Sap-green is a simple colour, but far inferior to verdigrease : it is prepared from the juice of buckthorn berries, evaporated to the consistence of a gum; but it frequently inclines to a yellowish colour.
Another green sometimes used is called terra verte, which is a native earth, probably impregnated with copper. It is of a blueish-green cast, much resembles what is called sea-green, but is gritty, and requires to be finely levigated before it is used. Its colour is durable, but not remarkably bright. See also p. 37.- 6. Green.
A durable green pigment has long been a desideratum among painters. M. Kinnman, a member of the Swedish Academy, has, at length, discovered and published the following process : Dissolve, in separate vessels, a portion of zinc in aqua fortis, and cobalt' strongly calcined, in aqua regia, till the liquors are completely saturated. When both solutions are prepared, mix two parts of the latter with one part of the former; then procure a hot and clarified solution of pot-ash, three parts of which will be required to precipitate the mixture above specified. After it has subsided, the fluid part should be decanted, and the sediment evaporated to dryness over the fire, till it assumes a green colour. Before, however, this pigment can be used, it ought to be repeatedly washed with filtered water: thus it will become fit both for oil and water-colours, as it is sufficiently fixed to withstand the effects of the air and sun; for the inventor has ascertained its superior durability by more than ten years experience. He adds, that painters may, by means of this preparation, combine their yellow and ultramarine, so as to form a very beautiful and permanent green.
Green. - The following method of preparing Brunswick Green, a beautiful colour, in great request on the Continent for oil-painting, and in the manufacture of stained paper, is inserted on the authority of M. KasteLEyn, He directs shavings of copper to be put into a close vessel, and sprinkled with a solution of sal ammoniac. - The metal first unites with the muriatic acid, and is dissolved, when it is precipitated by the disengaged ammonia, with which it combines. The precipitate is then to be washed, and dried either in wooden boxes, or upon an expanded cloth : the liquid now remaining may be repeatedly employed as a solvent for fresh portions of sal ammoniac, till it be completely saturated. M. Kasteleyn states, that three parts of such salt are sufficient for two parts of copper, and that the result will be six parts of colour. - This beautiful pigment is sold in Holland by the name of Friesland Green : and it sometimes forms an article of exportation ; in which case it is generally adulterated with white-lead.