Silk has a strong affinity for galls, and advantage is sometimes taken of this: for silk, being a valuable article, is often galled to excess, merely to increase its weight.
Cotton has a strong affinity for iron, and iron has the same for gallic acid, wherever it may be found; therefore, in sumach, alder-bark, &c., iron unites with the acid, whenever both are connected by the medium of water. Tannin, doubtless, has also some share in such dyeing processes, although what does not even now appear to be well understood.
Black, Macquer observes, is rather difficult to be dyed upon silk; or, at least, there is reason to think so, from the numberless experiments which have been found necessary to the attainment of a good black, as well as from the multitude of heterogeneous ingredients which Macquer admitted into the composition of his various processes for this dye, some of which consisted of arsenic, corrosive sublimate, litharge, antimony, plumbago, and about ten other ingredients! we shall not, therefore, detail such preposterous mixtures; one, however, we may just put down by way of showing what the art was in Macquer's time.
Take twenty quarts of strong vinegar, one pound of black nut galls pounded, and five pounds of iron filings; these ingredients are to be mixed in one vessel.
To dye skein silk BLACK for velvets, Genoa process, (from Macquer.)
The silk should be ungummed by boiling it four hours with a quarter of its weight of white soap, and afterwards to be well cleared from the soap.
Take, for every hundred pounds of silk, twenty pounds of galls in powder, and boiled one hour; two pounds of sulphate of iron; twelve pounds of iron filings; and twenty pounds of gum arabic or senegal.
This process is very simple: here are the gallic acid and tannin of the galls, and the iron of the sulphate and the filings. But we must proceed to a more modern process.
To dye silk BLACK, the London process.
Take of wove silk, twilled sarsenet, one hundred and fifty yards. Boil, for three hours, of alder bark one bushel and a half; of logwood fourteen pounds; and of iron filings one pound. Then let the fire be damped; dissolve four ounces of sulphate of copper in water; wet out the silk in hot water; after which put the solution of sulphate of copper into the liquor and stir it only; then put the silk into the copper, and work it from end to end four times; after which take it out in the air; now put it in again and work it as before; take it out again and let it be aired on the floor, opening it from time to time till it is cold; repeat the same thing twice more, in all four times. This is termed four wets. While the last wet is cooling and airing, dissolve and put into the copper three pounds of sulphate of iron, and then give the silk two more wets, which make the number of wets six. The drugs are now left to boil as much as they will during the night, being left so to do, because in a large business, this part of the process would close the day's work.