Page 163 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

nitrogen oxides, whose effect may not become apparent for many years. Recent studies have
identified a large number of corrosion products on exposed statuary, and it is apparent that gen­
eralized statements about their formation may represent an oversimplification of a very com­
plex problem. For example, each exposed bronze may be influenced by a special microclimate
within the overall climatic parameters of the atmosphere. Consequently, additional studies are
needed to piece together all available information and produce a more comprehensive under­
standing of the deterioration of exposed bronzes.

H I S T O R I C A L R E F E R E N C E S TO C O P P E R S U L F A T E S

Sasyaka (blue vitriol) has the play of colour in the throat of the peacock. Mayaratuttham is
an emetic, an antidote to poisons and a destroyer of the whiteness of the skin.— VAGBHATA 2
Blue vitriol, or what the ninth-century Indian physician and writer Vagbhata calls "sasyaka"
(Ray 1956), is copper (II) sulfate pentahydrate. It was well known in antiquity as a mineral whose
uses may have included pigment and medicinal preparations, and would have been found in an
impure form as a secondary mineral from the leaching of various primary ore deposits. In rich
copper mines, particularly in Cyprus, the leaching of copper sulfide ore deposits resulted in
a variety of copper (II) sulfate minerals. Blue vitriol was called "chalcanthum" or "chalitis" by
other writers from ancient times. From these names come the modern one for this mineral:
chalcanthite, CuS0 4 -5H 2 0.
Dioscorides describes chalcanthum (chalcanthite) as "appearing as a concretion of liquids
that filters drop by drop through the roof of mines. It is for this reason called 'stalacton' by those
who work in the mines of Cyprus." This copper sulfate exudate is similar to a compound that
3
Pliny the Elder describes as occurring

in Spain in wells or pools, the water of which holds it in solution. This solution is
mixed with an equal volume of fresh water, concentrated by heat and poured into wooden
tanks. From beams fixed permanently over these tanks hang ropes kept taut by stones,
and on these the slime deposits in glassy berries, not unlike grapes. This material is taken
out and dried for thirty days. It is blue, with a very notable brilliance, and may be mis­
taken for glass. 4

This lustrous botryoidal appearance is an apt description for basic copper-sulfate salts that
could be either chalcanthite or partially iron-substituted copper sulfates such as pisanite, which
is essentially a copper-rich melanterite, (Fe,Cu)S0 4 -7H 2 0. Pliny also refers to copper sulfates
when he writes, "The Greeks by their name for shoemakers black have made out an affinity
between it and copper: they call it chalcanthon, 'flower of copper/" 5 The allusion to shoemak­
ers' black is not accidental, since using copper sulfate solutions to treat many organic materials,
such as felt or matting, will result in a black color.

C H A P T E R F I V E
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