Paints Made of Copper. Schweinfurter green.

Manufacturer and builder 5, 1871

The pure Schweinfurter green consists of 32 parts of oxide of copper, 60 parts of arsenious acid, and 10 of acetic acid. As before mentioned, (see page. 63,) confusion exists in the names given in different localities to this class of paints, and the manner of manufacture we are now going to describe applies to the original Schweinfurter method.

Two large kettles are placed in such a way over the brick furnaces that the contents of both may be drawn off. into a lower receiver. The largest kettle serves for time solution of the arsenic, and most, in case 70 pounds verdigris are to be manipulated at once, be able to contain at least 200 gallons of water with 100 lbs. of arsenic; while the other kettle need only to contain 60 gallons of water to 100 pounds of verdigris. The lower vessel must be large enough to receive time contents of both kettles and admit of time mixing. Time best size is about 300 gallons. Time work is commenced in time evening by placing in time smallest kettle 70 pounds of crushed verdigris with 60 gallons of water. Time next morning time large kettle is filled with 200 gallons of water, (a gauging mark is made in time kettle, to indicate time exact quantity,) and 100 pounds finely-pulverized arsenic is added; fire is then made under this, and afterward under time smaller kettle. Time contents of the larger are brought to the boiling-point, and the boiling contin-ued for five to six hours, or until all time arsenic is dissolved. When this is nearly reached, time evaporated water is replaced by fresh water, so as to retain time ori-ginal bulk, 200 gallons. Time verdigris is then heated to a temperature of about 185° Fahrenheit, but not higher. Of course, time contents must be stirred often, in order to promote time solution of time solids in the water. When time arsenic solution is ready, and kept boiling for half an hour after time supply of time evaporated water, a large copper sieve is placed under the cock of the verdigris kettle, and both cocks are opened full at once. Time solutions are mixed by continuous stirring; but one third of time arsenic solution is kept in time kettle, and only added after time mixture has rested quietly for two or three hours, when time wimole is stirred up again.

It is better to allow the two solutions to settle before mixing, in order that small, solid particles of arsenic may not be carried into the mixture; therefore time fires must be nearly extinguished for about fifteen minutes, so as to stop all motion in time liquids; time verdigris solution is, however, never clear, but resembles a thin paste, and time first precipitate formed in time mixture always has a dirty yellowish-green color. On time top, bubbles show themselves of time bright green color of time paint desired; but time wimole is in a continuous interior agitation, wherefore most of the precipitate remains suspended. Two to three hours, and often sooner, after time remnant of time arsenic solution has been added, time precipitate contracts, and time more it does this, time more beautiful time color will be. At last it forms a crust, adhering firmly to time bottom, and over time same is a blue-green liquid. On top of which a skin of time green paint also floats. The precipitate is crystalline, and its lustre and intensity of color depends on time size and granulation of the crystals; the larger they are, time darker and more intense will be the color. After twenty-four to thirty-six hours, the liquid is drawn off, the paint passed through the proper hair-sieves, drained on linen filters, then dried, pulverized, (which latter is very easy,) and once more sifted in closed boxes, so as not to expose the workmen to the highly poisonous dust.

From 70 pounds verdigris are, in this way, obtained 70 to 80 pounds of Schweinfurter green of the most superior quality. The verdigris here mentioned is of the common kind, which, packed in leathern bags, is an article of extensive commerce in France. It is a basic salt, and contains, besides cupric acetate, also a basic salt, insoluble in water. It produces, therefore, more paint than the pure cupric acetate or the so-called distilled verdigris. (See page 16.) If the latter is used, it is necessary to take 100 pounds in place of 70, in order to obtain 70 or 80 pounds paint; but then the result is better, the crystals are much larger and purer, as the impurities of the common verdigris are absent. The Schweinfurter green obtained in this manner is usually called distilled Schweinfurter green; but frequently the common paint is sold under this name, when accidentally the color turns out a. little better than usual. If made from the distilled verdigris, it need not be passed through a sieve, either wet or dry, because there are no impurities to be removed.

The above is not the simplest manner of operating. However, it is the best, as by the slowness of the process the crystallization goes on without disturbance, which always interferes with perfect crystallization.

If the latter is not considered essential, one may perform the operation by dissolving the arsenic in the large kettle in the proportions given above, and then adding the verdigris paste; and either draw off at once, when the yellowish-green precipitate takes the right color already during the pouring, or it may first be boiled for a short time, when all time color is formed in time kettle, from which it is afterward drawn, or the verdigris may be pulverized, passed through a sieve and mixed in the arsenic solution, or the boiling solution may be poured into time powdered verdigris. Every variation of time methodus operandi will give as result a different shade in the green paint obtained.

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