from the Cambodian Buddhist Temple of Angkor Waf; top right: Mt. Rushmore. ANALYTICAL CHEMISTRY, VOL. 64, NO, 5, MARCH I , 1992
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moved from burial and placed in drier environments, the salt deposits can crystallize and pulverize the stone (Figure 1). Since the Italian Renaissance, and possibly as far back as Roman times, efforts have been made to preserve stonework (I). Early preservation methods included the application of wax- or oil-based coatings to the surfaces of stone objects to repel water and strengthen the deteriorated matrix. This strengthening process is known as consolidation. The modern analogues of Renaissance consolidants are synthetic acrylic and epoxy resins. Although many times stronger than their precursors, these newer consolidants deteriorate when exposed to UV light and therefore are unstable outdwrs. When they break down, synthetic organic eonsolidants not only lose their ability to reinforce, they can actually damage the stone. This debility has generated a search for consolidant systems with greater photostability.
lnorganlc consolidant systems In attempts to develop new consolidants, some workers have examined inorganic salts such as barium sulfate as possible alternatives to organic resins. Unlike the molecularlevel adhesion typical of organic polymer systems, inorganic salts act more like a mortar, creating a matrix of interloeking aggregates that reinforce the porous network of the stone weakened by chemical or mechanical degradation. The barium salt is introduced in a soluble form (barium ethyl sulfate) and hydrolyzed in situ to produce barium sulfate (2).Such inorganic consolidants have narrow ranges of application. For example, barium sulfate is effective only with limestones, and even then it exhibits poor depth of penetration and is difficult to apply.
A second formulation combines barium hydroxide and urea to produce a deposit of barium carbonate (3). Although by itself this deposit provides some consolidation, the carbonate is eventually converted into the less soluble sulfate when it reacts with acid rain. A third formulation involves the application of saturated solutions of lime (4).The low solubility of lime makes repeated applications necessary to deposit an effective amount of consolidant.
Alkoxysilane consolidants A second, somewhat more promising, line of research is the use of alkoxysilane monomers or oligomers to form cross-linked silicate polymers within the matrix of stone objects. This approach is especially attractive because, like acrylic and epoxide consolidants, alkoxysilanes may produce a binding polymeric network in some stones. However, unlike the C-C and C-0 bonding in the backbone of organic polymers, Si-0-Si linkages of alkoxysilane polymers are stable in UV light. The synthesis of tetraethoxysilane, the first monomer widely used to produce silicate-based consolidants, was reported by von Ebelman in 1846(5).Fifteen years later, von Hoffman (6) suggested that tetraethoxysilane be used as a stone consolidant. In fact, tetraethoxysilane exhibits several attractive features. It has low viscosity and low surface tension for easy and rapid penetration into the interstices of porous media, and it is hydrolyzed by water to form silanols, Si(OH),, which undergo condensation polymerization to form a nonreactive silica gel that is inert in UV light. Little came of attempts to exploit von Hofian’s prescient suggestion until the 19608, when workers a t Wacker Chemie patented a consolidant based on the catalyzed poly-
The photographs show the conditions of an Egyptian limestone relief from Abydos (ea.1315 B.C.) 6mn alleritwasexcavatetin1911 (len)andin1982(rigM).
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5. MARCH 1.1992
merization of tetraethoxysilane (7). Although this product, known as Sandstone Strengthening Agent OH, is still used today, its use is not restricted to sandstone. In the 1970s a related alkoxysilane monomer, methyltrimethoxysilane, was explored by Hempel and Moncrieff (8). Following their work, Arnold and Price developed a consolidant system, Brethane, which is based on the catalyzed polymerization of methyltrimethoxysilane (9). The use of catalyzed polymerization is important because of the volatility of some alkoxysilane monomers, particularly in the case of methyltrimethoxysilane, which has a vapor pressure of 31 mm Hg. Without the addition of a catalyst, the rate of polymerization of neat methyltrimethoxysilane is too slow and the bulk of the monomer evaporates long before it can react with sufficient atmospheric moisture t o polymerize. Catalysis of methyltrimethoxysilane or tetraethoxysilane, however, produces brittle gels that crack during the advanced stages of polymerization and produce a consolidant with poor mechanical properties. The objective of our research at the Objects Conservation Department of the Metropolitan Museum of Art was to develop a reactive alkoxysilane consolidation system that produces reasonable amounts of polymer gel dep o s i t s w i t h good m e c h a n i c a l properties.
The role of solvent To better understand the chemistry involved in alkoxysilane plymerization, we studied t h e methyltrimethoxysilane reaction in the absence of catalysts. Both acidic and basic catalysts were excluded because acids dissolve calcite, and bases, if left in the stone, can gradually combine with atmospheric CO, to form carbonate salts that can crystallize and damage the stone. Methyltrimethoxysilane polymerizes by reacting with moisture:
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ANALYTICAL CHEMISTRY, VOL. 64. NO. 5. MARCH 1. 1992
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ANALYrICAL APPROACH To prevent methyltrimethoxysilane from evaporating before sufficient polymerization takes place, reactive formulations were developed that incorporate water along with the alkoxysilane. Methyltrimethoxysilane and water are immiscible, and a solvent is required to create a homogeneous system. After evaluating 15 different solvents, we developed individual formulations based on methanol, ethanol, and isopropanol. In addition to having excellent solvating properties for both water and methyltrimethoxysilane, these compounds are inexpensive and readily available. Having settled on the basic system components, we next explored the effect of varying the relative ratios of monomer, water, and solvent. By visually inspecting the condensation products, we determined that gels with little or no cracking could be produced with formulations ranging between an upper limit of 4 mol of water, 1 mol of methyltrimethoxysilane, and 3 mol of solvent (41:3), and a lower limit of 2 mol of water, 1mol of methyltrimethoxysilane, and 2 mol of solvent (2:l:Z). In developing optimal formulations, it was important to determine whether the tendency for a gel to crack was a function of the degree of condensation in the gel (i,e., the type and relative proportion of siloxane linkages [Si-0-Si]) or of the nature of the solvent in the reaction mixture. To clarify the relationship between solvent and gel properties, three gels prepared from reaction mixtures-each differing in type of alcohol solvent and cured at 21 "C and 40% relative humidity-were analyzed by solid-state "Si NMR spectroscopy. Depending on the extent of hydrolysis and subsequent condensation, the methyltrimethoxysilane reaction generates producta classified in one
of four general types of chemical species; these can be differentiated using *'Si NMR spectroscopy (Table I). The results show little difference in the proportion of linear and threedimensional linkages formed with each alcohol solvent; therefore, the degree of condensation cannot be responsible for the amount of cracking in the gel. Because slower drying is known to reduce cracking (lo),it is likely that the reduction in cracking is a result of reduced solvent vapor pressure. Indeed, the degree of cracking in the gel shows a trend consistent with the vapor pressure of the alcohol solvent. Role of minerals Stones vary greatly in physical and chemical properties. In the past, the approach to consolidant development was to apply the same formulation to all substrates without regard to the particular mineralogy. However, given the varying composition and chemistry of different stones, it is likely that consolidation systems will have to be tailored to the nature of the substrate. The next phase of our research addressed differences in stone mineralogy. Although researchers in the field of stone conservation study rocks with a wide range of mineralogies, much of the sculpture that needs to be preserved is composed of either sandstone or limestone, two rocks that consist primarily of quartz or calcite, respectively. GClMS was used to examine the possible influence of these minerals on the polymerization of methyltrimethoxysilane. This technique has been used to separate oligomers formed by the reaction of alkoxysilanes (11,12). Because successful GC requires volatile analytes, it was necessary to maintain volatility of the many species in a reactive alkoxysilane mixture by limiting the extent of poly-
Condsnsalion type (%)
lcohol Went
Gd a m e a n n w CH,SI(R),
-(SI-ObSI(R)CH.
-(SI-O),SI(R)CH,
-(SI-O)rSICH,
me. R indicates OCH, or OH: SI indicates silicon atom detected by NMR spectroscopy.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1,1992
I
merization. I n t h e reaction of methyltrimethoxysilane, the amount of water added provides the desired element of control. A 1:l:l molar mixture (water, methyltrimethoxysilane, and solvent) produces volatile molecular species that can be analyzed with standard gas chromatographic temperature programming. This mixture was used as the control in the absence of any mineral. The chromatograph (HewlettPackard 5890 GC coupled to a 5970B mass selective detector) was equipped with a 25-m column consisting of a cross-linked 5% phenylmethyl silicone stationary phase, which provided separation both for polar OH-bearing and nonpolar, fully methoxylated reaction producta. The chromatogram for the control is shown in Figure 2a. (The mass spectral identification of the components follows the approach of Coutant and Robinson 1131 as well as previous work [141). Test solutions with either quartz or calcite were prepared in a manner identical to that of the control but with the addition of 40% wlw of the powdered mineral. By comparing the chromatograms of the control and the quartz mixtures (Figure Zb), it is evident that the addition of quartz retards the reaction of methyltrimethoxysilane. As a result, the quartz reaction product mixture is richer in the total amount of both monomer and dimer (-42% for the quartz mixture vs. 8% for the control) and the formation of oligomers is reduced. The addition of calcite retards the reaction much more than quartz, further enriching the relative amounts of both monomer and dimer (75% for the calcite mixture) while decreasing the amounts of oligomer formed. This result is confirmed by the chromatogram (Figure 2c). In addition, there is a higher concentration of silanols (retention time of 5.1, 12.2, 16.2 min) compared with the control and the quartz mixtures. Calcite appears to specifically retard the condensation reaction whereby two silanol groups condense to form a siloxane linkage. Under these conditions, much more of the methyltrimethoxysilane could be expected to evaporate before polymerizing to form a consolidant in objects made from calcitebearing rock. This result is borne out empirically: It is well known that methyltrimethoxysilane does not consolidate limestone as well as sandstone. (Note that FT-IR analysis of the mineral additives revealed an
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ANALYTICAL CHEMISTRY, VOL
1:1:1 molar mixture of water, methyltrimelhoxysilane, and methanol aner 24 h. (a) Control. Note that
- 92% of the readion mixture is composed of molecular species that are lrimers or larger, leaving Only - 8% monomers and dimers. (b) With 40% whv powdered quanz, only - 50% of the reaction mixture consists 01 trimers or larger molecular species compared with - 92% in the absence of quartz. (c) With 40% whv powdered calcite, only - 25% of Ihe reaction pmducts are trimers or larger moiecular species. NO. 5, MARCH 1,1992
absence of surface deposits of methyltrimethoxysilane-derived reaction products.) Future work
We are trying to improve the consolidation of calcite-bearing rocks by adjusting formulations to reduce evaporation of methyltrimethoxysilane while forming gels with good mechanical properties. Another important objective is the improvement of consolidantlstone substrate adhesion. Although there may be good adhesion between alkoxysilane-derived gels and silicate-based minerals such as quartz (one would anticipate a bonding affinity between silicate rocks and the Si-0 linkages formed by the reactive alkoxysilane), adbesion-promoting chemical compatibility is absent in calcite-based rocks. One approach to remedy this debility is to employ a chemical intermediary that might bond both to the calcite and to reacting multifunctional, multicomponent adhesion promoters. We thank Fred Osterholtz of Union Carbide for making the *$SiNMR spectroscopy work possible. We also thank Laura Cerruti (HewlettPaekardj for her assistance in interpreting and producing the graphics for the gas ehmmatograms. We would particularly like to express our gratitude to Anita Ciriello (HewlettPaekardj, whose support and encouragement brought this article to fruition.
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ANALYTICAL APPROACH J. D.: Ulrich. D. R.. Eds.: Wile": New York; 1987; pp. 819-25. (13) Coutant, J. E.; Robinson, R. S. In Analysis olSilicnnes; Smith, A. L., Ed.; Krieger: Malabor, FL, 1988: pp. 325-48. (14) Wheeler. G. The Chemistry of Four Alkoxyrilanes and 'Their Potential%; Use as Sfone Consolidants; U n i v e r s i t y M i c r o films: Ann Arbor, MI, 1987; pp. 98-104.
George Segan Wheeler rrcmi,pd a B.A. degree from Muhlenberg Collrge (PA), an M.A. degree in art history from Hunter College (Mi). and an M.A. degree and a Ph.D. from the chemistry department at New York University. He is also the recipient ofa Certificate in Conservntionfrom the New York University Conservation Center. In 1979 he joined the staffat the Metropolitan Museum of Art, where his research focuses on the preservation of stone sculpture, monuments, and buildings.
Alan Scheiii rrrrii,rd ii 1i.S. degree from Brooklyn Collrgr. Hr Is a sripnce journalist and a tecli)iical communications consultant focusing on analytical, environmental, and biological applications.
C. Scot! Blarka~rllrrwii'rd a B.S. degree from the Uiiicrrsify of North Carolina, Chapel Hill, and a Ph.U. from MIT. His work centers on NMR spectroscopy.
S. H. Su recrii i.11 a N S drxrPefrom National Taiwaii Vormal UiiiL,ersityand a Ph.D. from Michigan State University, and joined the staff at Union Carbide in 1981. Her research interests lie in the area of organo-functional silanes.
Gretchen Shearer rrcriupd a Ph.D. in archaeology from thr Uiimersity College of London. She was an I,. W Frolich Fellow at the Metropolitan Museum ofArt before moving to the University of town, where she is workingon analyticalchemistryapplications in enzymology,
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