Page 297 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

all the essential ingredients for its formation are found in the recipe: the copper ions from the
mortar and pestle, the sodium from the soda, and the carbonate from the soda.
Urine, which is called for in Pliny's recipe, is often mentioned in connection with several
pigment recipes and may play a complex role in the formation of a range of products. The prin­
cipal components of urine (in mg/ioo ml) are I820 mg of urea, H 2 NCONH 2 ; 53 mg of uric acid;
a nitrogen heterocyclic compound of empirical formula N4C5O3H4 with three carbonyl groups
and four nitrogen atoms linked to hydrogen atoms; 196 mg of creatinine, a single-ring nitrogen
heterocycle with empirical formula N 3 C 4 H 7 0 ; and small amounts of sodium, potassium, cal­
cium, magnesium, chloride, bicarbonate, sulfate, and phosphate ions (Shier, Butler, and Lewis
1996). This complex mixture is capable of dissolving verdigris and corroding copper to some
extent. It is much more active as an ingredient than water alone, which results in a very com­
plex mixture of organic and inorganic salts.
I TESTING FOR IMPURITIES Even in Roman times, verdi­
gris was not cheap to produce, and the pigment was sometimes adulterated with powdered
marble, pumice, gum, and even shoemaker's black. To test for powdered mineral additives,
Dioscorides recommended the following procedure based on knowledge of the relative solubil­
ity of verdigris:

[F ]or some do mix it with ye pumice stone, some with marble, but others with Calcanthum.
But we shall find out the Pumice stone, and ye marble, by wetting the thumb of ye left hand,
and by rubbing some part of ye rust with the other. For it happens that ye rust of it runs
abroad, but ye parts of ye Pumice stone and ye marble remain undissolved and at last it
grows white with ye long rubbing and with ye mixture of ye moisture. And not only so, but
by the crushing of ye teeth, for the unmixed falls down smooth and not rough. 13

The problem of detecting adulteration with shoemaker's black was nicely dispatched by
a test described by Pliny that was based on knowing that the Roman shoeblack contained iron
sulfates: "It is also detected by means of papyrus previously steeped in an infusion of plantgall,
as this when smeared with genuine verdigris at once turns black." This is perhaps the first
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recorded chemical spot-test, and it is quite accurate because the gallnut extract forms a strong
black colorant with iron salts; this mixture was long used as an iron gall ink.
Less accurate is the statement by Pliny that the presence of verdigris can also be detected
by heating the substance in question on a hot fire-shovel to see if the compound remains
unchanged. The idea that verdigris will remain unchanged also exists in an earlier text by
Dioscorides. Tests show, however, that after heating verdigris on an iron shovel, the copper salt
first turns red and possibly black under strong heat, first forming cuprite and gradually trans­
forming to tenorite. What might have been meant, and was perhaps misconstrued from some
lost original source, is that the presence of verdigris as a copper compound could be determined
by heating to see f the characteristic red of cuprite and the black of tenorite would form.
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