Natural Science in Archaeology

274 11 Building, Monumental, and Statuary Materials
The deterioration of stone monuments is largely the result of: (1) chemical dissolution,
(2) mechanical disintegration caused by freezing of water in pores and
cracks, (3) abrasion due to wind-driven particles, (4) exfoliation due to rapid heating
and cooling, (5) disintegration resulting from the activities of organisms, (6)
the formation of crystals on the surface, and (7) damage resulting from ill-advised
efforts at conservation or restoration.
When buried, an artifact enters a new environment in which it must find a new
stable state. When excavated, the artifact is again exposed to a different set of physical,
chemical, and biological conditions. Rapid transformation to a new stable state
often destroys the artifact in the process. The extent of alteration caused by burial or
excavation depends on the structure and composition of the material and the severity
of the contrast between the old and new environments. The oxidizing potential,
acidity (pH), moisture, and soluble salt content of each environment all affect the
stability of the artifact.
All materials have a stable state for the environment in which they exist. A significant
change in this environment may result in a transformation to a new stable
state. Water, ice, and steam are each a stable state of H 2 O in different temperature
and pressure environments. Minerals formed at high temperatures under anhydrous
conditions weather rapidly under moist conditions at the earth’s surface.
Chemical breakdown of feldspars provides a good example of the weathering
process. Feldspars make up nearly 60% of the earth’s crust. As feldspars weather,
potassium, calcium, and sodium are lost, leaving newly formed clay minerals
behind. Because of its high solubility, most of the sodium finds its way into the
oceans. Most of the potassium remains in the soil. Some potassium becomes part
of new minerals such as illite, and some becomes part of growing plants. Calcium
has an intermediate fate, with some ending up in the oceans and some remaining in
groundwater systems where it is precipitated out as carbonate and sulfate minerals.
Calcium is the most common cation in fresh water, and the precipitation of calcium
carbonate crusts on sherds is common.
The presence of chlorides, sulfates, and nitrates of calcium, magnesium, and
sodium in masonry is deleterious because these salts, by repeated solution, crystallization,
and hydration, generate sufficient pressure to fragment and spall the
masonry. Although these salts are seen commonly as efflorescence on buildings
in all climatic conditions, they are far more deleterious in arid regions, where the
lack of precipitation results in accumulation of these salts. In wetter climates they
are washed away by rain. In arid regions, condensation on stone surfaces during
the night, followed by evaporation at sunrise, leaves behind a small but significant
amount of the harmful salts, which crystallize as the evaporation proceeds.
In damp conditions, underfired or low-fired earthenware will gradually rehydrate
to clay, which leads to crumbling. This is especially true where the fabric of
the earthenware is coarse and porous. Crumbling is exacerbated in acidic conditions
by loss of calcite or other carbonate components. High-fired ceramics are
reasonably stable in most burial conditions. Even well-fired ceramics may become
softened under alkaline conditions by dissolution of any vitreous phase. Deposits of
both soluble and insoluble salts readily form on buried ceramics. Porous pottery is