Home › Recipes › To dye wool and woollen cloths of a blue colour

To dye wool and woollen cloths of a blue colour

Dissolve one part of indigo in four parts of concentrated sulphuric acid, to the solution, add one part of dry carbonate of potass, and then dilute it with eight times its weight of water. The cloth must be boiled for an hour in a solution, containing 5 parts of alum, and 3 of tartar, for every 32 parts of cloth. It is then to be thrown into a water-bath previously prepared, containing a greater or smaller proportion of diluted sulphate of indigo, according to the shade which the cloth is intended to receive. In this bath it must be boiled till it has acquired the wished for colour. The only colouring matters employed in dyeing blue, are woad and indigo. Indigo has a very strong affinity for wool, silk, cotton, and linen. Every kind of cloth, therefore, may be dyed with it, without the assistance of any mordant whatever. The colour thus induced is very permanent. But indigo can only be applied to cloth in a state of solution, and the only solvent known is sulphuric acid. The sulphate of indigo is often used to dye wool and silk blue, and is known by the name of Saxon blue.

It is not the only solution of that pigment employed in dyeing. By far the most common method is, to deprive indigo of its blue colour, and reduce it to green, and then to dissolve it in water by means of alkalies. Two different methods are employed for this purpose. The first is, to mix with indigo a solution of green oxide of iron, and different metalic sulphurates. If, therefore, indigo, lime, and green sulphate of iron, are mixed together in water, the indigo gradually loses its blue colour, becomes green, and is dissolved. The second method is, to mix the indigo in water, with certain vegetable substances wich readily undergo fermentation, the indigo is dissolved by means of quick lime or alkali, which is added to the solution. The first of these methods is usually followed in dyeing cotton and linen, the second, in dyeing wool and silk.

In the dyeing of wool, woad and bran are commonly employed as vegetable ferments, and lime as the solvent of the green base of the indigo. Woad itself contains a colouring matter precisely similar to indigo; and by following the common process, indigo may be extracted from it. In the usual state of woad, when purchased by the dyer; the indigo, which it contains, is probably not far from the state of green pollen. Its quantity in woad is but small, and it is mixed with a great proportion of other vegetable matter. When the cloth is first taken out of the vat, it is of a green colour; but it soon becomes blue. It ought to be carefully washed, to carry off the uncombined particles. This solution of indigo is liable to two inconveniences, first, it is apt sometimes to run too fast into the putrid fermentation; this may be known by the putrid vapours which it exhales, and by the disappearing of the green colour. In this state it would soon destroy the indigo altogether. The inconvenience is remedied by adding more time, which has the property of moderating the putrescent tendency. Secondly, sometimes the fermentation goes on too languidly. This defect is remedied by adding more bran, or woad, in order to diminish the proportion of thick lime.