The silk, before it is put into this bath, may be dipped in a solution of alum, in which it should remain only a very short time. Silk, dyed in this manner, is free from the reddish shade given by the blue vat, and from the greenish cast of com mon Saxon blue.
Of dyeing Cotton and Linen blue.
The vat for dyeing cotton and linen blue should contain, according to M. Pileur d'Apligny, about 120 gallons. The quantity of indigo used is generally from six to eight pounds. This indigo, after being pounded, is boiled in a ley drawn off clear from a quantity of lime equal to the indigo, and double its weight of pot ash. The boiling is continued till the indigo is thoroughly penetrated with ley, which should be carefully stirred all the while.
During the boiling of the indigo, an equal weight to it of quicklime is to be slacked; about twenty quarts of warm water are added, and in this is dissolved sulphate of iron, twice the weight of the lime. When die solution is completed, the liquor is poured, into the vat previous ly half filled with water. To this the so lution of indigo is added, and the rest of the ley not used in boiling it. After this the vat must be filled up to within two or three inches of the brim, and be raked two or three times a day, till it is fit for dyeing ; which generally happens in 48 hours, or sooner, according to the tern perature of the air. Some add to this vat a little bran, madder, and woad.
In the process used at Rouen, which is simpler, 20 pounds of indigo, macerated for a week in caustic ley, which will float an egg, are ground in a mill ; three hogs. heads and an half of water are then put into the vat, and afterwards twenty pounds of lime. When the lime is thoroughly slacked, the vat is raked, and thirty-six pounds of copperas are put in. When the solution of this is complete, the ground indigo is put in through a sieve. On the same day it is raked seven or eight times : and after having stood 36 hours it is fit for dyeing.
Bergman recommends a still simpler bath, composed in the proportions of three drachms of powdered indigo, three drachms of copperas, and three drachms of lime, to two pints of water. This being well raked will, in the course of a few hours, be fit for use.
The solution of indigo in sulphuric acid has been hitherto only used for dyeing wool and silk. The affinity of ve getable substances for indigo is not suffi ciently strong to separate it from the sulphuric acid. It cannot therefore be
employed to advantage in dyeing cotton or linen.
Attempts have been made to dye cloth with Prussian blue, but no method has yet been found to make this colour apply it self evenly, sufficiently certain and per fect for general use. This process de serves farther experiments, as the colour produced by it was very beautiful, and not liable to change, though exposed to all the vicissitudes of the air. But dust and rubbing injure it ; and any touch of an alkaline liquor destroys it altogether. The process in which stuffs, previously impregnated with alum and copperas, are submitted to a solution of Prussian al kali, seems that most likely to succeed in diffusing the dye equally, if improved by farther trials. Perhaps also a solution of caustic alkali might form a sufficient solvent for the Prussian blue, if ground with it, to admit of its being used in some what the same way as indigo.
Of dyeing Wool red.
Red colours are of various shades, ac cording to the nature of the colouring matters used. They all require mor dants to render them permanent. The principal shades of red are, scarlet, crim son, and madder red. ' Madder red is only employed for dye ing coarse woollen stuffs. To produce this red, the stuffs are first boiled for two or three hours with alum and tartar ; they are then left to drain, slightly wrung out, and then put into a linen bag, and car ried to a cool place, where they are to remain a few days. Some recommend five ounces of alum and one of tartar to each pound of wool ; by increasing the proportion of tartar, a deep and perma nent cinnamon colour is produced instead of red ; others advise to use only a se venth part of tartar. madder bath should not exceed the temperature which the hand can bear ; if let to boil, the co lour will be different from that required. `When the water is at this heat, Hellot re commends half a pound of grape madder to be put into it for every pound of wool to be dyed. It is to be well stirred before the wool is introduced, which should re main in it for an hour without boiling, except for a few minutes towards the end of this period, to make the combination of the colouring matter with the stuff more certain.