Page 320 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

the light blue corrosion products mentioned earlier, was characterized as sodium acetate trihy-
drate, CH 3 COONa-3H 2 0. The source of the acetic acid was clearly from the display and stor­
age materials being used in the Burrell Collection. The source of the sodium is more prob­
lematic. Sodium acetate trihydrate crystals were previously observed on the interior surface of
wooden drawers covered with glass. The presence of sodium in conjunction with bronzes may
be associated with previous conservation treatments or with sodium-containing corrosion prod­
ucts, either of which is quite possible, especially with ancient Egyptian bronzes. A similar Egyp­
tian tripod vessel, conserved at the Institute of Archaeology, London, in 1982-83, was found
to have light blue corrosion patches that were identified as chalconatronite by X-ray diffrac­
tion. The data obtained were so detailed that more lines were determined than reported in the
ICDD file 22-1458. This tripod had been conserved before 1982 with the sodium sesquicarbonate
method. This appears to be another example of chalconatronite occurrence as a consequence of
conservation that was first mentioned by Horie and Vint in 1982.

C O N S E R V A T I O N T R E A T M E N T S

In 1948 a virulent outbreak of corrosion attacked bronzes from the Fitzwilliam Museum in
Cambridge, England. These bronzes had been in long-term storage, packed in (probably moist)
wood shavings, in caves in Wales during World War I. Evans (i95i) studied these bronzes
I
and was convinced that acetic acid released from the wood shavings had corroded them by a
mechanism analogous to basic lead carbonate formation on lead objects exposed to acetic acid;
this turned out to be an astute observation. A detailed account of the Evans study is given in
Dawson (i988).
Evans devised a localized electrochemical treatment procedure for the objects: an incon­
spicuous area of bare metal was exposed by scraping and was attached by a wire to the positive
terminal of a milliammeter. A zinc nib, cut from sheet, was attached to the negative terminal.
The nib was dipped in a dilute hydrochloric acid solution (1 HCl : 3 H 2 0 ) and applied to the cor­
rosion spot. This created a circuit, and the milliammeter registered a current rise as the corro­
sion was dissolved. When no further increase was recorded, excess acid was blotted away, and
the procedure was repeated with the nib dipped next in syrupy phosphoric acid and finally in a
20% (w/v) aqueous solution of sodium carbonate. No external source of electric current was
used for this treatment. The Evans technique forms a circuit drawing chloride ions away from
the object, and the chloride ions are dissolved by the hydrochloric acid. Next, the phosphoric
acid precipitates relatively insoluble copper phosphates, and the sodium carbonate neutralizes
any remaining acid on the object.
Although relatively successful, the Evans method was never used outside of Cambridge. It
was certainly preferable to total electrolytic reduction, which was still extensively in vogue in
Europe at the time. This treatment reduced many important bronzes to shapeless lumps of bare

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