Paints Made of Copper. - Distilled Verdigris.

Manufacturer and builder 1, 1871

The so-called distilled, or rather crystallized, verdigris was formerly produced in France, but now also in many other countries, in a variety of ways. The main condition, however, necessary for its production on a large scale and in a profitable manner is a cheap supply of pure vinegar. The old method of making dis-tilled verdigris is to dissolve the impure verdigris in an excess of distilled vinegar. The liquid is then slowly evaporated in copper pans, in order to crystallize it in the same way as the solutions of acetate of copper hereafter described.

A solution of the acetate of copper may be also obtained by decomposing a solution of sulphate of copper (blue vitriol) by acetate of lead (lead-sugar) or acetate of baryta. The metals interchange their acids, not on account of greater chemical affinity of a base for a certain acid, as most text-books in chemistry still teach, but for the simple reason that by this change an in-soluble compound can be formed. The force of cohesion of the molecules of this insoluble sulphate of lead is greater than that of the soluble sulphate of copper, and hence the lead yields up its acetic acid, which forms with it a soluble compound, and takes possession of the sulphuric acid which was combined with the copper, forming in this way an insoluble sulphate of lead. The free acetic acid then immediately attacks the oxide of copper, old forms with it an acetate of copper, which remains in solution while the sulphate of lead is precipitated.

It is a well-established chemical law that, whenever the interchange of the acids of two soluble chem-ical compounds will cause one of the bases to unite with the acid previously combined with the other base, and form an insoluble compound, this reaction always takes place whenever the solutions are brought together, regardless of the chemical affinities of the two bases. As this law explains many chemical phenomena formerly supposed to be rather strange, we think it worth while to direct our readers' attention to it.

There are several other ways of producing the solution of the acetate of copper; but as the salts required are not as cheap as those mentioned, the old process is still preferred. It is not necessary to use the crystallized blue vitriol or the lead-sugar. The pure concentrated solutions may be made from the copper and lead, and used in place of the solutions made from the crystallized salts. There is no difference in the result, whether lead-sugar or acetate of baryta is used, except that in the first case the incidental product is the precipitate of sulphate of lead, a substitute for white-lead, and in the second case, sulphate of baryta, or heavy spar, a substance largely used for adulterating white-lead.

To produce the precipitate in the best manner, the solution of the acetate is slightly heated, placed in the precipitating vessel and constantly stirred, while the solution of sulphate of copper is slowly added. A white precipitate immediately commences to fall down, and the addition of the copper solution is continued as long as it produces any precipitate; consequently it must be added with care toward the end of the operation, in order to avoid an excess of the copper solu-tion. If an excess has been added, a quantity of the acetate solution must be added slowly till this fails to produce a precipitate, so that we may be sure that no excess of either salt is present. In order to test this more fully, a small quantity of the mixture is filtered into a test-glass, and a few drops of either solution added. If a few drops of one produce a cloudiness, we know that we have added too much of the other to the mixture. The intensity of this cloudiness indicates to the experienced workman the amount of the solution which must be added to the mixture to avoid an excess of either salt. When the required correc-tions have been made, the solution is allowed to settle for several hours, so that the supernatant fluid is perfectly clear. This is then drawn off by copper or wooden stop-cocks at the side of the vessel, or by means of siphons. This liquid is then placed in large copper pans, and evaporated at a temperature of 140° to 180° Fahr. As soon as a small portion of the liquid, taken out by: means of a small dipper and cooled, commences to deposit crystals, the fires are withdrawn from under the pans, and strings and sticks are suspended in the liquid. On cooling, crystals of verdigris will deposit themselves upon the sticks and strings as well as on the bottom and sides of the pans.

In the paint manufactories, when the acetate of copper is used for other purposes, the solution is used but not concentrated and crystallized. In this case, the question arises, How much crystallized verdigris is contained in a given quantity, for instance, in ten gallons of the solution? There are, of course, very correct chemical methods of determining this point, but these are not as useful for practical purposes as the following simple method, which is, therefore, universally adopted.

The specific gravity of the solution is ascertained by means of Beaumes hydrometer, and the temperature noted; then a small quantity of pure acetate of cop-per, say one pound, is dissolved in as little water as possible, and then water added till a solution is obtained of the same specific gravity and temperature as the solution, of which the degree of concentration is required. By partially immersing the vessels contain-ing the concentrated solution of the crystals, and the water with which it is to be diluted, in the original solution, the equality of temperature is easily obtained. Suppose now that the pound of acetate of copper has been dissolved in a quart of water, in order to bring it to the sane specific gravity as the ten gallons of solution to be tested, it is clear that both solutions would contain acetate of copper at the rate of four pounds per gallon, or forty pounds in the ten gallons.

Some manufacturers do not make the acetate and copper salts, but buy them in crystallized form. For the benefit of these, it is easy to give, once for all, the relative amount of material required to produce a given quantity of verdigris without excess or loss of either material, or without requiring the trouble of testing whether there is any excess of one or the other of the generating salts. According to the theory of chemical equivalents, a solution of one hundred and twenty-five parts, by weight, of sulphate of copper has to be mixed with a solution of one hundred and ninety of lead-sugar, or one hundred and fifty-four and a half of crystallized acetate of baryta, in order to form a complete decomposition, and time recomposition of, theoretically, exactly one hundred pounds of crystallized verdigris, with a precipitate of two hundred  and fifteen pounds of sulphate of lead, or one hundred and seventy-five and a half pounds of sulphate of baryta.

Practically, however, the lead or baryta precipitate carries down with it some of the acetate of copper, and both have therefore a greenish color; there is, there-fore, some loss of acetate of copper. By repeatedly washing the precipitate, it may be purified from the adhering verdigris. In this way the precipitate becomes more valuable, while the water used for the washing, and containing some of the copper compound, may be used for the succeeding solutions, and in this nranner the verdigris may also be saved.

Recently, it has become possible to obtain acetate of soda, in crystals or in solution, in large quantities and at low prices, and this gives occasion to another economical way of making a concentrated solution of acetate of copper. The acetate of soda may be obtained by saturating one hun-dred and forty-four parts of crystallized soda with vinegar, filtering through animal carbon in case it is colored, and concentrating by evaporation. Then one hundred and twenty-five parts of sul-phate of copper are dissolved in the same, and a clear solution will be obtained, in which the soda, the copper, the acetate and sulphuric acid are present, but from which a large quantity of acetate of copper may be obtained by crystallization. The strong tendency of this substance to crystallize being in this case the cause of the combination of the copper with the acetic acid, the sulphate of soda, which is very soluble, and hence does not crystallize so easily, will remain in the solution. This solution may, however, be used for many purposes without crystallization.

Also, if twenty-nine parts of pure quick lime are slacked with water, and dissolved in vinegar, the solution concentrated by evaporation, and mixed with a solution of one hundred and twenty-five parts of sulphate of copper, we obtain a precipitate of sulphate of lime, (plaster of Paris, gypsum,) and a solution of acetate of copper, which may be easily separated from the precipitate by decantation and filtration. However, as the gypsum is slightly soluble in water, (one part gypsum in three hundred of cold water) the solution will contain it in this ratio, and can therefore only be used to produce a color less brilliant than that made from pure verdigris. The great trouble in this case is, that the gypsum is less soluble in warm water than in cold water, and that by heating the solution for crystallization the gypsum is deposited with the crystals of verdigris.

Crystallized acetate of copper is also not very soluble in cold water. Four hundred parts of water will dissolve only one part of the crystals. It is, however, much more soluble in hot water; but if boiled for some time it is decomposed, some of the acetic acid being driven off, and causing the solution to become turbid. A bluish basic salt, that is to say, a compound which contains a double amount of copper oxide, is thrown down. Its production may be avoided by never heat ing the solutions to the boiling-point, but keeping them at a temperature between 140° and 180° Fahr., as mentioned above.

The crystallized acetate of copper is seldom used as a paint, while the basic impure verdigris described in our former number (page 334) is much more adapted for this purpose. It comists of forty per cent of oxide of copper, fifty-one per cent of acetic acid, and nine of water. Its chemical formula is Cu O, (C₄H₃O₃) +HO, which expresses most beautifully this composition according to the definite amount of matter expressed by each symbol. Cu always represents thirty-two parts of copper; O, eight parts of oxygen; C, six parts of carbon; H, one of hydrogen; and for the benefit of students in chemistry, we close this article on the most important of all copper compounds for the ma-nufacture of paints, with a simple calculation of the amount of each ingredient entering in its composition.

It serves at the same time as an illustration of what was commenced to be taught in a former number of The Manufacturer and Builder.
Oxide of Copper, (Cu O) = 32+8 =40
Acetic acid (C₄H₃O₃)=4 x 6+3+3 x 8 =51
Water, (HO)=1+8 = 9
Acetate of copper, Total, 100