276

DYEING AND CALICO PRINTING.

only deprived of its entire quantity of water a little below red heat. Itssolubility in water varies considerably, according to the temperature :—

100 parts of water at o° dissolve 3*39 parts of alum.

1* » 20 0 15-13

i) 4®° >) 3°'9 2 n ”

5, 5, ioo° „ 357*48 „ »

The solution of alum has a sweetish astringent taste, and exhibits an acidrea&ion. A concentrated aqueous solution of alum destroys the blue colourof ultramarine.

Ammonia-alum (NH 4 0,S0 3 -j-Al 2 03,3S03 +24HO) contains ammonia, 3*89percent; alumina, 11*90; sulphuric acid, 36*10; water, 48*11.

100 parts of water at o° dissolve 5*22 parts of this kind of alum.

„ „ 20 0 „ 13*66 „ ,, „

„ 5 » 4 °° 55 27*27 „ „ „

I) 55 IOO ,, 421*90 ,, . 5) 55

When ammonia-alum is strongly heated, sulphate of ammonia, water, andsulphuric acid are driven off, while pure alumina is left behind : this propertymay, of course, be readily applied as a test.

Neutral or cubical alum (K 0 ,S 0 3 +A1 2 0 3 .2S0 3 ) is obtained when 12 partsof alum and 1 part of slaked lime are boiled in water : on cooling, cubical-shaped crystals are deposited, which are valued in dyeing operations onaccount of their freedom from acid. The composition of such an alum is—Sulphuric acid, 33*95 ; alumina, 11*40; potash, 9*04; water, 45*61.

Sulphate of alumina—also known as cake alum, patent alum, and concen-trated alum—is a salt manufadured on the large scale by dissolving previouslycalcined alumina (a pure clay, as much as possible free from iron) in concen-trated sulphuric acid, the quantity of which is so regulated that there is ratherexcess of alumina. The formula of this salt is AlaC^SC^-f- 16HO. Not-withstanding yellow prussiate of potassa is purposely used for purifying thissalt from iron, it is often found to contain some of that metal, and it is notunfrequently contaminated with Prussian blue. As regards its use in dyeingand calico-printing, it makes good red liquors, the technical name for thosealumina mordants which yield red dyes with madder. It is stronger, weightfor weight, than alum, containing at least a third more alumina, and conse-quently going further; but its composition is not constant, and the experienceof all pra&ical dyers and printers is that, although its use effects a saving ofone-fourth of acetate of lead in the preparation of acetate of alumina, it isnot by any means a suitable substitute for alum (potassa or ammonia), sinceit has been found that acetate of potassa and acetate of ammonia are of realuse in the mordanting process: it is, moreover, by no means so easy anoperation to get sulphate of alumina quite free from iron. According toWildenstein, some of it contains even as much as 11 per cent of sulphate ofprotoxide of iron.

Nitrate of alumina and muriate or hydrochlorate of alumina (chloride ofaluminium) are sparingly employed : they are made by mixing, in the firstcase, alum and nitrate of lead; and in the second, alum and chloride of cal-cium, known also as muriate of lime in the trade. Nitrate of alumina iseasily decomposed, parting with its acid at a comparatively low temperature.