Conservation of Historic Stone Buildings and Monuments (1982) / Chapter Skim
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Problems in the Deterioration of Stone
Problems in the Deterioration of Stone
Pages 108-119
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From page 108... ...
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, subflorescence, 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.
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From page 109... ...
Most of the coping stones, the cover stones of the wall, are FIGURE 1 A sculpture at Herten Castle near Recklinghausen, Westphalia, West Germany, carved of Baumberg sandstone in 1702. The photograph on the left was taken in 1908; the one on the right in 1969.
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From page 110... ...
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.
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From page 111... ...
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 coherence of the grain bond, while the cement itself moves outward, developing a case-hardened surface that readily scales or develops honeycombs.
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From page 112... ...
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 window covering honeycombs in sandstone.
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From page 113... ...
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 progressive dissolution of the coarse calcite grains along the grain boundaries and along cleavage and twinning planes wherever they are exposed to the rain.
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From page 114... ...
The loss of supporting grain cement beneath the hardened surface skin causes scaling. The process is conFIGURE 6 Detail of weathered protruding rib showing dislodged calcite grains and vertical cracks at the Field Museum of Natural History, Chicago.
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From page 115... ...
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.
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From page 116... ...
Masonic Cemetery, Fredericksburg, Va. to soft talc, leaving hoi-es of about the original size of the mineral grains.
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From page 117... ...
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.
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From page 118... ...
The diameter of the spelled area on the Martin Luther monument in Worms, West Germany, 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 Manhattan, New York.
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From page 119... ...
d~ E (~ preset we Stone Exposure Test WaU Aver 30 Years of Exposure, National Bureau of Standards.
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