Pigments through the Ages - Manufacture Recipes

How Viridian is made:

Artificial variety of pigment	Calcining a mixture of of boric acid and potassium bichromate and washing the material thus formed.

19th century recipe	Mix 3 parts of boracic acid and 1 part of bichromate of potassa, heat to about redness. Oxygen gas and water are given off. The resulting salt when thrown into water is decomposed. The precipitate is collected and washed. This is a remarkably fine color, solid and brilliant even by artificial light.

In the lab
Materials needed:	Potassium dichromate (K₂Cr₂O₇₎, boric acid (H₃BO₃₎
Safety (MSDSs):	boric acid, potassium dichromate (at Fisher Scientific)
Method:	5 g boric acid and 2,5 g potassium dichromate are mixed together and heated to 500°C for 6 hours. The cooled mixture is then pulverised, added to cold water and stirred until bright green colour is visible. The pigment is then filtered off and dried.

Making viridian in the laboratory:


Boric acid and potassium bichromate are being homogenized		The reaction mixture prior to heating


The mixture at the start of the heating stage		After 6 hours at 500°C


The cooled chromium(III) oxide prior to reaction with water. The color is dull green		The hydratation of chromium(III) oxide brings about the characteristic fiery green color of viridian.