538

DYEING AND CALICO PRINTING.

receipt may be followed :—Let down ordinary double aquafortis to half-strength(32° Tw.) with water. Add to this gradually scrap iron, which should be cleanand free from rust, so long as it is dissolved briskly with rise of orange fumes.It will stand at 43 0 Tw.

If a deeper blue, of a rich “ bloomy ” tone, is required, either of the two

following receipts will serve the purpose :—

No. 1. Nitrate of soda, freed from chlorides.24 lbs.

Oil of vitriol .20 ,,

Water.15 gals.

Iron, scrap, as much as required.

Pour the water upon the nitrate of soda, and stir to promote solution. Whendissolved, add the acid and the iron by degrees, not letting the mixture gettoo hot.

No. 2. Nitrate of soda. ..16 lbs.

Oil of vitriol .20 ,,

Cold water. 13 gals.

Scrap iron as before.

Mix the ingredients as above stated, and keep up a steady afiion, taking carenot to let the heat get excessive. It is generally preferable not to add thewhole of the oil of vitriol at once.

Both these “ blue irons ” are unstable, and if kept many days are apt todeposit a large amount of sediment.

In “ black irons ” perfedt saturation of the acid is a matter of greatmoment. In black-dyeing no substance is employed which, like the prussiateof potash, is able to neutralise any excess of free acid, which, if present,damages the colours, the goods, and, in case of printing, the colour-dodtors,and even the copper cylinders. Still it is needful to avoid the oppositeextreme, for if the nitrate is overloaded, so that the oxide of iron is notslowly and regularly delivered to the fibre, the goods will, when completed,appear spotty and cloudy, or may even be smeared with buff patches, whereoxide of iron has been deposited too irregularly and rapidly to combine withthe tannin. For blacks, as a general rule, a nitrate of iron made from copperasis preferable to one prepared from the metal. The receipt given above for ablue iron for cotton skeins will serve very well for blacks, the nitric acid, forthe weight of copperas given, being reduced to 120 lbs.

For blacks, it is not necessary that the whole of the iron should be per-oxide. Better shades can, in fadt, be produced with a nitrate of iron in whicha proportion of proto-salt is still present. The result of the process, as workedwith the above modification, is diluted with water to about 50° Tw., if fordyeing, but to 8o° if for printing.

Some makers add to black nitrate of iron a certain amount of brown sugarof lead, which should not exceed 95 lbs. to 138 lbs. of the copperas originallyused. The result is that the oxide of lead combines with its equivalent of thesulphuric acid present, and falls down as insoluble sulphate of lead, whilst theacetic acid, formerly combined with the lead, seizes upon the oxide of iron inits room. Should, however, any excess of nitric acid be present, this seizesthe iron in preference to the acetic acid, a portion of which remains free. Asfree acetic acid is much less injurious to fibres and to colours than free nitric