Manufacturer and builder 11, 1870
The oxide of copper combines with the acid principle of vinegar, acetic acid, in different proportions, and thus forms different compounds, of which, however, only two are used in the preparation of paints. One is the so-called common and the other the distilled verdigris.
The common verdigris comes almost exclusively from the south of France, and is there prepared by spreading on scraps and plates of copper the refuse of the pressed grapes, and exposing it to the air. The juice still remaining in the mass gradually changes by contact with the air in the presence of the copper into acetic acid. The action of the copper is due to a chemical law that the presence of a base predisposes the formation of such acids as will readily combine with that base. The copper is oxidized, and the oxide of copper combining with the acetic acid forms a salt correctly named a subacetate of the oxide of copper.
This compound may also be made experimentally on a small scale, by moistening from time to time a copper coin with vinegar, which occasions the production of a green coating.
This French verdigris is brought into the market in leather bags or in hard halls. That contained in leather bags is usually the purest. It is very hard, difficult to break, and in breaking gives rise to considerable dust, which, when inhaled, causes a very disagreeable feeling and vomiting, and may even cause poisoning, the antidotes to which are enumerated in another part of this number.
The common French verdigris in question is properly a mixture of several compounds of copper and acetic acid, namely, simple acetate of the oxide of copper, subacetate of the same, another with still less acetic acid, an acetate of the suboxide of copper, and, finally, metallic copper and impurities. It contains, almost always 44 per cent of oxide of copper, 28 per cent it acetic acid, 25 to 28 per cent of water, and 1 to 2 per cent impurities.
Owing to this mixture of different compounds, the chemical relations of common verdigris are more complex than those of many other similar substances. If it is mixed with a small quantity of water, it swells up rapidly to a bluish-green soft mass, which may be heated to the boiling-point without change. If in this condition it is pressed through a hair-sieve, many impurities mechanically mixed, such as grape-skins, metallic copper, etc., will remain behind. If a little more water is added, the mass is decomposed in a solution of crystallizable verdigris, or an acid acetate of copper and a nearly neutral salt, while one third of a basic acetate of copper remains undissolved. And we must here mention that water acts in this way on several salts, namely, decomposing it into two different compounds, an acid salt which is dissolved, and a basic salt which is either precipitated or remains undissolved. If still more water is added, and the mass heated, all these compounds are changed in such a way that simple acid acetate of copper remains in solution, and a black-brown compound of the oxide of copper with a little acetic acid, mixed with about one third acetate of copper, remains unsolved. The latter may be again partially decomposed in the same way by repeated washing and boiling.
It is thus seen that verdigris does not change its composition if mixed with a little water; but that when mixed with more water, it is decomposed partially into a solution of crystallizable verdigris and other salts, which, losing their green color, obtain that of oxide of copper. Therefore, if we wish to heat verdigris with water, without changing its constitution or color, we must use only very little, not more than four times the amount of the paint.
At the present time verdigris is little used as an oil-paint, except in Russia. For making an oil-paint, it is rubbed up with pure white-lead, by which treatment a decomposition is superinduced into carbonate of copper and basic acetate of lead. Carbonate of copper forms with that portion of the white-lead which is still unchanged a light blue color, which, however, after being put en as an oil-paint, gradually changes into the very handsome green which is characteristic of the green roofs everywhere seen in Russia, and which improves continually by age. It is seen that when used in this manner verdigris possesses great advantages over many other green pigments, of which only those made of copper are permanent. Even the expensive sesqui-oxide of chromium, or pure chrome-green, (see MANUFACTURER AND BUILDER, Vol. I., p. 211, and Vol. II., p. 34,) does not improve by age when exposed to the weather, and never possesses the bright color of the verdigris, treated after the Russian method described.
Ei kommentteja :
Lähetä kommentti