Some manufacturers modify their "black-irons" by adding a solution of brown sugar of lead, in proportions below 95 lb. to 138 lb. of the copperas originally employed. In consequence, the lead precipitates its equivalent of the sulphuric acid present, its place being taken by the acetic acid previously present in the sugar of lead. If any free nitric acid exist, it will combine with some of the iron, formerly held by the sulphuric acid ; whilst a corresponding proportion of acetic acid will be set free. A black-iron thus corrected will neither injure tissues, colours, nor printing machinery. Hence it may safely be used for burl or burr dyeing.
When woollen pieces have been dyed, they are often found more or less spotted over with small grey specks, upon which the dye, as mixed for woollens, has not taken effect. These are due to small portions of vegetable matter, which have become entangled in the wool. To render the piece of a uniform black, it must therefore undergo a second process, which is, in fact, cotton dyeing. For this purpose, the " hurling " iron is required, and the nitrate of iron must be very accurately pre pared. If too acid, it may discharge or modify the colour of the wool ; and if too dead, it may occasion rust-smears.
"Common iron " is used for " saddening," as it is called, suob colours as olives, dark-browns, drabs, &c. It is sharper than black-iron, and is a perfect per-salt (ferric salt).
It may be obtained by the process just recommended for black-iron, using 130-135 lb. of nitric acid to 5 cwt. of copperas. If any copperas remains in the cask unattacked, the liquid is drawn off before diluting with water.
A saddening-iron for drabs is often made as follows :—Ordinary nitric acid is let down to 34° Tw., and of this, 100 lb. are placed in a large stoneware pan. In it, are dissolved, firstly, 4 lb. of clean sorap-iron, and afterwards, as much copperas is gradually added as the acid will take up, which is generally about I cwt. It is finally set at 60° Tw. with water.
So-called " blue-irons " were formerly very much used for dyeing Prussian blues upon cotton yarns, and upon the cotton warps of mixed piece-goods where the weft had been dyed with an aniline blue, and in producing a blue base for greens. These uses are now of very much leas importance, since the discovery of coal-tar colours, which work well upon vegetable fibre.
Blue-irons are sharper than black and common irons, as their acidity is to a great extent neutralized by the prussiate of potash. A blue-iron for cotton yarn and unmixed cotton piece-goods may be made by the first process for a common iron, viz. 5 cwt. copperas to 130 lb. nitric acid.
For light-blues on the warps of coburgs, delaines, &c., double aquafortis (nitric acid) is let down to 32° Tw., and clean scrap iron is dissolved in it as long as there is a good action, and reddish fumes are thrown off. It should mark 42°-43° Tw.
For darker blues, verging towards a violet, put 24 lb. nitrate of soda, freed from common salt, into 15 gal. water, and stir till dissolved. Then add by degrees 20 lb. of oil of vitrol, feeding with scrap iron as required. The heat must not be allowed to get very high. In cold weather, both the
oil of vitriol and the iron are added more rapidly than in summer. This nitrate of iron contains sulphate of soda, and, if kept for any length of time, deposits a sediment.
Analytical processes are of little avail in finding the practical value of an " iron," since the quality of any sample depends not merely upon the proportions of the ingredients, but upon the manner in which they are combined.
Pyrolignite of iron, likewise called acetate of iron, black-liquor, and iron-liquor, is leas nsed in dyeing than in calico-printing, where it plays a great part, especially in the madder-style. It is generally prepared on a large scale at wood-vinegar works. Scrap-iron is submitted to the action of raw acetic (pyroligneous) acid in a series of vats, till thorough saturation is obtained. It has been perfectly well ascertained that the purified acid, free from tarry matters, gives a much less satisfactory product. These impurities retard the conversion of the iron into a salt of the peroxide (ferric salt), in which state its efficiency is much lessened. Black-liquor is also sometimes prepared by the consumer by a process of double decomposition.
A solution of copperas is mixed with one of brown sugar of lead, or crude acetate of lime, and after settling, the clear liquid is drawn off for use. (For partieulare concerning both these processes, see p. 31.) The specific gravity of black-liquor, as met with in commerce, ranges from down to about 10° Tw. Its properties are very considerably modified by the degree of strength at which it is used. At 6° Tw., it gives with madder a full and fast black. If diluted down to 4° Tw., or lower, it yields with Madder and artificial madder colours various shades of purple and lilac. These colours are called by Continental writers violets. The words purpur (German) and poupre (French) are applied to shades between crimsons and reddish-violets. Artists in England use the word "purple" in the same sense, and not as synonymous with "violet" Used in combination with red-liquor in different proportions, it gives shades of chocolate.
Along with black-liquor, may be mentioned a proposed substitute, which possibly deserves more attention than it has yet received—hyposulphite of iron. It may be prepared by mixing a solution• of copperas with crystals of hyposulphite of soda, or by decomposing copperas with hyposulphite of lime. According to Kopp, the fixation of the base of this mordant upon the fibre is very slow and very intimate, and the tissues are not in the least injured. Other authorities, among them C. O'Neill, maintain that the action of the hyposulphite is irregular, and that the colours produced are consequently uneven.
It has been proposed by Persoz to dissolve pyrophosphate of iron in ammonia. The solution, if printed on cotton, and dried very slowly, gives, when dyed with the madder-colours, exceedingly pure purples and lilacs. It is said that cloth thus mordanted can be successfully dyed in becks which no longer give any colour on cotton prepared with black-liquor.