COPPER. (Copper Sulphate, Verdigris, blue vitriol, blue-stone.)
Copper is rarely used as a mordant. It is usually applied as a saddening agent, that is, the wool is dyed first, and the mordant applied afterwards to fix the colour. With cream of tartar it is used sometimes as an ordinary mordant before dyeing, but the colours so produced have no advantage over colours mordanted by easier methods.
Examples.—6 per cent. of copper is used as a mordant for weld to produce an olive yellow. 4 to 5 per cent. is used with old fustic for yellow. 10 per cent. of copper gives to wool a reddish purple with cochineal.
Mordants should not affect the physical characteristics of the fibres. Sufficient time must be allowed for the mordant to penetrate the fibre thoroughly. If the mordant is only superficial, the dye will be uneven: it will fade and will not be as brilliant as it should be. The brilliancy and fastness of Eastern dyes are probably due to a great extent to the length of time taken over the various processes of dyeing. The longer time that can be given to each process, the more satisfactory will be the result.
Different mordants give different colours with the same dye stuff. For example:—Cochineal, if mordanted with alum, will give a crimson colour; with iron, purple; with tin, scarlet; and with chrome or copper, purple. Logwood, also, if mordanted with alum, gives a mauve colour; if mordanted with chrome, it gives a blue. Fustic, weld, and most of the yellow dyes, give a greeny yellow with alum, but an old gold colour with chrome; and fawns of various shades with other mordants.
TANNIN.—(Tannic Acid.)—Tannins are used in the dyeing of cotton and linen. Cotton and linen possess the remarkable power of attracting tannins from their aqueous solution, and when these substances are prepared with tannins, they are able to retain dyes permanently. Cotton saturated with tannin, attracts the dye stuff more rapidly, and holds it. Tannic acid is the best tannin for mordanting cotton and linen, as it is the purest and is free from any other colouring matter. It is, therefore, used for pale and bright shades. But for dark shades, substances containing tannic acid are used, such as sumach, myrobalans, valonia, divi-divi, oak galls, chestnut (8 to 10 per cent. of tannin), catechu.
Cotton and linen are prepared with tannin after they have been through the required cleansing, and if necessary, bleaching operations. A bath is prepared with 2 to 5 per cent. of tannic acid of the weight of the cotton, and a sufficient quantity of water. For dark shades, 5 to 10 per cent. should be used. The bath is used either hot or cold. It should not be above 60°C. The cotton is worked in this for some time, and then left to soak for 3 to 12 hours, while the bath cools. It is then wrung out and slightly washed.
The following gives the relative proportions of the various substances containing tannin:—1 lb. tannin equals 4 lbs. sumach, 18 lbs. myrobalans, 14 lbs. divi-divi, 11 lbs. oak galls.
A few examples taken from various recipes of cotton dyeing:—
For 10 lbs. cotton use 12 oz. tannin.
For 50 lbs. cotton use 10 lbs. sumach.
For 40 lbs. cotton use 10 lbs. sumach.
For 20 lbs. cotton use 2 lbs. yellow catechu or black catechu.
For 20 lbs. cotton spend 3 lbs. of catechu with 3 oz. of blue vitriol.