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Physical Properties of Building Stone EUGENE C. ROBERTSON Porosity and permeability seem to be the most important physical properties affecting weathering and deterioration of building stones by water and gases. Thermal and mechanical properties, because of their effects on permeability, chemical reactions, strength, and stability, can be important In diagnosing decay processes in stones; but optical, electncal, and magnetic properties have little significance in deterioration processes. Measurement of physical prop- erties on small laboratory samples of any rock type range widely because the composition and character of every rock type differ according to locality. Var- iations in physical properties because of compositional and textural inhom- ogeneities can be seen in small to large quarried blocks or in rock in place and can be more significant in explaining rock deterioration than laboratory tests of the physical properties of selected small samples of rock. Examples of in- homogeneities are intercalated shaley layers, calcite, limonite, or clay cements; thin to thick bedding; mineral variations within beds in sedimentary rocks; foliation; induration; microfractunng; and incipient to open jointing. Certain physical properties of building stones are very important in determining the susceptibility of stones to natural weathering or de- terioration caused by pollution, whereas other properties have negli- gible influence. Most physical properties are discussed in this paper, Eugene C. Robertson is Geophysicist, U.S. Geological Survey, Reston, Virginia. 62

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Physical Properties of Building Stone 63 and distinctions are drawn between physical properties measured on small laboratory samples and those observed on stone in place or in large blocks. Chemical properties of building stones need to be con- sidered in conjunction with physical properties in studying processes of deterioration, and they are covered elsewhere in these proceedings. The comprehensive book on properties and durability of stone by Erhard Winkleri has been helpful in preparing this report. Other books containing data on physical properties of building stones are those of Bowles,2 Merrill,3 Schaffer,4 and Winkler.5 PHYSICAL PROPERTIES Samples of one type of rock obtained from different localities, inev- itably differ considerably in their properties because of variability in the composition and texture of the rock among the localities. Even though the chosen samples with the same rock name might appear form, their measured properties would vary so widely that an av- erage value for that rock type would be: misreading. Therefore, it seems appropriate to give only ranges of values for physical properties of common building and monument stones (Table 11. The names of rocks are quite general in their geologic usage As in this report), and are even less specific in stone industry usage. For a few rocks for which only a few measurements have been published (e.g., soapstone and serpen- tinitel, the range limits in Table 1 were estimated by comparing values with those of other types of rocks. Porosity and permeability are probably the most important physical properties of rocks for studies of decay and corrosion of building and monument stones. This is because these properties characterize the accessibility of water to the interior of the stones and because water in all of its three phases is perhaps the most important substance affecting the weathering and deterioration of the stones. Thermal and mechanical properties are next in importance in the decay of rocks. Optical, electrical, and magnetic properties have very little importance. Physical properties depend primarily on the origin and geologic his- tory of each rock. Because mineral composition and texture differ ac- cording to the varying geologic histories of rocks, the values of physical properties range widely {Table 11. Building stones are polycrystalline mineral aggregates, not single crystals. Thus intergranular bonding, pore shape and size, and fabric are more important than the physical properties of individual mineral grains, even including their anisotropy.

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Physical Properties of Building Stone 65 Aggregation Properties The density, porosity, and permeability of rocks are different measures of the state of aggregation of the mineral grains that make up the rock and are measures of the accessibility of fluids, pnncipally air and water, into and through the rock. Geologic history determines the aggregation. Density Grain density, PG, is the ratio of grain mass to grain volume of rock having no porosity. The values in Table 1 are bulk densities, PB, which are the mass of grains divided by pore volume plus grain volume. Measurements of PA by immersion methods on whole samples can be in error by as much as 10 percent owing to incomplete saturation of inaccessible pores; PB measurements would be more accurate. The p,: measured on a crushed sample would give a more reliable value. Moen put lower and upper density limits of 1.7 to 2.2 g/cm3 on commercial stone that would be favorable for preparation and work- ing.6 By his criteria, stones having a density greater than 2.2 g/cm3 are too hard to work easily with masonry tools, and stones having a density less than 1.7 g/cm3 are too soft and easily weathered. However, those having densities above 2.2 g/cm3 resist weathering better than stones with lower densities and can be worked with modern abrasives and machines. Porosity Porosity, +, is the ratio of pore volume to bulk volume. Porosities of common building stones are listed in Table 1. Porosity of igneous and metamorphic rocks is low, usually less than 5 percent, but it can be as high as 40 percent for sedimentary rocks. Pores are important in rock decay because they are receptacles for fluids and sources of weak- ness for ambient stresses. Hudec found that weathering of stone is enhanced by decrease in pore sized Mineralogy ~nc3 degree of metamorphism cause sizes and shapes of pores to differ in sandstone and shale and to differ in quartzite and slate. Clay and mica minerals make up half of most samples of shale and slate; they occur as closely packed, parallel flakes, which make for tabular and very small pore spaces, about 0.01 ,um across in most shales. Porosities of shale and slate range from 30 to 0.1 percent, de- pending on the degree of compaction, diagenesis, and metamorphism. In sandstones, porosity does not depend on grain size but does depend

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66 CONSERVATION OF HISTORIC STONE BUILDINGS on sorting of the original sand grains. For example, in unconsolidated sand beds, porosity ranges from 40 percent in very well sorted sands to 25 percent in very poorly sorted sands. In addition, compaction of sand- stone and deposition of silica or other mineral cement in quartzite can reduce markedly the pore size, porosity, and permeability (Table 11. Permeability The ease of flow of fluid through a rock is defined empirically by Darcy's law, in which the flow depends on the permeability of the rock and on the pressure and viscosity of the fluid: Q = ,uP/v (L/A), al) where Q is discharge in cubic centimeters per second, ~ is permeability in darcies, P is pressure difference in bars, v is fluid viscosity in cen- tipoises, L is distance of flow in centimeters, and A is cross-sectional area In square centimeters. Intrinsic permeabilities obtained by laboratory measurements of in- tact samples of building and monument stones are listed in Table 1. Joints and fractures increase the permeability of rocks in place at shal- low depths by 10 to 100 times the intrinsic values for samples of solid rock given in Table 1. This increase is described in a very compre- hensive review of the permeability of rocks. Water at 200 C, flowing through granite having 10-3 darcy crack permeability, dissolved and reprecipitated enough silica in one month to reduce the permeability to 10-4 darcy.~3 Silicate and carbonate rocks are susceptible to such sealing by dissolution and reprecipi- tation of silica and carbonate ions. Most research on intrinsic permeability has been performed on sedimentary rocks; workers have studied the effects of porosity, pore size and shape, and mineralogy. Data were obtained in studies of the migration of oil and gas. Very few equivalent data are available for igneous and metamorphic rocks. The permeabilities of stones range from several hundred darcies in river sand, through 0.1 darcy for common sandstone of 20 percent porosity, to 10-9 darcy for common shale. Clay minerals in shale or sandstone reduce perme- ability markedly because the pores between clay particles are very small; smectite clays in a rock expand by water absorption, further reducing permeability. Figure 1 presents points from measurements on five sandstones in rows labeled by their porosities, which range from 8 to 22 percent;

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Physical Properties of Building Stone 10 10-1 - c~ au . _ cat 10-2 J m LO 10-3 10-4 10-5 1- _ 10-6 10-3 67 // Porosity 9~ //// 50% o/ 33% 10% 19 15 8 10-2 10-1 PORE RADIUS (,um) 1 10 10+2 FIGURE 1 Effect of pore size on permeability of five sandstones having porosities as shown; a mercury injection technique was used on cores. The symbols aligned hori- zontally with each porosity percentage in the right column are for measurements on a separate rock. The percentages marked on the lines represent the proportion of pores larger than the pore radius for a given permeability on the ordinate axis Redrawn from Blatt et al.~.~4

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68 CONSERVATION OF HISTORIC STONE BUILDINGS the permeability, A, is plotted against pore size, eliminating grain size, sorting, and cement characteristics. The percentages on the lines show relatively how many pores are larger for any point on the line. The line on the right is for sandstones in which 10 percent of the pores are larger than those found on the abscissa for a given ,u" on the ordinate; the line on the left is for sandstones in which 10 percent of the pores are smaller for a given A. As might be expected intuitively, the permeability of sandstones varies exponentially with porosity and with pore size. Figure 2 shows the effects resulting from differing contents of two clay minerals on the permeabilities of several sandstones of similar 1 10-1 ._ cat - _ 10-2 J m LU UJ cot /Kaolinite / Kite l ) / 10-41 1 1 r 1 1 1 -1 0 4 8 12 16 20 24 28 POROSITY (%) FIGURE 2 Variation of permeability with two types of clay con- tained in several sandstones of sirn~lar porosities Imodified from Blatt et al.~.l4

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Physical Properties of Building Stone 10 10-, m us ~2 ~ 1u llJ 10-3 10-4 HA ~ 4~ 1 ( ALL 0 10 20 30 1 1 1 1 POROSITY (%) FIGURE 3 Vanation of permeability with specific in- ternal surface areas (shown in cm2/cm3 inside enclosed areas) of six sandstones having a range of porosity {modified from Blatt et ~.i.~4 69 porosities. The specific surface areas (cm2/cm3) of six sandstones of varying porosity are shown inside the enclosed areas in Figure 3. Specific surface area is high for fine-grained, low-permeability sand- stones; it is Tow for coarse-grained, high-permeability sandstones. Physical Models for Aggregation Properties The intrinsic density, porosity, and permeability of a sample would appear to be closely related, judging from their definitions. However, measured values of these properties differ from absolutely accurate values enough so that they cannot be calculated exactly from each

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70 CONSERVATION OF HISTORIC STONE BUILDINGS other. The differences are probably due to the effects of isolation, small size, en c! irregular shape of some pores, leading to incomplete satu- ration by the measuring fluid en c] diminished accuracy of measure- ment. The following discussion and equations are meant to provide the reader with some understanding of- what physical characteristics are important and to permit calculations for comparison purposes. The porosity, , of a dry rock that is, the ratio of pore volume to bulk volume is given in terms of densities by: (> (PG PB)/PG. (2) A good value of pore volume is needed to obtain PA, but it is not easy to measure closely. However, as PA can be calculated from the rock's mineral composition, and as PB is more easily measured, a reasonable estimate of ~ can be calculated from equation 2. A relation between porosity, , and permeability, A, was found em- pirically by Kozeny:~4 it= 106~/2t2S2, t3) where ~ is in darcies, ~ is in percent, t is tortuosity (usually taken as 2 to 3), en c! S is specific surface area in cm2/cm3 of grains in a rock. The interdependence of ,u and ~ is not clearly understood. The value of s is obviously strongly affected by grain size, so that ,u for a coarse- grained sandstone can be an order of magnitude higher than for a fine- grained sandstone, although both have the same .~4 The Kozeny for- mula has limited use because s and t are difficult to estimate. Absorption of fluids in rocks depends on the connected, effective (i.e., permeable) porosity. Connected pores in building stones can be visualized as a system of capillary passages in which the surface tension of water becomes important. The surface tension by of a fluid in a capillary crack of width c] is given by: A= C3h, (4) where C is a constant and h is the height of rise of the fluid. For a given fluid, "y and C would be fixed; therefore, the smaller the crack width c] the greater the rise h in a capillary passage. Crack widths of 5 ,um have been measured in building stones; one epoxy that was injected into the pores of deteriorated stones to try to seal them has itself been found to contain crack widths of 2 to 10 ,um. Lewin discusses capillary flow in detail in these proceedings. He points out that the

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Physic~lPropertiesof Building Stone 71 volume of flow is proportional to the radius of the capillary to the fourth power. Saline water has been observed to rise 4 to 10 m in capillaries in sandstone and other masonry materials.~5 THERMAL PROPERTIES The effects of diurnal and seasonal heating and cooling on deterioration of building and monument stones can be significant on a microscopic scale. These effects involve conduction of heat by solids and induced thermal stresses. Some of the measurements that have been made of thermal stresses and their effects on rock in place are described below. Thermal Expansion Thermal stresses resulting from changes in the temperature of ambient air can be large enough to produce microfractures in and between the mineral grains of a rock. This can happen even in temperate climates because of anisotropy and differences in the thermal expansions of the minerals. An important feature is that the fracturing is irreversible, and thereafter the permeability will be greater and will allow greater penetration of water. Ide found that although the volume of several common rocks did not increase perceptibly upon heating, microfractures formed by dif- ferential expansion of mineral grains, resulting in a very marked and irreversible decrease in the elastic modulus.~6 For example, a 25-fold reduction, from 0.8 to 0.03 M bar, in the elastic modulus, E, of a granite resulted from heating to 500 C (see Figure 4; note that E varies as velocity squared). Ide found that only a 2 percent reduction in the modulus resulted from heating to 100 c.~6 Griggs found no spelling or extension of cracks in photomicrographs after heating and cooling a granite block between 32 C and 142 C for about 20,000 cycles.~7 However, Ide's result at 100 C indicates that some microfractures would have formed, although they would have been undetectable at the magnification Griggs used. Hudec shows that water in the pores weakens rock, making thermal stresses more effective.5 A property like elasticity could be used to reveal the extent of damage from ther- mal cracking. Modem ultrasonic, acoustic, or mechanical velocity- logging devices can be used to measure the expected decrease in elas- ticity and could be adapted to measure the weathering of monument stones. Hooker and Duvall, in a quarry at Mount Airy, N.C., measured a 70-bar increase in stress in granodiorite resulting from a 25 C tem-

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72 CONSERVATION OF HISTORIC STONE BUILDINGS 4 3 2 1 O ~ \ IN GRANITE 0 100 200 300 400 500 TEMPERATUR E ( C) FIGURE 4 Irreversible change in longitudinal sonic velocity (a measure of the elastic modulus E) of a Qliincy granite sample on heating to 270 C and then to 500 C ride. perature change between February and August (see Figure 51.~8 The thermal stress equation is: if= 0`E(Ti To)/~1 - v), (5) where or is stress in kilobars; ax is coefficient of expansion in reciprocal degrees Celsius; E is elastic modulus in kb; To and To are final and initial temperatures in degrees C; and v is Poisson's ratio, which can be taken as about 0.25. Stress change per degree temperature rise was measured at 3.1 bars per degree Celsius {Figure 5) and was calculated

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76 CONSERVATION OF HISTORIC STONE BUILDINGS strength of rock, but their relations to rock decay are probably min- imal. Elasticity The ratio of stress to strain is Young's modulus of elasticity, E; it is usually accepted as constant. Most rocks are brittle and behave elas- tically to an elastic limit, which is usually near the stress at which the rock fails. As noted under Thermal Expansion {above), microcrack- ing changes the elasticity of a rock, so changes in elasticity may be used to detect increased porosity, permeability, and susceptibility to deterioration. Compressive Strength Walsh discusses compressive strength in detail in these proceedings, but a brief review may be in order. The unconfined, or uniaxial, strength of rock in compression is the maximum stress attained before the rock fails, usually by brittle rupture at strains of a few percent. Generally, igneous rocks, quartzite, and some state are strong rocks; schist and marble are moderately strong; and the porous sedimentary rocks are weak. Limestone and rock salt, under respective confining pressures of 1 kb and 0.2 kb or higher, will deform plastically under differential stress above the elastic limit; they can deform by creep to 20 to 30 percent before large cracks form. The uniaxial strengths of porous sandstones, limestones, and shales depend on porosity; the strength of these rocks increases about threefold as porosity decreases from 35 to 1 percent. These rocks also show a small increase in strength as grain size decreases. In a feldspathic sandstone cemented by calcite, an increase in quartz content from 1 to 60 percent increases strength fourfold. The effect of water, especially if under pressure, is to weaken rocks; this effect and those of confining pressure and other physical conditions on graywacke and other rocks were reviewed by Robert- son.20 Modulus of Rupture The modulus of rupture of rock is measured by a simple bending test on an unconfined sample. It is approximately equal to the tensile strength, in that the sample fails by tension in the extended elements of the beam. Tests strictly of tension in rocks are quite difficult to do

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Physical Properties of Building Stone properly. As can be seen~in Table 1, the modulus of rupture is one- third to one-tenth of the compressive strength. The modulus as a test of tensile strength is useful in applications involving failure in tension of building and monument stones because of thermal or mechanical microfracturing. 77 Fnability A rock or mineral is said to be friable if it crumbles naturally or is easily broken. Examples of friable rocks are soft or weakly cemented sandstones and shales. The friability of rocks can be considered a gra- dational mechanical property, which is perhaps best measured by an abrasion-hardness test. Such tests use a grinding powder and a lap or wheel applied to the specimen under a standard load. Relative values from such tests could be useful in detecting and monitoring the de- terioration of building stones. No values of abrasion hardness are given in Table 1 because the values range widely among rocks and overlap from one rock to another. Friability depends on the strength of the weakest of the major mineral constituents and on the strength of the bonding between the mineral grains. Friability is low in dense, igneous rocks and high in porous, sedimentary rocks; it depends on the char- acter of the intergranular bonds, from the weak bonds of a poor cement to the strong ionic bonds of silica tetrahedra. OPTICAL PROPERTIES Color The colors and patterns of monument, facade, and other building stones are important for artistic reasons. However, aside from changes that indicate the extent of weathering, color is not important in stone deterioration. Transmittance and Reflectance Some stones like marble, travertine, and chalcedony are selected as facade stones for their transmission and reflection of light, because of their interesting layered patterns and colors, through thin slabs and from polished surfaces. Light reflected from mineral cleavage, twin- ning, or grain surfaces, as in calcite, labradorite, and mica, may indicate

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78 CONSERVATION OF HISTORIC STONE BUILDINGS locations of cracks, which would enhance permeability and lead to decay. ELECTRICAL PROPERTIES The resistivity and dielectric strength of stones are not affected directly by decay, but both properties are influenced strongly by the pemlea- bility and saline-water content of the pores in the stones. Measure- ments of electncal properties could be used to estimate permeability and approximate water content. Brace found that the resistivity of a wide range of crystalline igneous and metamorphic rocks decreased as 107 1o6 105 04 103 en In LL cr 1o2 10 1 ~ , . ~S ^~ ': _ 4~ \ ~~ - Tap Water 50Q-m \ Salt Solution 0.3Q-m 0.1 1 10 100 POROSITY (percent) FIGURE 7 Decrease in resistivity of many crystalline igneous and metamorphic rocks with increase in porosity to about 5 percent, for saltwater and tap water saturating the samples under 4 k bar confining pressure {Bracel.2i

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Physical Properties of Building Stone 79 water content increased (see Figure 71.2i Resistivities decreased from 106 ohm-m at ~ = 0.1 percent to 102 ohm-m at ~ = 5 percent in samples saturated with saline water; resistivities were one-tenth as high with tap water. If Brace's results were extrapolated to the\higher porosities (and corresponding water content) of sandstones, the resis- tivity would be about 1 ohm-m at ~ = 40 percent. Thus, resistance decreases rapidly as porosity and water content increase. Good resis- tivity measurements are easy to make, using four-probe geophysical techniques, and con detect small changes in ~ and water salinity and saturation. MAGNETIC PROPERTIES The magnetic-susceptibility and remanant magnetism of rocks are closely tied to the magnetite content. However, magnetite is a minor constituent of rocks and without importance to decay processes in stones. BUILDING AND MONUMENT STONES WinMer reviewed the specifications for stones of the American Society for Testing and Materials.1 The ASTM tests are guidelines to proper selection of stone for specific uses, although they need to be brought up to date. In addition, the comprehensive works of Bowles2 and Barton7 provide very useful descriptions of the properties of building and mon- ument stones and of criteria for selection. The tests and descriptions of criteria for selection inherently provide information on susceptibil- ity to deterioration, but the physical and chemical mechanisms of deterioration need thorough study. Where structures and monuments have already deteriorated, the physical properties of their stones will need study in properly diagnosing and solving problems. Physical properties are measured on small specimens of stones, but minor and subtle features of rock in place con be of overriding im- portance. Merrill reported on a new firm that started up an abandoned but formerly successful quarry and lost nearly $1 million because the new operators failed to observe imperceptible defects in the rock in the new quarrying zone.3 He said that, as a consultant, if he were restricted to either field examinations or laboratory tests, he unhesi- tatingly declares that, with good natural outcrops or quarry openings of Tong standing, he would choose the field examination, no matter how elaborate the other tests might be. At the time of writing, he was probably correct, but today, presumably, careful sampling and com- plete testing of physical properties can detect small but critical differ-

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80 CONSERVATION OF HISTORIC STONE BUILDINGS ences in the characteristics of building stones and thus reinforce the field examinations. Igneous Rocks Granite In geologic usage the name "granite" refers to rocks of various origins, a range including felsic igneous and metamorphic rocks that vary con- siderably in mineral composition. These rocks usually are dense and range in grain size from fine and equigranular, through medium and equigranular or porphyritic, to coarsely granular. Porosity and perme- ability are usually low, and the granite has high resistance to weath- ering and corrosion unless it is highly jointed, microfractured, or fol- iated. Granites range in jointing from those having no definite rift, like the granites at Charlotte, North Carolina, and VinaThaven, Maine, which can occur in blocks 90 m by 6 m by 3 m (300 ft by 20 ft by 10 ft), to a Wisconsin granite having joints at 20 cm t8 in.) spacing, too close for a building stone. Gneissic granite is strong perpendicular to its foliation but can be split into stabs for curbing an paving stones. Incipient joints, which are actually planes of microfracturing, occur in the granite. of Essex County, New York; although the granite is ac- ceptable for buildings, the incipient joints would open up on prolonged exposure and deface a monument. Calcareous layers in mica schists continue into contiguous massive granite gneisses in Vermont and Maryland and in time would be sources of deterioration. Gabbro bodies are seldom quarried in the United States because the rock is hard and difficult to work. Rhyolite and Andesite Rhyolite and andesite are volcanic rocks that occur as massive rock and as porous or welded tuff. The porous tuff is usually poorly con- solidated, has bedding partings in places, and has a density less than 2 g/cm3. It is subject to permeation by rainwater but drains well and. has the virtues of easy workability and good standing strength; the effects of frost can be severe. The welded tuff is very hard and difficult to work. Indurated rhyolite and andesite do not take a polish and are seldom used for buildings in the United States. They have been used in the past in Europe, however, because of their easy workability.

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Physical Properties of Building Stone Basalt 81 The dense varieties of basalt are relatively impervious to water, but they are hard and lack a rift. Columnar jointing is found in certain basalt flows, and the columns have been used in a few buildings. Basalt in massive flows is dark and does not take a polish, so it has little aesthetic appeal. Jointed basalt will have crack permeability, but ves- icular basalt may not be permeable owing to isolation of vesicles. Metamorphic Rocks Quartzite The strong, dense quartzites are usually cemented by silica, are fine "rained, and are almost impervious to moisture. The Dakota quartzite is a good example. It takes a fine polish, although only after consid- erable grinding; it is unique in that it has almost perfect rift and grain cleavages. These properties make the stone desirable for ornamental as well as building uses. Silica-cemented, fine-grained quartzites do not deteriorate, but if shaTey layers or close jointing occur, they will constitute planes of weakness and high permeability. Marble Both calcitic and dolomitic marble are massive rocks but commonly have moderate intrinsic permeability. ~ fact, moderate friability some- omes develops after only a few years of weathering, especially in coarse- grained marble. Tremolite laths, which are ubiquitous in marble, weather out once leave pocks. Marble occurs in a variety of colors and polishes well; however, it is usually jointed and fairly pe~eable and therefore can be subject to rapid chemical decay. Blasting and rough mechanical Treatment create microcracks easily in marble and are avoided in good quarrying practice. State The obvious cleavage of fissile state provides permeability for water penetration. Good roofing state is fine "rained, smooth, and tough. The quarried slabs have good bonding and Tow permeability across the unbroken cleavage and are quite resistant to deterioration.

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82 Sedimentary Rocks Sandstone CONSERVATION OF HISTORIC STONE BUILDINGS The type of intergranular cement determines the physical character- istics-of sandstones. There are four common cementssilica, limonite, calcite, and clay minerals. Silica-cemented sandstone, even though porous, may resemble a quartzite in its high hardness, strength, and resistance to decay. Sandstone cemented by limonite is so* to work; in a fairly dry climate it will season to a harder, stronger rock, resistant to weathering and chemical disintegration. CaTcitic cement is suscep- tible to the same kinds of chemical decay that affect limestone, and sandstone containing it may be greatly weakened. Clayey cement ab- sorbs water, and sandstone containing it is easily broken, either by freezing or because clay minerals form poor intergranular bonds. The reddish-brown, porous sandstone from Seneca, Maryland, used in the Smithsonian building in Washington, D.C., is limonite-ce- mented and has stood up reasonably well. The sandstone in Potsdam, New York, has both limonite and silica cement, and so it is soft to work and also holds up against deterioration. The Berea, Ohio, grit is fairly porous and easily worked; it has very little cement, probably silica. The material is cohesive, but slightly friable, and is used for grindstones because the grit contains none of the other cements that would glaze the surface and stop the cutting action. Inhomogeneities resulting from interbedding are important to ob- serve in checking sandstone formations for use as building stone. Ad- jacent layers may differ considerably in type of cement or plagioclase and mica content, or shale beds may be intercalated; if so, the sand- stone, whether soft or hard, may be unusable for buildings. Also, the bedding may be too thin for usable blocks. Such variation can be very subtle, and close observation is needed. Because the porosity of com- mercial sandstone for building use ranges from 2 to 15 percept, 7 the permeability will be fairly high and the stone will stand up to decay only if interbedded layers are not permeable. Quarrying of sandstone is often stopped in winter because water deep inside fresh blocks would freeze and split them, and freezing of near-surface water could cause spelling.

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Physical Properties of Building Stone Limestone 83 The relatively easy workability of limestone makes it a favored stone for construction and monuments. However, almost all limestones, even those of low porosity, have relatively high intrinsic and micro- crack and bedding permeability. Thus they are susceptible to weath- ering by permeating water and gases and especially by the well-known process involving conversion of sulfur dioxide to gypsum. Bedford, Indiana, limestone is soft but moderately strong; it has no rift and can be worked in any direction, so it is a much-used building stone. Dense colitic limestones are commonly varicolored, compact, and easily po- lished, so they are used as veneer or ornamental stone. Normal lime- stone is usually impure, containing quartz, mica, clay, other silicate minerals, and carbon; shaTey layers along the bedding can form partings as a result of weathering. In fossiliferous limestone in Kansas, the space around the fossils is not filled, and cellular breakage occurs. The soft limestone at Caen, France, is easily carved and, being moderately strong, is widely used for buildings in Europe. However, it deteriorates rapidly in the more severe U.S. climate because of its relatively high pe~n~e- ability. Shale Shale is inherently friable in that it is not lithified well enough to resist abrasion. Shale has very low intrinsic permeability because of its clay mineral content, but some shales have pronounced bedding planes and jointing, which provide permeable channels if the shale is under very low confining pressure. Invasion of shale by water often results in almost complete disintegration; adobe and mud for walls, which react to water by disintegration, are essentially shalelike in mineral composition. Soapstone, Travertine, and Serpent~te Soapstone, travertine, and serpentinite are relatively soft and take a good polish; they are used for omaments, statuary, or facades. They rarely occur in large blocks. Travertine usually is soft just after quar- rying and becomes hard on standing; of course, it is porous and subject to the deterioration characteristic of such calcitic stones. Verde antique is ornamental serpentinite and usually consists of white calcite veins running through the variegated green serpentine. It is used as a veneer stone. The calcite, of course, can be corroded by atmospheric moisture and gases.

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84 CONCLUDING REMARKS CONSERVATION OF HISTORIC STONE BUILDINGS Knowledge of physical properties can be useful both for initial selection and for diagnosis of the deterioration processes of building stones. We need not only laboratory measurements of physical properties, but also observations on the rock in place. For example, laboratory measure- ment of the strength of rock is not an adequate index of the stability of stone under weathering or other decay processes. Knowledge of lack of homogeneity in macroscopic to microscopic features needs to be obtained by quarry-site inspection and by microscopic observations of petrographic and textural discontinuities. Once identified, inhomo- geneities can be tested in the laboratory on carefully selected samples; there are many examples of such testing.) 5 7 ~ 9 i0 ii In stone to be used for buildings or monuments, such physical prop- erties as rift (cleavage parallel to foliation or bedding) and grain (cleav- age perpendicular to rift) should be identified. As has been repeatedly mentioned above, veins, layers, or cements of calcite, clay, talc, mica, or shale, whether thick or thin, can be expected to weather out or be corroded and, together with bedding and cross joints, can increase permeability. Pore size and shape in stone can vary from one part of a quarry to another, even within short distances. Such variation can influence the effective porosity or pem~eabflity, and the pem~eabflity may vary along and across bedding or foliation in a single quarry. Thus, induration, foliation, microfracturing, variation in mineral composi- tion among or within layers, and jointing at small to large intervals in rock in place can be more significant than laboratory tests in de- termining the susceptibility of building and monument stones to de- terioration. These characteristics also are important in diagnosing de- terioration processes affecting stones in use. Quarrying methods can affect the durability of stones. Blasting can create cracks that become permeable channels for water. The same effect can be produced by imperfect splitting along rift and grain. It can also result from zones of small-scare microfracturing, which can form when existing tectonic stresses are concentrated by the quarrying operation until they exceed the strength of the rock and it fails. For those analyzing deterioration processes in particular stones, geo- physical techniques could provide useful measurements of the physical condition to supplement laboratory tests of physical properties. Geo- physical exploration techniques based on electrical resistivity and acoustic velocity can probably be helpful in diagnosing stones undergo- ing corrosion or decay. Resistivity varies with water content and sa- linity and so would be sensitive to increases in permeability and po-

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Physical Properties of Building Stone 85 rosity resulting from dissolution or microfracturing. Ultrasonic-wave- velocity methods can be used on a small scale (to 10 cm) to detect spelling or microfracturing from thermal or other stresses in the stones, but acoustic-wave-velocity measurements could be used for deeper penetration To 100 m). Hudec found that water saturation decreased velocity in "sound" rocks and increased velocity in "unsound" rocks (which are more susceptible to weathering),S so saturation must be accounted for in interpreting velocity studies. In general, to obtain a fuller explanation of each deterioration process in building stone, collaboration will be needed among the following people: the preservationist, who knows where these processes take place, what stones to study because of their architectural and historic significance, and what remedies have been tried; the geologist, who knows the origin and mineral content of the stone and its probable geologic inhomogeneities; the specialist in rock mechanics, who knows the measurement and the significance of physical properties; the geo- physicist, who knows exploration techniques and their application to characterizing the extent of stone decay; and the geochemist, who understands the chemistry of weathering and knows what analytical techniques can be used to explain the detenoration process. With fuller explanation will come knowledge of what physical and chemical prop- erties to measure and how to measure them, leading to more satisfac- tory decisions on remedial measures. REFERENCES 1. Winkler, E.M., 1973, Stone: Properties, Durability ill Man's Environment, Sprin- ger-Verlag, New York. 2. Bowles, O., 1934, The Stone Industries, McGraw-Hill, New York. 3. Memll, G.P., 1903, Stones for Building and Decoration, John Wiley, New York. 4. Schaffer, R.J., 1932, The Weathering of Natural Building Stones, Harrison and Sons, London. 5. Winkler, E.M., ea., 1978, Decay and preservation of stone, Geol. Soc. Amer., Eng. Geol. Case Histories No. 11, 104. 6. Moen, W.S., 1967, Building stone of Washington, Washington Div. Mines & Geol. Bull. 55. 7. Barton, W.R., 1968, Dimension Stone, U.S. Burl Mines Infor. Circ. 8391. 8. Blair, B.E., 1955, 1956, Physical Properties of Mine Rock, Parts III, IV, U.S. Burl Mines Rep. Inv. 5130 and 5244. 9. Blair, B.E., 1956, Physical Properties of Mine Rock, Part IV, U.S. Burl Mines Rep. Inv. 5244. 10. Clark, S.P., Jr., 1966, Handbook of Physical Constants, Geol. Soc. Am. Mem. 97. 11. Windes, S.L., 1950, Physical Properties of Mine Rock, Part II, U.S. Burl Mines Rep. Inv. 4727.

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86 CONSERVATION OF HISTORIC STONE BUILDLINGS 12. Brace, W.F., 1980, Permeability of crystalline and argillaceous rocks, Islet. I. Rock Mech. Min. Sci. v. 17, no. 5, p. 241-252. 13. Morrow, C., Lochner, D., Moore, D., and Byerlee, J.D., 1981, Permeability of granite in a temperature gradient {abstract), EOS, v. 61, no. 52, p. 1238. 14. Blatt, H., Middleton, G., and Murray, D., 1980, Origin of Sedimentary Rocks, Prentice-Hall, Englewood Cliffs, N.J. 15. Torraca, G., 1976, Brick, adobe, stone, end architecturalceramics: Deterioration processes and conservation practices, in Proc. North Amer. Int. Reg. Conference, 1972, Preservation and Conservation: Principles and Practices, S. Timmons, ea., Smithsonian Inst. Press, Washington., D.C., pp. 143-165. 16. Ide, J.M., 1937, The velocity of sound in rocks and glasses as a function of tem- perature, I. Geol., v. 45, no. 7, pp. 689-716. 17. Griggs, D.T., 1936, The factor of fatigue in rock exfoliation, four. Geol., v. 44, pp. 78~796. 18. Hooker, V.E., and Duvall, W.I., 1971, In Situ Rock Temperature: Stress Investi- gatior~s in Rock Quarries, U.S. Burl Mines Rep. Inv. 7589. 19. Robertson, E.C., 1979, Thermal Conductivities of Rocks, U.S. Geological Survey Open-File Report 79-356. 20. Robertson, E.C, 1972, Strength of metamorphosed graywacke and other rocks, in The Nature of the Solid Earth, E.C. Robertson, ea., McGraw-Hill, New York, p. 631- 659. 21. Brace, W.F., 1971, Resistivity of saturated crustal rocks to 40 km based on lab- oratory measurements, in The Structure arid Physical Properties of the Earth's Crust, J.G. Heacock, ea., American Geophysical Union Monograph 14, p. 24~255.