After having washed and beetled the silk, and wrung it out with the jack and pin, in order to separate any soap it may have retained, it is immersed in the alum liquor for eight or nine hours, after which it is wrung out by band over the vat, and washed in a stream of water.
The above quantity of liquor will be sufficient for one hundred and fifty pounds of silk ; but when it grows weak, which is known by the taste, twenty or twenty-five pounds of alum, dissolved as before, must be added, and this addition must be repeated,till the liquor acquires a disagreeable smell, and then it may be employed for stuffs intended for browns, marones, and other dark colours, till it has lost all its strength. The preparation of silk with alum is always made in the cold, because if the liquor should be em ployed hot, the lustre of the silk is liable to be impaired.
Of Cotton.
The several species of cotton differ principally in the length oftheir filaments, their fineness, their strength and colour. They are of different shades, from a deep yellow to a white. The darkest cotton comes from Siam and Bengal, and is of ten made into stuffs in its natural colour. The most beautiful is not always the whitest: it is necessary to bleach it. Processes similar to those employed for • linen may be employed ; but those in which oxygenated muriatic acid has been used are more expeditious, produce a more beautiful white, and prepare the cotton better (according to M. Decroi.
sille) for the reception of a fine colour in dyeing.
In order to dispose cotton to receive the dye, it must be first scoured ; some boil it in sour water, but more frequently alkaline ley is used; the cotton must be boiled in it for two hours,and then wrung out; afterwards be rinsed in a stream of water, till the water comes off clear, and then be dried. The cotton stuffs, which are to be prepared, must be soaked for some time in water, mixed with at most one fiftieth of sulphuric acid, after which they must be carefully washed in a of water and dried. The acid employed in this operation has been observed to take up a quantity of calcareous earth and iron, which would have injured the colours.
Aluming and galling are generally ne cessary in dyeing cotton and linen.
In the preparation with alum, about four ounces of it are required to each pound of the stuff. It must be employ ed with the precautions mentioned in the last article ; sonic add a solution of soda, in the proportion of one-sixteenth of the alum, others a small quantity of tartar and arsenic. The thread is well impreg nated by working it pound by pound in this solution. It is then put altogether into a vessel, and what remains of the liquor is poured upon it. It is left there for twenty-four hours, and then removed to a stream of water, where it is suffered to remain for an hour and a half, or two hours, in order to extract a part of the alum, and then it is washed. In this ope ration the cotton gains about one-fortieth of its weight.
In the operation of galling, different quantities of galls, or other astringents, are used, according to the quality of the astringents, or the effect desired. The galls, powdered, are boiled tor about two hours in a quantity of water proportioned to that of the thread to be galled ; the li quor is then suffered to cool to. a tempe rature which the hand can just support, after which it is divided into a number of equal parts, that the thread may be wrought pound by pound, and what re mains is poured upon the whole toge ther, as described in the process of alum. ing. It is then left for twenty-four hours especially when intended for maddering for black, but for other colours twelve or fifteen are sufficient. After this it is to be wrung out and dried. When stuffs are to be galled, which have already re ceived a colour, the operation must be performed in the cold, that the colour may not be injured. Cotton which has been aimed, acquires more weight in the galling than that which has not un dergone that process. Although alum adheres but in small quantities to cotton, it gives it a greater power of combining both with the astringent principle, and the colouring matter.
Of Flax.
As flax and hemp possess the same pro perties, as far as relates to dyeing, the di rections for one will succeed equally well for the other.