The Textile Mercury, 13.12.1890
When a solution of tannin is added to a solution of a basic coal-tar dye, the latter is precipitated as colour lake, insoluble or nearly so. The complete precipitation is in some cases prevented by the acid which is present in combination with the colour-base, which is duo to the solubility of the colour take in this acid; by neutralising with soda this action can be prevented and the colour completely precipitated.
Justus Koechlin has found that for 4 parts of magenta, there are required 5 parts of tannin and two parts of soda for complete precipitation of the dye-stuff; for 4 parts of malachite green, 5 of tannin, and 1 of soda are required; and for parts of methyl green 10 parts of tannin and 4 parts of soda.
G. H. Hurst, in his work on "Aniline lake making," gives from 1 to 1½ parts of tannin for each part of colouring matter used, says nothing of soda being required, and further states that the whole of the colouring matter is deposited. If the tannin is greatly in excess there is a slight tendency for the colour lake to be re-dissolved.
The application of tannin in the dyeing of cotton is carried out as follows:
—The yarns or tissues are first passed through and allowed to steep in it solution of tannin containing ¼ lb. of tannin in a gallon of water, using the bat bat from 50° to 60° C., and allowing the fabrics to remain in six hours. For delicate colours it is necessary to use aa pale a tannin as it is possible to get; for dark shades the tannin may be replaced with economy of to cost by about twice its weight ot sumac extract or five times its weight of sumac. No strict rules can be laid down as to the quantity of tannin to be used, as it depends much on the quality of the tannin materials employed and on the kind and quantity of dyestaff used. Generally about 2 to 3 per cent. of tannin is employed for medium or pale shades, and 4 per cent. for dark shades, these quantities being calculated on the weight of cotton to be dyed.
If the tannined cotton were entered directly into the dye-bath, some of the tannin would become dissolved off the fibre, and, passing into the bath, would precipitate some of the colouring matter out to the bottom of the bath. This would also get on the fibre. and being loose would cause the latter to be dirty or rub very much. To prevent this the tannin is fixed on the fibre by means of a metallic mordant, such as antimony or tin. This ia done by passing the tannined cotton through a solution of a metallic salt. A large number of these are and have been used, alum, tin crystals, antimony chloride, and other antimony preparations, etc. Of all these, tartar emetic has been found to be the one that gives the best results, and is the almost universal mordant or fixing agent for tannin now in use. This is because it contains a fairly large proportion — about 43 per cent. — of valuable metal, is neutral, and any free acid that may be formed front it does not appreciably affect the colour or prevent its dyeing properly, aa happens with other dyeing preparations and salts of antimony. Many substances have been proposed and offered as substitutes for tartar emetic, but as yet none have been found to equal it.
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