(4) Other recipes for making mountain greens have been published which bear no relation to the composition of the original article, e. g. by mixing a solution containing potash and arsenic with a solution of bluestone; or, as a much more complicated example, treating a solution of bluestone first with slaked lime, then with a solution of arsenic and soda obtained by boiling in water, and finally with tartaric acid.
The advantages attendant on so much trouble in producing what is at best an unstable pigment are not very apparent.
Paris Green.—This is another name, used especially in America, for the emerald greens described on p. 121.
Prussian Green.—A name often applied to class b of the Brunswick greens (see [p. 114]), or in other words those which are prepared from Prussian blue.
Rinmann Green.—The first cobalt green (see [p. 119]), put on the market was made by Rinmann, and hence it is still often called by his name.
Sap Green.—This vegetable pigment or lake is closely allied to the Chinese green or lokao, described on p. 129.
It consists of the solidified juice extracted from the berries of the common buckthorn shrub (Rhamnus catharticus), which is obtained either by allowing the berries to undergo slight fermentation for about a week in wooden tubs, then pressing and straining; or by boiling the berries, and straining off the juice. In either case the clean juice is boiled down to a syrupy consistence, and a little alum (about ½ oz. to the pint of thickened juice) is added, the liquor being then evaporated to dryness, or very nearly to that point, the drying being left to complete itself after the pigment has been ran into bladders.
The quality of this green is liable to serious fluctuation, owing to the neglect or ignorance of certain simple precautions. Thus, for a true green the berries should be selected before they have quite reached maturity. The more nearly ripe the berries are, the more yellow will be the tint of the green afforded by them. The boiling of the berries, if followed, and the evaporation of the juice, must be done at a low temperature, and the final stages of the evaporation cannot safely be done with direct fire heat, but should be effected in a water bath. The only substance incorporated with the juice should be potash alum. Sometimes it is replaced by carbonate of magnesia (which destroys the transparency of the pigment); or by carbonate of potash (which introduces a stickiness or viscosity).
Sap green possesses too little body and is too translucent for use as an oil paint; but being non-poisonous, and in fact perfectly harmless, it finds many useful applications outside of water colour and pastel painting, viz. in colouring alimentary substances such as drinks and sweets. Its true colour is a leaf green, glossy and translucent. In durability it is not remarkable.
Scheele’s Green.—For more than a century has Scheele’s green been a familiar pigment, but the reputation it enjoyed in its early days has long since departed, and it is now to be classed among the inferior green colouring matters. It consists essentially of a basic arsenite of copper, and contains from 8 to more than 40 per cent. of arsenic, according to the mode of preparation, of which there are several, as follows:—