Page 296 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

EARLY V E R D I G R I S R E C I P E S
Recipes from Pliny records several recipes for verdigris, showing that there
Pliny the Elder was probably a long tradition of making the pigment by the first
century, although some of the compounds were simply corro­
sion products of copper rather than specific recipes for the preparation of copper acetate. The
following are two of Pliny's recipes for making verdigris:

[I]t is scraped off the stone from which copper is smelted or by drilling holes in white
copper and hanging it up in casks of strong vinegar which is stopped with a lid; the verdi­
gris is of much better quality f the same process is performed with scales of copper. Some
i
people put the actual vessels, made of white copper, into vinegar in earthenware jars, and
nine days later scrape them. Others cover the vessels with grape-skins and scrape them after
the same interval, others sprinkle copper filings with vinegar and several times a day turn
them over with spattles until the copper is completely dissolved. Others prefer to grind cop­
per filings mixed with vinegar in copper mortars. But the quickest result is obtained by
adding to the vinegar shavings of coronet copper. 10

There is also another kind of verdigris called from the Greek worm-like verdigris, made by
grinding up in a mortar of true Cyprian copper with a pestle of the same metal equal weights
of alum and salt or soda with the very strongest white vinegar. This preparation is only
made on the very hottest days of the year, about the rising of the Dogstar. The mixture is
ground up until it becomes of a green colour and shrivels into what looks like a cluster of
small worms, whence its name. To remedy any that is blemished, the urine of a young boy
to twice the quantity of vinegar that was used is added to the mixture. 1 1

Attempts by researchers at the GCI and elsewhere to duplicate some of these recipes mostly
resulted in the production of neutral verdigris, or a mixture of neutral and basic forms. These
are essentially similar to the recipes discussed in the following paragraphs.
I REPLICATION EXPERIMENTS The first of Pliny's recipes
given here results in the manufacture of various copper acetates. It is not apparent what range
of compounds would result from the second recipe, however, which is a precis of an earlier
account taken from Dioscorides. To investigate this more closely, a replication experiment was
12
conducted at the GCI Museum Research Laboratory using a mortar and pestle made from pure
copper, which is what Pliny implies by specifying "cyprian copper" (copper from Cyprus,
known to be a supplier of relatively pure copper metal in Roman times). The alum used in the
replication experiment, which is given in APPENDIX B, RECIPE 12, was potassium aluminium
sulfate, and the salt was sodium chloride. The product formed using this mixture was the chlo­
ride atacamite; when soda was substituted for the sodium chloride, the principal product was
the carbonate chalconatronite. Although the formation of chalconatronite is slightly surprising,

T H E ORGANI C SALT S O F C O P P E R
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