Scientific American 18, 28.4.1860
The famous Chinese green or lokao, when upon silk, is the finest green yet known, and is principally remarkable for its brilliancy when seen by candlelight or gaslight. The Chinese have produced this dye for some time past, but their process has remained a secret until very lately. The subject has been investigated by some very celebrated chemists, and it results from their labors that we can not only produce the dye as the Chinese do, but we can in a great measure explain its formation. It was first of all discovered that the lokao was obtained by the Chinese from two species of exotic buckthorn (Rhamnus), with the bark of which a sort of decoction was made with lime, &c., and that the fabrics, on leaving the bath, were exposed on the grass to the action of light and air. Since these facts were known, experiments have been made upon the colors furnished by our common buckthorn (Rhamnus frangula, and R. cathartica); and the result has been that these plants, like the Chinese species, can be made to furnish a green color, produced by the action of light, and doubtless identical with the lokao. The bark of the pergative buckthorn is boiled for half-an-hour with a sufficient quantity of water. After cooling, the clear liquor is decanted off, and to it is added its own volume of lime water; the next day a saturnted solution of alum is poured in, and twenty-four hours later some carbonate of soda. After an hour or two of quiet, the clear liquor is decanted or filtered off. The solution is then fit for dying green; it is of a yellow color, and when exposed in shallow basins to the action of the sun, it deposits the lokao, which, like that of the Chinese, is soluble in acetic acid, by which means it may be purified, as it is precipitated again by ammonia. The substance which gives birth to this green dye is an unknown colorless body, which, by the influence of light, becomes green. In France, 10,000 francs were offered for the production of lokao, but I do not think the prize will be awarded, as the subject has been investigated by so many scientific men, and with such remarkable results, that the 10,000 francs could only be divided among them, and the sum is too small for such a division.
An admirable quality in colors such as those just spoken of is, that, being produced by the direct agency of light, they cannot be decomposed or spoiled by exposure to it, as is the case with many of our most costly dyes produced by other means.
-London Photographic News.