Viridian

Synonyms:

Viridian, Emerald Oxide Of Chromium, Vert Pannetier, Vert De Guignet, Vert Emeraude, Mittler's Green, Feuriges Chromoxyd

About the year 1838 Pannetier and Binet began to make a beautiful chromium green by means of a secret method. Many years afterwards M. Guignet discovered and patented a process by which this admirable pigment could be manufactured. About 8 parts of crystallized boracic acid and 3 parts of potassium bichromate are thoroughly mixed and calcined. The mass obtained is treated with cold water, washed by decantation, ground wet, washed with hot water, and carefully dried. The product is a hydrated chromium sesquioxide, in which a variable amount of the boracic constituent is often retained. Viridian is, however, essentially a hydrated ses-quioxide of chromium, having the formula Cr₂O₃2H₂O. In the preparation of this pigment it often happens that sufficient care is not taken to wash it perfectly free from all soluble salts. I have known the metal tubes in which this moist water-colour has been kept to be corroded by these salts and to fall to pieces in a few months. The presence of such saline matter is easily detected by mixing the moist or cake colour with water and noting if coagulation or curdling takes place.

It would be difficult to exaggerate the value of this addition to the artist's palette. The colour of viridian is a very deep bluish green of great purity and transparency. It furnishes, with aureolin on the one hand and with ultramarine on the other, an immense number of beautiful hues, adapted to represent the colours of vegetation and of water. It is quite unaffected by sunlight and sulphuretted hydrogen, and it has no action on other pigments. Moreover, it may be safely used with all the different painting media, and upon all kinds of painting-grounds.

Viridian is the name by which this pigment is perhaps now best known in England. It is unfortunate that it should be called 'Vert Émeraude' in France, since it has little in common with the poisonous emerald green of our colourmen. This pigment is sometimes adulterated with baryta yellow in order to modify its hue. This substance may be readily detected by moistening the pigment with dilute hydrochloric acid, when a yellow solution is obtained.

Arnaudon's chrome green is of a somewhat opaque green hue, rather like that of Schweinfurt; it is a phosphate of chromium. A similar product is Mathieu-Plessy's chrome green; but under the name of 'chrome green,'

'green cinnabar' and 'grüner Zinnober,' spurious pigments are constantly sold, the commonest of them consisting of mixtures of chrome yellow and Prussian blue. They are worthless in comparison with viridian.

A clear and bright green called 'vernalis' is one of the potters' pigments introduced to the notice of artists by Mr. Wm. Burton. It is formed at a very high temperature and is perfectly permanent. It contains lime but owes its colour to chromium.