(1) A mixture of 2 parts of commercial carbonate of potash and 1 part of powdered arsenious acid (white arsenic), are dissolved in 35 parts of boiling water; the solution is filtered clear, and then added gradually and while still warm to a filtered solution of 2 parts of sulphate of copper until no further precipitate goes down. This latter is collected, washed with warm water on a filter, and slowly dried without excess of heat.
(2) The preceding formula is modified by making one solution of the arsenic and the sulphate of copper, and precipitating by adding the carbonate of potash solution till the colour is fully developed, agitation being constantly maintained.
(3) Another variation is to mix the arsenic with soda crystals in boiling water, and to pour the arsenite of soda solution thus formed into the bluestone solution, the boiling being kept up for a few minutes.
Scheele’s green has a pale yellowish cast, and mixes well with either water or oil, but it lacks brightness, durability, and covering power, in addition to being highly poisonous, and though once much employed in staining wall papers, is now generally discarded.
Schweinfurth Green.—This is an old-fashioned name for emerald green, which has been described on pp. 121-125.
Terre Verte.—Rendered into English, the name terre verte means “green earth.” It is applied to a number of green-coloured earths found widely distributed in rocks of various ages, but especially in those of a basaltic or porphyritic character. In commercial quantity it occurs notably in Cyprus and near Verona in Italy; the latter locality is so important that the pigment is often known as “Verona earth.”
Notwithstanding minor points of dissimilarity in samples from different sources, there is a great family likeness among them, sufficient to indicate that the essential constituent is a silicate of iron and magnesia. The other ingredients vary with the locality producing the mineral. The same may be said of the physical characteristics, some specimens being soft and earthy, while others are hard and glassy. All possess the peculiar soapy touch of the magnesian earths, and a clay-like odour. Analysis of a Verona earth gave:—
| Per cent. | |
| Silica | 51·21 |
| Iron protoxide | 20·72 |
| Magnesia | 6·16 |
| Water | 4·49 |
| Alumina | 7·25 |
| Soda | 6·21 |
| Manganese protoxide | trace. |
While a Cyprus earth showed:—
| Per cent. | |
| Silica | 51·5 |
| Iron protoxide | 20·5 |
| Magnesia | 1·5 |
| Water | 8·0 |
| Potash | 18·0 |