Chemistry Reference
In-Depth Information
slowly disfigures and eventually destroys ancient structures and statues
made from any of these types of stones (Bubernick and Record 1984).
17.4.
INTERACTION OF MATERIALS WITH THE ENVIRONMENT
Most materials are affected by physical, chemical, and/or biological factors
in their environment that may alter their composition and cause either decay
or conversion to other materials. It is not always possible, however, to make
a clear-cut separation between the different forms of interaction of materials
with the environment; some chemical agents may affect materials physically,
physical agents may cause their chemical alteration, and, under certain cir-
cumstances, the same agent may induce physical or chemical changes and
even both simultaneously. Water, for example, may erode and wash away
particles from solid matter, thus affecting it physically without chemically
altering its composition; but it may also dissolve the washed-away particles
and later even cause their chemical alteration. Two or more physical and/or
chemical agents may act simultaneously, often in ways that are interrelated,
and the effect of one may even exacerbate that of the other, thus causing an
overall intensification of the decay process. Thus it is customary nowadays
not to classify the agents of decay into separate (physical or chemical)
categories, but to consider an all-embracing category of
physicochemical agents
of decay
. Some naturally occurring substances such as ozone and water
vapor, in addition to human-generated atmospheric and water pollutants,
are among the most active physicochemical agents affecting antiquities.
Temperature Effects
Changes in temperature alter most of the physical properties of materials
and affect the rates of chemical reactions. Thus heat and cold may advance
or inhibit the deterioration of antiquities (see Textbox 75).
TEXTBOX
75
THE THERMAL PROPERTIES OF SOLIDS
When a solid is heated, the heat is transferred throughout its mass by
con-
duction
: the heat energy passes from one part of the solid to another
without the solid undergoing any mass changes.
Heat conduction
is the
flow of energy from a spot of higher temperature to one of lower
