Practical Receipts. Borate of Copper a Beautiful Green.

Scientific American 47, 12.8.1848

Prepared by a German Chemist for the Scientific American.
The public have been often and repeatedly cautioned against the poisonous effects of the green paints which are produced by arsenic, and the dangerous application of them for wall papers has especially been pointed out.

Dr. Elsner, in Berlin, proposed as as substitute for the arsenic colors, to make decoctions of woad and quercitron with solutions of the carbonate of soda, to precipitate the same with a solution of the sulphate of copper, and to dry the precipitate thus obtained after repeated washing by a temperature of 44 R.

The most beautiful green, which can produce a great variety of shades and can meanwhile be applied for porcelain and oil painting although it is somewhat more expensive than the previous mentioned, is the borate of copper. It is a much clearer and much more saturated green than the chromate or green ultramarine. It is produced in solutions, one of borax and the other of blue vitriol in such proportion as will correspond about with the chemical equivalents of the two substances (16 sulphate of copper and 24 borax.) The two filtred solutions are mixed together, the light green precipitate is collected on a filter and repeatedly after washed with cold water. It is then at first dried in common temperature and heat applied only towards the end of the process. Cold washing is required, because hot water would decompose the precipitate, extracting the boric acid from it, by which means the separated oxide of copper would cause a dirty blackened appearance; the same evil occurs if a high temperature is applied to the wet or moist precipitate. In a heated state the water deprives the copper soon of its boracic acid and dark spots are immediately produced. As soon as the precipitate is dried in the air, in which state it appears as a dark green horny, shining mass, it is pulverized in a wedgewood mortar, and heated in a Hessian crucible until it commences to get red hot, (it must not melt.) The borate of copper loses by this process the rest of its water, the small particles are deprived of their horn-like appearance and gloss, and the color will be of a deep or agreeable yellowish green, according to the longer or shorter continued application of heat. The color is then ground and prepared.

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