No one, therefore, should attempt to have a woad vat or vats, unless he can keep them nearly always at work. When worked down in a moderate time, and replenished with lime, woad, indigo, &c., working out and replenishing in, there can be no danger. On the other hand, in proportion as the vat is out of condition, although partially recovered, it must always be with more or less loss.
Methods of dyeing BLUES.
Whether the goods be cloth, or skeins of yarn, they must, in all cases, be first wetted out and wrung, and then put into the vat, worked in it, taken out and aired, that they may turn from green to blue; and, if necessary, they must be put in again.
There is no difficulty in dyeing dark blues, by repeated dippings; but if light blues be dyed in vats which are nearly exhausted, they will not be bright.
Blue vats, upon a large scale, are now mostly heated by steam; they are then, with little trouble, always in a state for working, without the necessity of re-heating. They are very convenient for light colours, even after they become very weak. In some instances, in order to dye light colours to the best advantage, it would be advisable to set a vat on purpose, which should be strong in woad and weak in indigo; because the colour would be given more slowly, and the light colour obtained from them with much more facility.
To dye wool with lac-dye, SCARLET and CRIMSON.
We have mentioned lac-lake and lac-dye in page 12. Lac-lake is of very uncertain quality, having many heterogeneous substances mixed with it. Lac-dye is very superior to lac-lake. Lac-dye is much used for dyeing woollen yarn scarlet and crimson, for carpets and hearthrugs. It is used with a peculiar spirit, which may be purchased of the dry-salters. Some think that this colouring material is nearly equal to cochineal; the author has, however, never seen any thing dyed with it equal to the colour obtained from cochineal, although it affords, nevertheless, a good scarlet.
Lac-dye is used by being powdered and put into a stone pan, (the quantity must be in proportion to what is likely to be used), with a portion of the above-named lac-spirit sufficient to make it about as fluid as treacle; it must be stirred with a glass-rod or a tobacco-pipe. Some use alum and tartar as a preparation, and some not. After putting the mixture of lac-lake and spirit in the copper with a proper quantity of water, add the goods and work them at a boiling heat. For scarlet add quercitron bark, for crimson, archil.
Lac-dye may be, however, prepared for dyeing, by submitting it, in powder, in a leaden vessel, to the action of sulphuric acid, in the proportion of not more than one part to two of the dye; and after the lac-dye is dissolved, the acid may be neutralized by carbonate of soda. With suitable mordants to the cloth or yarn, the colour may be then applied. Other processes for the employment of this dye are also adopted, but we have no room to detail them. (See Ure's Notes on Berthollet.)