Natural Science in Archaeology

276 11 Building, Monumental, and Statuary Materials
of the megascopic calcite crystals oriented in multiple directions in the marble cause
microcracks along the edges of the crystals allowing penetration by pollutants.
To understand the archaeomineralogy of stone structures, it is necessary to comprehend
the complete history of the rocks even before they were removed by quarrying.
Unloading of confining pressure as a rock formation migrates toward the
earth’s surface due to erosion, along with a secondary unloading during quarrying,
lead to jointing and microfracturing of the rock. These phenomena affect the
later durability of the rock. After shaping and construction, the rock undergoes the
effects of natural weathering as well as various treatments and mistreatments by
human agents. There are many ways that earth scientists examine structural stone
including visual inspection, microscopy, x-ray diffraction, chemical analysis, and
the determination of physical and mechanical properties.
All materials expand and contract with temperature changes. Simple expansion
and contraction is not harmful. Mechanical disintegration is caused by dissimilar
expansions or contractions where two different materials are combined. For example,
lime mortar has a coefficient of linear thermal expansion that is approximately
50% greater than that for bricks. In making ceramics, it is necessary that any additives,
such as temper, have a coefficient of expansion similar to that of the clay
matrix. Many sedimentary rocks used in buildings and monuments contain clays
that cause differential swelling when wetted.
Fire has damaged a large percentage of ancient buildings. Sippel et al. (2007)
tested limestones, sandstones, granites, tuffs, gneisses, marbles, and gypsum to
determine the effects of fire. Every rock has its own thermal expansion and thermal
shock characteristics. Some rocks incur damage from phase changes in constituent
minerals. Others suffer dehydroxylation reactions, and some are subject to cracking
(e.g., feldspars in granites). The response of building stones to fire is controlled both
by mineral composition and by fabric.
The amount of water vapor that is absorbed by a rock depends on both the relative
humidity of the air and the porosity of the rock. Frost damage in building stone
is common where temperature variations around the freezing point cause cycles of
freezing and thawing. The influence of frost action on dry stone is limited, but it
can be substantial on wet stone, because, when water freezes, it expands by about
10%. When freezing occurs in a confined or limited space, the resulting pressures
are enormous. Therefore, building stone with high porosity is more vulnerable than
compact stone.
Most damage occurs during the process of drying out, not during absorption, as
mineral salts recrystallize and cause structural damage and discoloration. A related
process that occurs during drying is the crystallization of soluble salts. As salts crystallize,
they can cause cracking similar to frost action. Salts also form crusts on the
exterior of the stone. The most common salts occurring on walls are CaSO 4 ·2H 2 O
(gypsum) and Na 2 SO 4 (sodium sulfate) in various hydration states. Calcium sulfate
(CaSO 4 ) dry deposits are difficult to remove. The damage caused by sulfates is the
result of different hydration states. As moisture conditions change, one hydration
state converts to another. Transformation to more hydrated states leads to increased
pressure on the walls of the pores in the stone.