ALUM.— Manufacture from Alum Stone.
155
atmosphere, while, simultaneously, the iron is oxi-dized, and ferrous sulphate and aluminium sulphateare produced. These are separated from the rest ofthe mass by lixiviation with water. The solution isthen concentrated and potassium chloride added.The iron and potassium salts, are transformed intoferrous chloride and potassium sulphate, and thelatter combining with the aluminium sulphate, con-stitutes the alum of commerce.
Alum may be prepared by decomposing clay withsulphuric acid. The decomposition is effected bycalcining pure clay, grinding the mass to powder,and mixing it with about a half per cent, of sulphuricacid. This mixture is then to be heated in a furnacetill the mass becomes very thick; afterwards left torepose for a month or more, and then lixiviated withwater to wash out the aluminium sulphate. Theaddition of potassium sulphate converts it into alum.
Manufacture of Alum. —Two alums only arcapplied in the arts; these are composed of aluminiumsulphate, A1 2 (S0 4 ) 3 , in combination with potassiumsulphate or ammonium sulphate, thus:—Potassio -aluminic sulphate, A1 Iv(S 0 4 ) 2 12H 2 0; ammonio-alu-minic sulphate, A1(OTI 4 ),(S0 4 ) 3 12II0.
The first of these, potash alum, is by practical mensimply called alum, whilst the second variety is dis-tinguished by the name of ammonia alum. The latteris easily distinguished from the former by its giving. off ammonia when triturated with quicklime. P otaslialum is preferred by turkey-red dyers; but both areused by dyers and calico-printers solely for their
Fig. l.
crystals; the purity of the alumina in this salt, andwhich is necessary in the applications of alum,enables the purchaser to pay for the sulphuric acid,water, and alkali, although they are useless exceptfor effecting the crystallization. Aluminium sulphateis only with great difficulty separated from the manyother extraneous salts (particularly ferrous sulphate)that accompany it in the manufacture; whilst alum,from the ease with which it dissolves in hot water,and its sparing solubility in cold, is readily separablefrom any adventitious substances.
Production of Alum from Alum Stone.— Alumis obtained in much larger quantity from alum rock,a formation of volcanic origin , than from any othersource. This is a massive, granular, only partiallycrystalline, transparent, and not homogeneous rock,which frequently incloses quartz, sometimes ironpyrites and manganese ore. Its colour is yellowish,passing into green, grey, red, or brown. The puremineral alum stone, alunite, sometimes occurs in it indistinct crystals, which have been found to consist ofa basic aluminium sulphate, with potassium sulphate,and is, therefore, a basic alum, or, more probably, acombination of neutral potassio-aluminie sulphate,with aluminium hydrate, A1K(S0 4 ) 0 A1 2 0 3 ,1I 2 0.—liammdsbcrg. It differs, therefore, from alum incontaining an excess of alumina. Alum rock is amassive alunite, but of a less pure character; it isfound at Tolfa, near Civita Veccliia; at Monti one,in the Comitats, Beregh, and Zemplin, in Hungary ;at Mont d’Or, in France ; and in the Greek islands,Milo and Nipoglio; but is not of very common oc-currence in other places. The analyses of this rockhave yielded the following results:—
From
Tolfa,
ty
From
Beregszaz,
by
From
Montione,
by
Descotil.
Cordier
Silica,..
56-5
62*3
_
28-4
Alumina,.
19*0
17-5
40-0
31-8
Sulphuric acid.
16*5
12*5
35-6
27-0
Potassa,.
40
1*0
13*8
5-8
Water,.
3-0
5*0
10*0
3-7
Sesquioxide of iron,.
—
—
—
1-4
Loss,.
1*0
1*7
0-6
1-9
100-0
100-0
100-0
100-0
alumina, and in this particular they are accountednearly of equal value; the other constituents beingalmost useless.
The property which alum possesses, of separatingfrom its concentrated solutions in large, well-defined
It will be seen from these analyses that, overlookingthe silica, there is chiefly a deficiency of potassa, and
. ™ also of sulphuric acid, in the rock, for the
formation of alum; but in the mineral fromTolfa, for instance,there is about 3 percent, of sulphuric acidand 14 per cent, ofalumina more than arerequisite to form alumwith the 4 per cent,of potassa. The alum
stone from Beregszaz also contains an excess of 9 percent, of acid and 16 per cent, of earth; that fromd’Or 6 per cent, of acid and 25 per cent, of alumina.At Tolfa, where the alum stone comes to the surface,a quantity of alum is produced proportionate to the