MORDANTS. 54*

crystallise. It gives results very similar to those produced by the nitrates ofiron made from copperas.

Alkaline Iron Mordants.

Certain compounds of iron are soluble in alkalies, and may be used asmordants :—

1. Take a very neutral persulphate of iron, dissolve it in water, and precipi-tate it with the phosphate of soda. The precipitate—a white powder—can bedissolved in ammonia, and constitutes the mordant in question. According toPersoz, a madder-bath, spent as far as the ordinary mordants are concerned,will still dye goods mordanted with this preparation.

2. A concentrated solution, either of metallic iron or of copperas in nitricacid, is mixed with a strong solution of carbonate of potash. The precipitatewhich first forms re-dissolves in an excess of the alkaline liquid, forming ared solution.

Lead Mordants.

Though several compounds of lead are used in dyeing and printing, thismetal can scarcely be said to give rise to true mordants. The acetate (sugarof lead) and still more the sub-acetate (basic acetate) have a great affinityfor colouring matters, and when the latter is added to the solutions of dye-wares everything dissolved is generally precipitated, and the supernatantliquid left colourless. These lakes are, however, of little value, being void ofdepth and brilliance. This arises from the fadt that a great variety of othersubstances besides the tindtorial principles are also precipitated. Hence thelead-lakes are not articles of commerce, and are not employed in pigmentstyles. The affinity of lead for vegetable fibres is not great, and the liabilityof all colours of which it forms a constituent to become blackened, or at leastdarkened, on exposure to sulphuretted hydrogen is a serious obstacle to its use.It is not a good mordant for wool, where it is liable to injury from the presenceof sulphur. In 1859 Messrs. Perkin and Gray patented a process for fixing theaniline purple or mauve by means of a lead mordant. In dyeing, the goodsare worked alternately in solution of acetate of lead and in ammonia, andthen dyed in the ordinary manner. In printing, acetate of leadis applied tothe cloth, and fixed by means of a mixture of soda crystals and ammonia.The process is not now in common use.

Nitrate of Lead is formed by dissolving litharge in nitric acid with theaid of heat, and allowing the solution to crystallise. It forms large whitecrystals, readily soluble in water. It is used in mixing murexide purple, and,like the acetate in forming chromate of lead in cotton goods, along with thechromates of potash.

Sulphate of Lead, although insoluble in water, has the property of at-taching itself to vegetable fibre, and can thus serve as a preparation for chromeyellows and oranges.

Alkaline Solutions of Lead.

Lead is also capable of playing the part of an acid and combining withalkalies. The plumbite of soda is formed by dissolving litharge in causticsoda as long as anything is taken up. The resulting liquid must be preserved,as far as possible, from contaft with air. The plumbate of soda is formed in