=Plessy’s Green= is essentially a chromium phosphate mixed with variable amounts of chromium oxide and calcium phosphate. It is obtained by boiling a solution of 1 part of potassium bichromate in 10 parts of water with 3 parts of a solution of acid calcium phosphate and 1 part of sugar, until the whole has become deep green. The chromic acid is reduced by the sugar, so that the precipitate contains chromium phosphate, chromium oxide and neutral calcium phosphate. These two chromium phosphate greens are very stable towards chemical reagents, and very durable towards atmospheric influences. =Schnitzer’s Green.=—Thirty-six parts of sodium phosphate are melted in its water of crystallisation, 15 parts of potassium bichromate and 14 parts of Rochelle salt are then added. A large dish must be used for the fusion, since the mass effervesces. The colour changes gradually from yellow to green. When it is pure green the heating is stopped, and, after cooling somewhat, as much hydrochloric acid is added as the mass will absorb; then, after standing some time, it is washed with cold and finally with boiling water. On account of their great durability, the chromium phosphate pigments are well adapted for paperhangings, calico printing and oil painting. =Chromaventurine= is a glass coloured green by chromium oxide. It is hardly used as a painters’ colour in the ordinary sense of the term; it is applied in porcelain painting as an under-glaze colour, and is largely used for colouring glass. Chromaventurine is most simply made by the process of Pelouze: 250 parts of quartz sand, 100 parts of soda, 50 parts of calcium carbonate and 40 parts of potassium bichromate are fused together. The presence of iron would have a very harmful effect on the shade of the product. Quartz sand generally contains a small quantity of ferric oxide, which should be extracted by treatment with strong hydrochloric acid when a product of the best colour is required. The calcium carbonate should also, as far as possible, be free from ferric oxide. =Chrome Blue= (Garnier).—A mixture of 48·62 parts of potassium chromate with 65 parts of fluorspar and 157 parts of silica is fused in a crucible lined with coal-dust. CHAPTER XXXIII. GREEN COBALT PIGMENTS. From mixtures of chromium oxide, cobalt carbonate and alumina in different proportions a series of pigments can be obtained varying in shade between pale blue and bluish-green. On account of the great stability of these colours at the highest temperatures they are of great importance in porcelain painting, for which they are principally used. It is necessary that the porcelain should be free from iron, since ferric oxide forms a black compound with chromium oxide, very small quantities of which would be sufficient to considerably damage the fineness of the colour.