Chemistry Reference
In-Depth Information
is of any interest, the layer should, whenever feasible, be removed, any traces
of rust eradicated, and the surface protected against further rusting. If the
rust is not removed and the iron surface is not protected, the layer eventu-
ally flakes away from the underlying metal and exposes fresh iron to more
water and oxygen; these, in turn, cause further rusting and after some time,
depending on the environmental conditions at the site, the entire mass of
iron is converted to rust.
Iron objects submerged under seawater, in which there is a free supply
of oxygen, are easily corroded: initially the surface and eventually the bulk
of such objects are fast transformed into rust. If and when submerged iron
objects are removed from the water and exposed to the atmosphere, the
rusting process is almost immediately accelerated; unless terminated by
appropriate conservation procedures, the rusting may continue until the
objects are entirely corroded and collapse. If submerged under water or in
the seabed under
anaerobic
conditions (where there is little or no free oxygen),
however, iron rusts slowly and may endure for long periods of time in a rel-
atively well-preserved condition.
Silver and Silver Alloys.
The surface layer of silver and its alloys is readily
tarnished by hydrogen sulfide, an atmospheric pollutant. On occasions the
tarnish may develop into a thick layer of black patina composed mainly of
silver sulfide. Silver retrieved from the ground is often encrusted with a gray
and stable layer, known as
horn silver
, composed of silver chloride formed
as a result of the interaction of the metal with chloride ions in the ground-
water. The formation of a thick horn silver layer surrounding a silver object,
or its penetration into the bulk of the metal, is occasionally accompanied by
strong forces generated by dimensional changes that can cause the defor-
mation of buried silver objects.
Lead.
Ancient lead objects excavated from the ground are often affected by
ions in the groundwater and become covered by a white layer made up of
a wide range of lead corrosion products (see Table 45). Sometimes, however,
the early formation of a corrosion layer of
cerussite
(composed of lead car-
bonate), provides the metal with a protective coating that prevents the
progress of the corrosion process.
5.10.
STUDIES ON ARCHAEOLOGICAL METALS AND ALLOYS
The scientific study of ancient metals and the remains of metallurgical activ-
ities is of obvious importance from the history of technology standpoint; the
conclusions and inferences drawn from such studies are often also helpful
