Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 3, 1802).djvu/249

Rh , in the "Annales de Chimie."

The three principal mordants used in this operation are, oil, galls, and alum.

The greatest caution is necessary in choosing the oil, which ought to be similar to that employed in painting, and to contain a large portion of the extractive principle. Hence, this oil should not be completely saturated with the alkali; but, previously to giving the red dye to the stuff, it ought to be combined with a weak solution of soda (or of pot-ash, if the former alkali cannot be easily procured); and the cotton duly impregnated with this preparation; by which every part of it will thoroughly imbibe the oil. The next process is that of galling, for which purpose, galls only should be employed, as no other vegetable astringent is equally efficacious.

The last mordicant is alum, which not only possesses the property of brightening the red tint produced by madder, but at the same time contributes, by its decomposition, and the fixity of its earth or aluminous base, to give solidity to the colour. In order to judge of its effects in dyeing cotton, it will be sufficient to mix a decoction of galls with a solution of alum. The mixture will immediately become turbid, and a greyish precipitate be formed, which, on being dried, is insoluble both in water and in alkaline ley. Great care, however, is requisite, that the aluminous solution be not too hot, lest part of the astringent principle, obtained from the galls, escape from the cotton, and the alum be decomposed in the immersion; a circumstance by which the power of the mordant is necessarily diminished, and the colour is impaired.

It is, therefore, to be attributed wholly to the united effects of the three principles (oil, the astringent principle, and the earth, or base of alum), which serve as a mordant in dyeing red with madder. If these be employed separately, they will neither produce the same fixity, nor afford a similar brilliancy of colour.  . See.  MOROCCO-LEATHER, the skins of goats tanned and dyed in a peculiar manner by the Turks; but which processes were originally invented in the kingdom of Morocco.

The skins are prepared in parcels, consisting of 36, divided into six bundles, which are folded in the middle, lengthwise, and thrown into a pit full of lime, whence they are taken out, rinsed in pure water, drained, and suspended in the shade to dry, till the hair becomes loose. After carefully taking off the latter, the skins are returned to the lime-pit, for two or three weeks, when they are again rinsed, and passed repeatedly through a decoction prepared of 30lbs. of dog's-dung, and an equal quantity of water. As soon as the skins are thus gradually impregnated, they are thrown into a large vessel for the space of twelve hours, after which they are cleaned with pure water, and immersed for seven or eight days in a watery decoction made of bran.

The skins are now wrung, and thoroughly washed in clear water, to render them soft and pliable: next, they are cured, by spreading a thick layer of common sea-salt, in the proportion of about half a