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Problems in the Detenoration of Stone ERHARD M. WINKLER Stone decay is determined by the type of stone and by the amount and source of moisture. The carbonate rock~limestones, dolomites, and marbles are attacked by moisture from the surface downward; limestones tend to form a relief between dense fossil shells and a less dense matrix, with a maximum surface reduction of 0.2 mm in 10 years of exposure to 40 in. (100 cm, of precipitation annually. Crystalline marble dissolves around the grains, result- ~ng in sanding and a rough surface relief. Secondary layers and crusts of gypsum may form by dissolution and redeposition in the presence of sulfate, a process often aided by bacterial action. The decay of silicate minerals and rocks is very slow, except for tremolite In some dolomite marbles and black mica in granites and some marbles. Black mica may form brown blotches around mica flakes, whereas tremolite decays to soft talc leaving craterlike holes in marble. Granitic rocks tend to separate into thin, even sheets parallel to the surface near ground level: Ground moisture combined with the action of salts and relief of stress from the weight of the building fowl this common spell, while the mineral components themselves remain unweathered. The weathering and weathering rates of stone depend on the routes of travel and the amount of moisture, as follows: corrosive rain and drizzle on the stone's surface with a pH range of 3 to 5; rising ground moisture of variable corrosiveness, a vehicle for salt transport leading to efflorescence, subfloresc- ence, and honeycombs; leaking indoor plumbing and gutters leading to uneven cleaning of the stone's surface and secondary deposits of calcite or gypsum, or both; and outward seepage of condensation water, leading to flaking, surface hardening, and honeycombs. Preventing the access of moisture is the most natural but most difficult way to preserve stone. Erhard M. Winkler is Professor of Geology, Department of Earth Sciences, University of Notre Dame. This study of stone weathering was made possible by grants from the National Bureau of Standards and the National Science Foundation. 108
Problems in the Deterioration of Stone 109 The rapid decay of stone buildings and monuments is well reflected in a pair of photos (Figure 1) of a sculpture in West Germany, taken at an interval of 60 years; the near exponential increase of the weath- ering rate since the beginning of industrialization is a stern warning to ah- of us, especially to those involved in the preservation of mon- uments. Test walls or similar means of monitoring susceptibility to such decay are clearly needed. The National Bureau of Standards (Nss) was aware of the need for a stone test wall when it occupied its previous campus only a few miles from downtown Washington, D.C., ~ 1948. The wall (Figure- 2) was moved from there in one piece 37 ft. 9 in. (11.5 m) long and 12 ft. 10 in. (4.4 m) highto the present, more rural campus of the Nag in Gaithersburg, Maryland, in 1978. Figure 3 shows details of weathering of 4 in. (10 cm) square blocks on a section of the front (south) face. Many sandstones were most vulnerable to weathering, as shown by crumbling and scaling, whereas many limestones have developed a surface relief of about 3/4 mrn between densely crystalline fossil-shell fragments and a much softer, fine-grained matrix of the same calcitic material. Most of the coping stones, the cover stones of the wall, are FIGURE 1 A sculpture at Herten Castle near Recklinghausen, Westphalia, West Ger- many, carved of Baumberg sandstone in 1702. The photograph on the left was taken in 1908; the one on the right in 1969.
110 CONSERVATION OF HISTORIC STONE BUILDINGS FIGURE 2 View of Nss stone test wall, south face. The wall is 12 ft. 10 in. high and 37 ft. 9 in. long. Indiana limestone, which is composed primarily of fossil fragments and oolites with a calcitic bonding cement. The exposure of the north, top, and south surfaces permits the development of surface relief to be monitored with depth micrometers; these data have been correlated with wind any rain data from the nearest airport, first from Washington National Airport and now from Dulles International Airport. The 2,400 stone samples built carefully into the Nss test wall, many in duplicate, are well protected against rising ground moisture and interior condensation. This is in contrast with stone in buildings and monuments. The origin of the moisture and its travel routes determine. its effectiveness in the decay of stone, as follows: 1. Rain and drizzle, often driven against a wall by wind, are generally corrosive and acidic. The attack is primarily superficial and the pH is between 3.0 and 5.8 (rainwaters are usually charged with carbon diox- ide and sulfate in urban and industrial atmospheres). The waters move in and out of the stone, dissolving ingredients widlin the stone and transporting them to the surface, where the waters are neutralized and redeposit the dissolved material as hard, secondary crusts. Carbonate rocks are readily attacked, with formation of a distinct surface relief.
Problems in the Deterioration of Stone 111 [IGURE 3 Nss stone test wall {see Figure 2), detail section. Some sandstones show weathering, discoloration, and salt efflorescence. Also readily attacked are porous sandstones with a calcareous grain cement; the dissolution of the grain cement may cause loss of coher- ence of the grain bond, while the cement itself moves outward, de- veloping a case-hardened surface that readily scales or develops hon- eycombs. 2. Ground moisture travels from the ground upward by capillary action, often climbing as high as 10 meters or so. Groundwater is potentially rich in ingredients from several sources: leaching from the soil, rain Inning down the building into the ground, or salts used to deice streets and sidewalks. The composition of the groundwater is thus variable. The salts are carried upward to the capillary fringe, where the moisture tends to evaporate, leaving the salts behind. A "wetline" can develop, often associated with a rim of efflorescence and invisible subflorescence beneath the stone's surface. Concentration of hygro- scopic salts around the wetline can lead to further attraction of mois- ture, especially at high relative humidities. At 90 percent relative hu- midity, masonry that contains 4 percent salt can retain 2i percent water. 3. Leaking plumbing, both outside and inside a building, and leaking roofs and gutters may concentrate water between ornaments and along
2 CONSERVATION OF HISTORIC STONE BUILDINGS joints. Continuous washing, or complete prevention of it, can clean and corrode portions of the wall while other parts remain covered with soot and grime. Crusts of calcite or gypsum or both, open form beneath the washed zone. Waters of this kind vary in composition, but tend to become neutralized in contact with mortar, dust, or soluble stone. 4. Indoors, moisture from saturated air condenses on cool walls. From there the water moves toward the warm outside surface, in the process dissolving ingredients from both stone and mortar. Figure 4 shows the resulting redeposition of lime as a crust, from the mortar joint downward; the sandstone is entirely free of calcite. Notwithst~n ding the foregoing description, it is difficult in most FIGURE 4 Freiburg Cathedral, West Germany, east wall, with lime crust {calcite) from joint near win- dow covering honeycombs in sand- stone.
Problems in the Deterioration of Stone 113 cases to identify the sources of moisture associated with decaying stone. PROBLEMS OF DISSOLUTION Carbonate rocks such as limestones, limestone marbles, dolomites, and crystalline marblesare readily attacked by rainwaters, especially waters charged with excessive carbon dioxide and sulfate. Limestone quarries often show channels and rills inflicted by dissolution. Ex- amples are the Indiana limestone quarries and the Tennessee Holston limestone-marble quarries. Dissolution is also apparent on the Georgia marble on the exterior of Chicago's Field Museum of Natural History (Figure 51. The large vertical columns framing the north and south entrances show pro- gressive dissolution of the coarse calcite grains along the grain bound- aries and along cleavage and twinning planes wherever they are exposed to the rain. Deep cracks abound along the ribs, though the foliation of the marble runs almost perpendicular to the vertical axes of the columns. No cracks can be observed on the stone that has been pro- tected from rain. Similar cracks may be observed in columns of coarse- FIGURE 5 Field Museum of Natural History, Chicago, south entrance. Photo shows deep weathering and vertical cracks that have developed along ribs on columns.
4 CONSERVATION OF HISTORIC STONE BUILDINGS "rained marble at other places with homier climates, such as New York City. The vertical cracks appear to be due to a combination of causes: relief of the stress of the overburden of the him columns and heavy roof; residual stress locked into the marble as a prestressed geological body (like a sleeping bag that expands when its cover is removed); and moisture-heat expansion and contraction, often combined with the action of frost. The disruptive factors are triggered by the dissolving action of acid rains; in turn, the disruption of the stone opens new channels for rain to enter and dissolve the stone. The surface reduction of the marble against unweathered hornblende shows well, although it was measured between the protecting ribs (Figure 61. The measured surface relief correlates well with the wind-rain rose. Dissolution of soluble minerals or mineral grain cement in a porous stone is followed by the transport of the dissolved ingredients to the surface. There the solvent evaporates, leaving a thin skin or crust of calcite or silica and also soluble salts like chlorides and sulfates of sodium, calcium, and magnesium. The loss of supporting grain cement beneath the hardened surface skin causes scaling. The process is con- FIGURE 6 Detail of weathered protruding rib showing dislodged calcite grains and vertical cracks at the Field Museum of Natural His- tory, Chicago. Scale in millimeters.
Problems in the Deterioration of Stone 115 sinuous after a scale has fallen off, another develops underneath, and so forth. The action of salt behind the hardened surface can accelerate the process of scaling. A hardened surface skin may also function as a semipermeable membrane, making way for a true osmotic pressure system. The solubility of calcite in water is well known to depend on the presence of carbon dioxide; in contrast, dissolution of silica depends on the temperature and degree of crystallinity. At 20° C crystalline quartz (silica) dissolves in pure water at only about 5 mg/l, and at 50° C about 15 my/; microcrystalline chalcedony is about twice as soluble as quartz, and amorphous silica fopal) has a solubility of about 100 mg/1 at 20° C and 120 mg/l at 50° C. It should thus not be surprising that silica dissolves readily on a stone surface saturated with capillary growing moisture in the hot desert sun or on a sun-drenched masonry wall. A headstone in the Masonic Cemetery, Fredericksburg, Virginia, shows intensive surface hardening toward the outer fringe on one side, but strong flaking with a present surface reduction of 25 mm in the center portion on the other {Figure 7~. The stone is soft Aquia Creek sandstone with only a little calcite in the grain cement; the rest is mostly silica. After about 200 years of exposure, the original tool mark- ings are still visible on the outside, while fresh stone is exposed near the center as a result of progressive scaling. In many sandstones, surface hardening may develop a honeycomb pattern in which differential hardening appears to follow a meniscus-like pattem, with crumbling occurring behind the crust and in the depressions where deepening is rapidly aided by the action of salt. Honeycombs are frequently observed on sandstones on buildings. Figure 4 shows honeycombs in a calcite- free red sandstone on the east face of the Freiburg Cathedral in West Germany. A crust of secondary calcite covers the honeycombs. The surface grain cement was introduced from the surface as calcite; it did not move to the surface to concentrate there. This case appears to be unique. WEATHERING OF SILICATE MINERALS Black mica, feldspars, and tremolite hornblende decay slowly, yet fast enough to enable the rate of decay to be recorded in a human gener- ation. Black mica tends to become rusty by the oxidation of iron, which also discolors the irnrnediate surrounding of the mineral flake. Feld- spars have a distinctly glassy Juster which gradually dulIs as they weather to clay. Tremolite hornblende, a calcium magnesium silicate, is a common constituent of some dolomite marbles; it hydrates readily
116 CONSERVATION OF HISTORIC STONE BUILDINGS FIGURE 7 Surface hardening on sandstone headstone at Masonic Cemetery, Fredericksburg, Va. Tool marking on up- per surface is hardened and preserved while the center is flaking. Six-in. ( 15-cm) scale at base. Masonic Cemetery, Fred- ericksburg, Va. to soft talc, leaving hoi-es of about the original size of the mineral grains. The white tremolite is difficult to locate in white marble when fresh. In contrast, a weathered dolomite-tremolite marble is peppered with small, craterlike holes; such damage can be seen well on the south wall of the U.S. Capitol in Washington, D.C. (Figure 81.
Problems in the Deterioration of Stone FIGURE 8 Pockmarked dolomite marble on the southwest corner of the U.S. Capitol Building, Washington, D.C. Tremo- lite weathered to talc is leaving holes and causing flaking of the granite beneath. SCALING OF GRANITES 117 Scaling is frequently observed on granites of medium or fine grain. Thin sheets separate readily from the stone block parallel with the outer surface, regardless of the mineral orientation of platy or prismatic components, such as mica or hornblende. The sheets are between 1 mm and 3 mm thick; their thickness is surprisingly even. Black micas and feldspars appear to be entirely fresh; they retain their original color and luster. These minerals are excellent visual indicators of the fresh- ness of granitic rock. Scaling of granite is generally found near street level. There is strong evidence that the scaling is physical in nature. The evidence suggests that the following variables are instrumental in the formation of scales in granites: · Expansion-contraction cycles of ground moisture entrapped in the pores of the granite.
118 CONSERVATION OF HISTORIC STONE BUILDINGS · The action of salts introduced with groundwater and from the street. Salts in masonry attract considerable additional moisture to the stone. Although crystallization of salts at the surface often disfigures stone with efflorescence, it may also roughen the surface, which is caned salt fretting. Subflorescence, the crystallization of salts beneath the surface, often leads to spelling. · Relief of stress from the load of the building (see columns in Figures 5 and 6~. · Relief of residual or dormant stresses. · Relief of stresses caused by machining and tooling the stone. Figure 9 shows spelling of granite at the base of the Tweed Court House in lower Manhattan, New York City. The scales are large and thin, the minerals fresh on the insides of the flakes. Irregular thin flakes can also be seen on the granite ledge at the base of the U.S. Capitol Building, Washington, D.C. (Figure 81. The diameter of the spelled area on the Martin Luther monument in Worms, West Ger- many, has increased from 25 cm to about 100 cm in only 28 years of exposure, as observed by this author. FIGURE 9 Strong flaking on granite at the base of Tweed Court House, lower Man- hattan, New York.
s ~ Me ^^~ ~ awe CONCLUSION 119 We decay of stone ~ ~ bag or moment is ~ extremely complex process OI combustion of processes art may Evolve severed ~ter- d~endent factory Even viable should be Ply understood before pIese~ation of ~ kind is attempted. We HISt add most inmost step should be to Mint the off of moisten ~ present ad in Gaels ~ou~out He mason. We pIes~adve should by c^Uy chosen' tag into account poach ~ cbe~c~ co~adbdRy with He parent stone ad its potential Mobility Odes the a~ process. BIBLIO CRAPHY Her, E.~., 1975, Same Lourdes, Du~~ty~ ~~? a, 2nd ed.> Sp^- ~ ~~ I. Hem E. 1977, He decay of Building stones: A literature review. havoc. ~s~- ~~- ~~ Bag-, 9{4), 5~1. Her, E.M, 1978, Stone preservation, Me each scientist's view. asset. ~~s~- - 1~2[ 11~12L e~ E.~., 1979, Role of sots in development of ~bc tow, Soup Aus~aba: A Discussion, ~~r ~ ~~ 87, 11~120 W~e~ E. 1980, Histodca1 implications ~ Me complexity of destructive salt ~eath- e~g Cleopat='s Needle, Nev Yolk. Music. ~s~b~ ~~ Blown, 12~2}, 9~102. W~e~ E.~., 1980, we National Bureau of Standards Stone Test WaU After 30 Yeas of E~osur~ Lesson ~ Stone Wea~e~g. ~~c~] fly C, ~~~ -~ ~ 12 {7t 551. Dew E.~., 1~ pressl, we eRect of residual stresses ~ stone. d~ E. (~ preset we Stone Exposure Test WaU Aver 30 Years of Exposure, National Bureau of Standards. Dew Em., we wea~e~g of Ceo=a maple, Chicago Field Museum of NatuIa Astor, m~schpt Ad posters ~ process.
