MORDANTS.

535

A double chloride of aluminium and ammonium applied to cotton yieldswith the woods a variety of browns, such as cinnamon and Esterhazy. Itwould probably repay a closer investigation.

Iron Mordants.

Copperas (Green Vitriol, Protosulphate of Iron, or Ferrous Sulphate) isgenerally manufadtured from the soft whitish iron pyrites, or sulphuret of ironfound in the coal measures, and familiarly known as “ brass lumps.” Whenexposed to air and moisture the latter is decomposed, hydrogen gas escaping,and oxygen being absorbed, whence the sulphide of iron is converted intosulphate of ferrous oxide. As the amount of sulphuric acid generated by theoxidation of the sulphur is often more in quantity than the iron is able tosaturate, old iron of all kinds is added to the liquid, which drains from theheaped-up beds of pyrites. Copperas forms semi-transparent crystals of apale greenish-blue, and contains 45 per cent of crystalline water, on the ex-pulsion of which, by careful heating, a white powder remains, harsh tothe touch. The crystals of copperas dissolve in about ij parts of cold,and in one-third its weight of boiling, water. At common temperaturesit oxidises and turns brown, being partially converted into ferric sulphate(sulphate of the peroxide of iron). This change takes place the more readilyin proportion as the copperas and the air are damp, and as the crystals havebeen formed from a neutral solution. Copperas should be chosen welldrained from the mother-liquor, forming bold, hard, clean crystals of adecided green colour. If it is soft, dull, and of a whitish or greyish green,known in the trade as “ milky,” the presence of alumina is to be suspedted.To ascertain this point a small portion is dissolved in water, and boiled withpure nitric acid. To this liquid, whether clear or not, a large excess of purecaustic soda, preferably that prepared from metallic sodium, is added. Thewhole is digested in a clean, covered iron vessel, diluted with distilled water,and filtered. If a white precipitate appears on adding to the clear filtrate asolution of pure sal-ammoniac, the copperas contains alumina. Most com-mercial samples will be found to contain this impurity to a trifling extent. Ifthe pyrites employed have been ill seledted and are contaminated with alum-shales, or if the copperas has been crystallised from a highly concentratedsolution, sulphate of alumina may be present to a very serious extent, and mayrender the copperas pradtically'worthless. The presence of alumina must beregarded rather as the result of mismanagement than of intentional fraud.Zinc and copper are rarely present, and never, probably, in injurious doses.The latter might be very readily detedted by dissolving a little of the sample,boiling with nitric acid, and adding to the liquid ammonia in excess. Theprecipitate which subsides is then filtered off. If the clear filtered liquid showsa blue tinge, copper is present. Lime is sometimes dusted over copperas togive it a brownish appearance, for which some consumers have a preference.The applications of copperas in dyeing and printing are less extensivein the present day than was formerly the case. In producing logwood blacksupon wool it has been largely replaced by the bichromate of potassa, at the cost,it is true, of the stability of the resulting shade. For saddening olives, drabs,clarets, &c., and for cotton blacks, it has been generally discarded in favour ofa nitrate of iron. Some dyers are very partial to an.article sold as “ calcined