There are a good many imitation emerald greens on the market, some of which are offered as genuine emerald greens, others as “emerald tint” green, which is much more honest. The composition of these greens necessarily varies greatly, some are prepared from coal tar greens, others by careful admixture of various green, blue, and yellow pigments. If the tint of these substitutes is right and they are sold for what they are, there is no reason why they should not be used in place of the real article, over which they have the advantage of not being poisonous, which is a great disadvantage of the genuine emerald green. Although one authority disputes this point, certainly the poisonous action of emerald green varies very considerably with different individuals. The genuine emerald green may be distinguished from the spurious by being perfectly soluble in acids and alkalies, which the imitations are not; the character of the latter must be inferred by the application of a few special tests, the nature of which will be readily deduced from what is said as to the properties of other green pigments. Emerald green should be assayed for purity and tint; this is important, as pure emerald green has a tint of its own, which is difficult to imitate, and sometimes really pure emerald green offered for sale is of a defective tint, due to some fault in the process of manufacture. Such samples should be rejected.
Guignet’s Green.—The greens of this class, which owe their colour to chromium oxide, are also known as “chrome greens,” a name which they share with a totally different group into whose composition chrome yellow enters as a constituent, and which have been already described under the synonym “Brunswick greens,” on pp. 114-118.
Though one of the simplest of chemical products, a great many ways of preparing chromium oxides have been proposed. One of the earliest for industrial application was that of Guignet, who has given his name to the pigment, and this may fitly commence the long list.
(1) The first method adopted by Guignet consisted in mixing bichromate of potash with three times its weight of boracic acid and moistening the mass with just sufficient water to form it into a thick paste. This paste is put on the hearth of a reverberatory furnace, which is carefully heated to a point never exceeding a dark red heat; if this precaution is neglected, the mass, instead of becoming porous, will fuse entirely, and the anhydrous oxide will be produced, which has a pale-green colour. The heated paste, while still red hot, is thrown into cold water and washed with boiling water, in order to remove borate of potash in solution; and this solution, when boiled down and treated with hydrochloric acid, can be made to yield up most of the boracic acid it contains. The filtered and washed residue is the hydrated oxide of chromium.
(2) A modification of (1), followed by Guignet, was to replace the bichromate of potash by chromate of soda, prepared by dissolving in boiling water 61 parts of neutral chromate of potash and 53 parts of nitrate of soda. For the neutral chromate of potash, also, may be substituted a mixture of 92 parts of bichromate of potash and 89 parts of crystallised carbonate of soda, the nitrate of soda remaining as before. On cooling, in either case, the solution deposits much nitrate of potash, which is commercially valuable. The chromate of soda present in the mother liquors is obtained by evaporating to dryness. The pigment produced by the chromate of soda process is lighter in colour than that obtained with bichromate of potash. It may be still further paled by adding a little alumina, baryta, or other white pigment to the bichromate and boracic acid mixture before calcining.
(3) Equal quantities of potash bichromate and potato starch are thoroughly mixed and then calcined in a crucible at a high temperature. The product is washed with boiling water, to remove the potash carbonate formed, and any remaining undecomposed bichromate. The precipitated chromium oxide is filtered, dried, and again calcined to drive off the water. The final result is a handsome pigment which flows well from the brush.
(4) On heating in a crucible a mixture of 3 parts of neutral chromate of potash with 2 parts of salammoniac, the two salts are decomposed, the result being formation of chromium oxide mixed with potassium chloride, which latter is removed by several washings with hot water. The brilliancy of the chromium oxide is enhanced by calcination at a dull red heat.
(5) Fuse together 3 parts of boracic acid and 1 part of potash bichromate at a dull red heat on the hearth of a reverberatory furnace. Thus is formed a borate of chromium and potash, with evolution of oxygen. The mass is repeatedly washed with boiling water, which causes decomposition, and consequent separation of hydrated oxide of chromium, and a soluble borate of potash. The chromium oxide is washed, and ground very fine.
(6) When a solution of potash bichromate is poured into a neutral solution of mercury proto-nitrate, it forms an orange-coloured precipitate, which is washed and gently dried, then powdered, and heated in a stoneware retort provided with an arm dipping into cold water, by which the mercury is distilled and condensed. The residue in the retort is a highly comminuted chromium oxide, of a fine dark-green colour.
(7) On calcining potash bichromate in a crucible at a very high temperature, it is decomposed, and results in chromium oxide and potash, the latter of which can be washed out. The chromium oxide thus obtained is very dense and of a dark-green colour resembling (6).