Viridian - manufacture recipes

/ vuh • rih • dee • uhn /
Viridian

1. Overview

2. History
of use

3. Making the
pigment

4. Technical
details

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.

Illustration of the process:



Making viridian in the laboratory:

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

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 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.

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.

The ground pigment:



Pile of ground Viridian: