Chemistry Reference
In-Depth Information
off by wood, a
moisture gradient
is established across the wood (since not all
the fibers of the wood swell or shrink at the same rate); the moisture gradi-
ent creates internal stresses, and these often result in warping of the wood.
Usually, the more moisture in the environment, the more serious its
detrimental effects on wooden objects, and the more rapidly they decay.
The strength and rigidity of wood is also greatly weakened by continuous
soaking in water, which results in the formation of very soft and flabby
waterlogged wood.
Large quantities of flowing water, in the form of rain, snow, hail, or river
or flood streams, exert strong erosive action on practically all materials.
Water also brings about the physical breakdown of materials through the
frost wedging
effect (see Fig. 45), which becomes active when water in the
internal voids and crevices of materials cools down; since below 4°C water
expands before freezing, the dimensional expansion of water during frost
wedging has a sort of localized explosive action that can reduce porous
stone, ceramics, and wood to dust (see Textbox 45).
Water can contribute to the decay and breakdown of some materials not
only by its presence but also by its absence; most organic materials require
some optimum moisture content to preserve their mechanical properties;
papyrus, skin, parchment, and leather that are too dry, for example, are
extremely stiff, brittle, and friable.
Air Pollutants
The atmosphere is a reservoir of aggressive air pollutants that may cause
serious deterioration to archaeological objects; some of the most active
pollutants are
sulfur dioxide
,
hydrogen sulfide
, and
ozone
.
Sulfur Dioxide.
Sulfur dioxide
is a dangerous air pollutant because of its
corrosive properties, which are causal factors in the decomposition of most
organic and some inorganic materials. Moreover, sulfuric acid, formed when
sulfur dioxide is oxidized and reacts with moisture, is one of the harshest
solvents of a wide range of materials: it dissolves for example all of the
metals known in antiquity, with the exception of gold, converting them into
metallic salts. Sulfuric acid also dissolves many common stones, such as
limestone and marble, which are widely used for building and statuary (see
Textbox 74). Sulfur dioxide can, however, also act as a reducing agent, and
as such, it causes textiles, particularly cotton, to be rapidly broken down,
and leather to change its texture, become brittle, and eventually break down
completely.
