Paints Made of Copper. Chlor Copper Oxide. Braunschweiger green.

Manufacturer and builder Volume 2, 1871

Chlor Copper Oxide

 There aro different compounds of chloride of copper and oxide of copper, or cupric chloride and cupric  oxide, as the now nomenclature  has it. One of thew, however, is exclusively used either as a paint or as a material for the manufacture of other paints. It i s made in the following manner:

One hundred pounds of cupric sulphate (blue vitriol) are moistened and ground up in a paint-mill with sixty punds of sodium chloride, (common salt,) so as to obtain  a pasty mass, consisting, by mutual decomposition, of natrium sulphate and cupricchloride, with an excess or remnant of common salt. This mass is now used to act on three hundred pounds metallic copper. For this purpase, pure cheat copper is cleaned and cut up in small stripes, which are placed in alternate layers with the mass previously described, is flat vessels, and put in a place such as a cellar, where the temperature is uniform. They aro kept there about three months being stirred about once a week. The result will finally be that the cupric chloride Cu Cl₂ in the pasty mass, by combining with more metallic copper, will be changed into cuproud chloride, Cu₂ Cl₂. The first is a  yellowish, brown deliquescent substance; the latter whitish, becoming blue on exposure to the air, whence it absorbs oxygen; and the action continues till all the cupric chloride, Cu Cl₂, has come in contact with so much copper that it is changed, first into Cu₂ Cl₂, and then into the insoluble chlor copper oxide, Cu Cl+Cu O. The test, therefore, for determining when the operation ia completed, is to throw some of the mass in water. As long as green solution is formed, the operiation is continued, and only suspended when the water remains clear. The blu-green powder is now separated by means of a sieve from the remaining strips of metallic copper, which are utilized in the next operation. The true formula of the compund is this:
Cu Cl +2Cu O +5 H O,
which, according to chemical equivalents, would give
(64+36) +2(64+16) +5(1+8),
or 100 + 16+ + 45,
which agrees well with the analysis of the substance, which given for 100 parts,
32 Cuproid chloride ... Cu Cl.
53 Cuprous oxide ... Cu O
15 Water ... HO
----
100

It behaves, with referrance to foreign substances, like other salts of copper; but has the advantage of containing only one quarter as much acid; for which reason, by precipitation by means of alkalies, copper oxide can be obtained in the most economical manner. Formally, green paste was used as a paint, and the so-called Braunschweiger green was made of the same, after it was partially changed into hydrated oxide of copper. This paint is now made by another  method, which we will now describe, remarking, however, that the demand for it has of late years considerably diminished.

Braunschweiger green

One hundred pound., cupric sulphate (blue vitriol) are dissolved tartrate an abundance of water and 2 pounds of potassic tartrate (salts of tartar) in a copper vessel. Three ounces of arsenious acid are also dissolved in water, with 10 pounds calcined potash in another copper vessel. Twenty-two pounds fresh burnt lime, which keeps a pure white color when slacked with water, are mixed with more water till a milk of lime is formed, left for a few days, and, when stiffened, well ground up in a paint-mill. The copper solution first mentioned is then precipitated, stirring it continually, by first pouring in the potassic arsenite solution, and the nthe milk of lime. After settling, the supernatant water is drawn off, and the precipitate washed with more water.

The common kinds used to be adulterated with ground barium sulphate, (heavy spar.) By adding 60 pounds of this material to the above, from 135 to 140 pounds of paint are obtained. This mass is pressed into long rectangular cakes, then cut and dried in the air.

It may be used as water-color, oil-paint, and for fresco. As lime-color, however, it is pale, and has little intensity. Used as an oil-paint, it at first looks pale, but becomes continually darker; as is also the case with the Bremen blue, described in Vol. II., pp. 142, 238, and at last ends with being a very beautiful green. We ought to remark that, in some localities, Bremen blue is called Braunschweiger green.