Artificial Green Earth (Green Ochre)
A product sometimes put on the market as green earth or green ochre has nothing beyond its name in common with green earth properly so called, except a certain similarity in colour. This pigment is prepared by mixing yellow ochre to a thin pulp with water and adding about 2% (of the weight of ochre) of hydrochloric acid. After a few days, a solution of 2 parts of yellow prussiate of potash is added, and if the liquor still gives a precipitate when tested with ferrous sulphate, this last-named salt is added so long as such a precipitate continues to form.
The deposit is washed, and dried in the ordinary way. When the right proportions have been taken, a pigment is obtained that coincides fairly in point of tone with true Verona earth. It is, however, inferior in point of permanence, the Berlin blue present being somewhat unstable and decomposing very quickly when brought into contact with lime. The reaction taking place in the production of so-called “artificial Celadon green” is that the hydrochloric acid used dissolves ferric oxide from the ochre, the addition of the yellow prussiate of potash then forming a blue precipitate of Berlin (Paris) blue which, in conjunction with the yellow of the ochre, gives a green-coloured mixture.