Verdigris

Synonyms:

Verdigris, Basic Copper Acetate, Vert-De-Gris, Verdet De Montpellier, Grünspan

This green copper pigment was called by the writers of the fourteenth century 'viride Græcum,' or, more simply, 'viride,' 'viride terrestre' being used for green-earth - that is, 'terre verte.' 'Vert-de-Grèce' - that is, verdigris - was used by the ancient Romans as a pigment, and has been detected in the wall-paintings of Pompeii. It occurs in early Italian tempera pictures; but it has frequently injured the gesso-ground on which it has been laid, forming calcium acetate with the calcium carbonate, and disintegrating the surface. The blackness of the shaded parts in many Venetian and Spanish pictures of the sixteenth and seventeenth centuries has been attributed to the changes which this pigment suffers in oil. The medieval writers on the practice of painting endeavoured to show how the peculiar liability of verdigris to change could be obviated by locking it up in some hydrofuge substance, such as a resin or balsam. But the problem actually possesses little practical interest to-day, though of real moment in the study of old pictures. In the modern palette the place of verdigris is taken by permanent greens derived from chromium and from cobalt: concerning the safety of these we need not be anxious.

The large proportion of resinous matter employed by early painters for the protection of verdigris from alteration, and the success of this precautionary measure, may be seen in the green drapery in several pictures by Van Eyck and Mabuse in the National Gallery and at Hampton Court. It should, however, be stated that the older processes for preparing verdigris often yielded a product much more alterable in the presence of damp than is the verdigris which for a century or more has been made at Montpellier.

Verdigris is commonly called in chemical language a basic acetate of copper. In fact, it is a mixture of three such acetates, its varying hues, ranging from green to greenish blue, being dependent upon the relative proportions of these acetates. The most blue basic acetate contains 1 molecule of copper acetate, and 1 of copper hydrate, with 5 molecules of water; the greenest has twice as much acetate. Average verdigris contains in 100 parts about 29 parts of anhydrous acetic acid, 43 of copper oxide, and 27 of water. It is nearly insoluble in cold water; but by continuous washing, or by continuous exposure to moist air, is ultimately decomposed.

The Montpellier process for making verdigris consists in exposing thin strips of metallic copper to the vapours arising from grape marc undergoing the acetic fermentation. The operation is conducted in a moist, warm atmosphere; finally, the whole substance of the metallic copper is transformed into verdigris.

An impure atmosphere containing sulphuretted hydrogen blackens verdigris; it is also affected by moisture and by carbonic acid. As a water-colour, it is quite inadmissible; in oil, it stands pretty well if 'locked up' in the way already described. But it acts energetically upon several important pigments, and is very poisonous.

For these reasons its employment in artistic painting ought to be abandoned.

Verdigris, if pure, dissolves perfectly in liquor ammoniæ, any gypsum or barytes present as diluents or adulterants remaining undissolved. If blue vitriol has been added to verdigris, it also will dissolve in the ammonia; but this falsification may be detected by acidifying the ammoniacal solution with hydrochloric acid, and then adding solution of barium chloride - a white precipitate of barium sulphate indicates the presence of copper sulphate.

There are many composite green pigments sold by artists' colourmen; none is of real value. Green lake, a mixture of quercitron lake and Prussian blue; Hooker's green - gamboge and Prussian blue; olive green - Indian yellow, umber and indigo; and olive lake, a mixture of quercitron lake, bone brown and ultramarine - all these belong to the same category. However, there is one mixed pigment, the so-called 'Cadmium-green' on which a favourable judgment may be passed: it consists of viridian and cadmium yellow.