Paints made of Copper. Some Practical Hints in Regard to the Economy and Details in Their Manufacture.

Manufacturer and builder 8, 1871

In some localities it is more economical to use the common sodic sulphate (Glauber's-salt) in place of the soda itself. In this case, the vinegar used, whether it be strong or weak, is first changed into calcic acetate, (acetate of lime,) by adding to a quantity containing 60 pounds of the hydrated acetic acid, dry hydrated lime until it ceases to redden litmus paper. About 42 pounds of lime .will suffice for this; but an excess of lime is to be guarded against. Then a concentrated solution of 165 pounds sodic sulphate is added. The great economy of this process lies in the fact that it may be impure, and even contain some iron. We have the same precipitate of calcic sulphate (sulphate of lime, gypsum, plaster of Paris) as described page 127. and this is to be separated by decantation or filtration, The liquid contains then the acetate of soda, and must be brought to the quantity of about 70 gallons by evaporation of the vinegar used, if the same be weak, or by the addition of water, if it be strong, because the amount of liquid thus obtained will be large or small in proportion as the vinegar used is weak or strong. We have now the solution of sodic acetate, with which we may proceed 'as before described. Circumstances must decide the economy of this process, which may be easily determined by making up an account of the expenses of Glauber's-salt, lime, fuel in evaporation, labor, etc., compared with the cost of the sodic calcic acetate when bought.

One point must not be forgotten, which is, if we dissolve the pure sodic acetate, we have the same in solution. and nothing else; while, when we follow the process here described, we have in the solution not only gypsum, which is soluble in 400 parts water, but also undecomposed calcic acetate, and even free sodic sulphate, because the mutual decomposition is never a complete one, as small quantities of the substances used remain together in solution in their original state. (This is a practical hint to young chemists who labor under the impression that the theory laid down in the elementary text-book is complete, when teaching the mode of mutual decompositions and recompositions.) When, now, a solution of cupric sulphate (blue vitriol) is added to such a mixed solution, we obtain, besides the substances mentioned, some more calcie sulphate, (gypsum and on adding the arsenic solution, a white precipitate will mix with the green, increasing the quantity obtained, but decreasing the quality. This, of course, is not the case when using the pare sodic acetate, or vinegar and soda, from which the brightest greens are always obtained. The practical question is, now, What method pays best under the circumstances, to make smaller quantities of a brighter color, at greater expense, for which a higher price may be obtained, or to make larger quantities of an inferior quality, at less expense, for which a lower price must be charged ?

An important point is, that the colors are brighter and of finer grain when they are formed rapidly in the kettles, and not slowly crystallized.

These colors are assorted by manufacturers, with reference to their light or dark shades, and from the above it is clear why the latter are somewhat higher in price.

 This difference in price is increased by adulteration, by the intermixture of gypstim and barytic sulphate, (heavy spar,) producing light shades, as this adulteration of a valuable paint with a comparatively worthless ingredient diminishes its real value in direct proportion to the amount of adulteration, which, However, may be easily detected by the following means:

To 100 grains of the paint, add liquid ammonia; when pure, and if enough ammonia be used, all will dissolve; bat what does not, is adulteration. Place the resonant on a small filter, wash it with water, and dry it; its weight will give the amount of adulteration, and consequently the quantity of pure material. (The filter should be weighed both before use and afterward with the precipitate, and its tare subtracted, a method always followed in similar manipulations.)

It has been found that the color sometimes turns out yellowish-green, when the verdigris used contains very little acetic acid, or when by accident the arsenic is not all dissolved. The Cassel green is a color of this kind. In general there is no great demand for these shades; they are nevertheless manufactured, but not in the manner stated, as the result is exceedingly uncertain; they are simply made by adding chrome-yellow, of the sulphur-yellow or lemon-yellow shade, the method of manufacture of which is described on page 52 of our first volume. In this way the tone of colof may be regulated to any extent.

For the benefit of painters we must here remark, that if the paint is unadulterated, they lose by paying a higher price for the dark, coarse-grained or crystallized qualities, because both, when rubbed up fine, are absolutely equal, the darker appearance being only caused by the coarse grains, which require considerable snore labor to bring to the right condition for use. The body of the paint and its covering qualities are the same.

In this country the chrome-greens have among the house-painters almost entirely superseded the copper compound. The largest consumers are now the dyers, calico-printers, carpet manufacturers, etc., who use it in large quantities for the production of most of the beautiful greens.

The case with these manufacturers is totally different from what it is with painters; as they do not rub up the copper compounds, but use solutions, and only being certain of obtaining the pure material when buying the dark, large-grained crystallized qualities, they do best by paying the higher price, in order to be sure of producing the snore beautiful colors, which in dyeing . can only be obtained by the use of the purest attainable materials.

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