Of dyeing Silk black.
It is necessary to cleanse silk that is to be dyed from the substance which ad heres to it, called its gum ; for though raw silk takes the dye with more facility, the colour is much less intense, and less durable, than when the silk is scoured ; which is done by boiling it four or five hours with a fifth of its weight of white soap, after which if is beetled and care fully washed.
After being cleansed, the silk must be galled ; for which nearly three-fourilis of its weight of galls are boiled for three or four hours, but their quantity must de pend on the kind used ; after boiling, the liquor is left at rest for two hours, that the galls may subside ; the, silk is then put into the bath, and left in it from twelve to thirty-six hours ; it is then ta ken out, and washed in the river. Silk is capable of combining with much of the astringent matter of the galls, which at tracts the colouring matter in proportion ; therefore, when what is called a heavy black is required, it is allowed to remain longer in the gall liquor, the galling is repeated, and it is dipped in the dye a greater number of times, and left in it also for a considerable time. This method neither improves the dye nor the silk ; but is merely used to give profit to the vender where silk is sold by weight.
Silk-dyers preserve the black bath for silk for several years ; when its effect be comes weak, it is renewed by adding more of its ingredients, and when the grounds accumulate too much, they are taken out.
While the silk is preparing for dyeing the bath is heated, taking to stir it occasionally, that the grounds, which fall to the bottom, may not acquire too much heat ; it should never be heated so as to boil. Gum and solution of iron are added, in proportions depending on the differ ent processes used : and when the gum is dissolved, and the bath near boiling, it is left to settle for about an hour. The silk is then dipped into it, being in ge neral at first divided into three parts, each of which is put in successively.
Each part is afterwards wrung gently three times, and hung up in the air after each wringing. '['he action of the air deepens the shade, and the wringing out the liquor prepares the silk to imbibe a fresh quantity.
After this the bath is again heated, and as much gum and sulphate of iron is put in as at first. The operation is repeated twice for light blacks ; but for the heavy blacks three times ; and after the last the silk is left in the bath for twelve hours. Sixty pounds of silk are commonly dyed at one operation.
After the dyeing is finished, the silk is rinced, by turning and shaking it in a ves sel of cold water.
Silk when dyed is extremely harsh ; to soften it, a solution of four or five pounds of snap to every hundred pounds of silk is poured through a cloth into a large vessel of water ; being well mixed, the silk is put in, and left about a quarter of an hour, after which it is wrung out and dried.
When raw silk is dyed, the galling is performed with cold liquor, to preserve its natural gum, and the elasticity which it causes. If the gall liquor is weak, the silk is left in it for several days ; liquor that has been employed for other silk is generally used, and silk which has natu rally a yellow hue is preferred. The raw silk thus prepared is dyed in the cold bath ; it takes the dye readily, and the water in which other silk has been rinced suffices to communicate it, if sul phate of iron be added. It requires more or less time to lie in the rincings, accord ing to their strength : sometimes three or four days are necessary, after which it is washed, and beetled once or twice ; but not wrung, that its elasticity may not be injured. It may be dyed more speedily by shaking it over in the cold bath after galling, and then airing it, and repeat ing these operations a few times ; after which it is to he washed and dried as above.
Macquer describes a more simple pro cess, with which they dye velvet black at Genoa ; it is as follows.