Manufacturer and builder Volume 4, 1871
We have mentioned, on page 142 of our second volume, that the minerals malachite and lazulite are beautiful green and blue carbonates of copper, (or cupric carbonates, according to the new nomenclature,) but rather expensive minerals to crush and use as paints; this was only done in olden times, before chemistry had taught us how to manufacture a much brighter and purer compound of the same composition, and it is at present, perhaps, impossible to obtain a good green or blue copper paint not artificially made, but of real mineral origin.
The natural carbonate of copper is the oldest green color known: the artificial article is also the oldest green color made by chemists. Soon, however, the so-called mineral green, when made artificially, had nothing in common with the natural green but the color and the fact that it was made from copper A carbonate was no longer made, because experience had taught that other compounds of copper could be made cheaper and of a finer color than the imitation malachite or carbonate. The paints now in the market, and sold under the name of mineral green, are, in reality, different combinations of copper, made in different ways, and imitated in a great variety of styles.
The oldest method was to take a warm solution of cupric sulphate free from iron, precipitate the copper as carbonate by adding a small excess of potassic carbonate, and wash and dry the latter. This powdered precipitate was by no means a brilliant paint; but when used with oil, it became gradually of a darker and more beautiful color. A later method was to take 100 pounds of cupric sulphate, 2 pounds of potassic tartrate dissolved in at least 600 pounds of water, and precipitated with a solution of 21 pounds arsenious acid, and 10 to 12 pounds of potash dissolved in about 600 pounds of water; then add 22 pounds of lime mixed with water, to form a milk of lime; and finally 60 pounds of very finely-ground heavy-spar. Wash and filter the precipitate; press in the ferns of cakes, and dry. By keeping for a long time, the color improves considerably.
It is seen that this method is identical with that described before, on page 85, for Braunschweiger green, only it contains more arsenic, and has therefore a better color. It is now sold in Germany under the name of Neuwider green. Sometimes Schweinfurter green, a similar compound, is mixed with it. So it is seen that, in fact, the so-called mineral greens at present in the trade are nothing but hydrated cupric oxide with an excess of lime, chalk, and heavy-spar, owing their fine color to the addition of cupric ar-senile and acetate.
These mineral greens may be used as water or oil. paints, but the finest kinds can not well be used on lime, while the Schweinfurter green, for example, under such circumstances, changes its color into a, greenish. yellow. It often contains, besides, an adulteration of calcic sulphate, or gypsum. Its value, therefore, is best tested by mixing it with white. The kinds which can stand the largest amount of white with the least amount of change to a paler green are the best. Chemically, however, the test is easy; all we have to do is to digest the powder with a solution of ammonium chloride (sal ammoniac) or with liquid ammonia, which will slowly dissolve all the copper and leave a residue of lime, heavy-spar, etc., which may be then dried and weighed. Of course, the larger this residue, the lower the value of the paint. The amount of copper is sometimes found to be as low as fifteen per cent.
Neuwieder Green, Neuwieder Blue.
One way of making this green is the same as in making the mineral green (p. 60) or the Braun-schweiger green, (p. 35,) and its tint may be varied by varying the amount of arsenious acid employed.
A better and improved way of making this green of a far superior quality was for a long time a secret of a few manufacturers, but, as in the case with all such secrets, in the course of time it became known. To make it they took a solution of blue vitriol, and added liquid ammonia gradually, till the precipitate formed at first was redissolved again, and a pure dark-blue liquid was obtained; to this was added milk of lime, stirring continually, till a small quantity of a precipitate appeared; then the warns liquid was filtered and cooled, when long, fine, needle-like crystals were formed, which were the concentrated blue coloring material, and of a most brilliant appearance. These have only to be mixed with an excess of lime, strained, and dried to make the Neuwieder blue.
To change it into green after the improved secret method, this blue, when still wet, is mixed with a cold solution of arsenious acid till changed to the right shade, which is totally under the command of the operator, by regulating the quantity of arsenic. After straining, pressing, and drying the deposit, it is kept in large vessels in cellars, where it slowly obtains a much finer and purer color, which penetrates from the outside of the pieces to the interior.
A third cheaper method, adapted to localities where acetic acid or acetate of lime is cheap, is the following:
80 pounds of calcic acetate, (acetate of lime,) or an equivalent mixture of acetic acid and slacked lime, (40 pounds of the acid and 30 dry lime,) is mixed with water and boiled in a vessel with 100 pounds arsenious acid, which is not dissolved, but combines in such a way that we obtain arsenite of lime and diluted acetic acid. Then 125 pounds of cupric sulphate is gradually added, stirring the mixture often. At first we obtain a yellowish color, which, however, after the addition of all the cupric sulphate, becomes a bluish green. As soon as this change is seen, the fire is extinguished, the liquid drawn off into another vessel and cooled, when the green paint is deposited, which is removed without washing, and dried in irregular pieces. To this paint pulverized heavy-spar is also frequently added as an adulteration.
Common vinegar may also be used in the preparation, but then the color has never the freshness and beauty given by using acetic acid; but the most essential condition, in all these, preparations, is, that the blue vitriol be totally free from iron, the smallest quantity of which greatly deteriorates the beauty of the paint.
It must be mentioned that the liquid drawn off still contains copper and acetic acid, which, however, may be utilized in the next operation.
When pure lime can not be obtained, but only a lime which is clayey or sandy, it is better to use fine chalk or ground Iceland spar, (calcic carbonate.) It requires then, however, 5 parts for every 3 of lime, in order to obtain the same results, because the atomic weight of carbonic acid is combined with the lime so that 50 parts only contain 30 of lime. The only inconvenience is the evolution of carbonic acid, which makes the liquid foam and run over when not mixed carefully and slowly.
A fourth method, adapted to localities where acetic acid or acetate of lime is scarce but French verdigris cheap, is the following:
100 pounds arsenious acid are boiled with a sufficient quantity of water 3 to 4 hours, or till dissolved. Then 70 pounds of ground hydrated white gypsum are mixed in, and immediately afterward a paste con-sisting of 100 pounds verdigris mixed with the required water. The formation of the paint takes place, as in the former cases. The liquid is drained off, the heavy-spar mixed in, in case the adulteration is desired, all well mixed, passed through a sieve, in order to separate the impurities of the verdigris on the sieve. This method gives for the quantities mentioned 170 pounds of pure green paint, which may be adulterated to suit circumstances.
The liquid drawn off contains some green in solu-tion; also arsenious acid, and free acetic acid. It may be again used for dissolving the arsenic in the next operation; but then it is only necessary to use 60 pounds arsenic instead of 100 pounds, about 25 pounds cupric sulphate, and a milk of lime of only 6 pounds fresh-burnt lime, after the 100 pounds of verdigris have been added. If the liquid is impure, or for some other reason can not be utilized in this way, it may be precipitated by potash or soda, and thus a clear green paint obtained, adapted to be added to mineral green.
This is a good water-color, but difficult to rub up fine in oil; there is not much body to it. On lime it can not be used, its acetic acid being abstracted, making it yellow. Otherwise its tone is much brighter than all the others of this class before described.
To determine the adulteration and value of a sample by analysis, the simplest way is to treat it cold with hydrochloric acid till the remnant is white. The liquid is then filtered to separate it from the remnant, the filter washed, and the filtrate treated with caustic potash till dissolved, then boiled till the precipitate is black, filtered, washed, dried, and weighed.
Being oxide of copper, its weight will show which paint contained the most copper and hiss the greatest value, as its price increases in the same ratio as the amount of copper contained in it.
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