In order to ascertain if any of the sulphur trioxide was vola- 
tilised, the residue was dissolved in 5 per rent, caustic soda solution, 
the sulphur content of which was known, and the sulphur trioxide 
determined. The results proved that there was no appreciable loss 
of SO, 
NaOIT soluble SO., in raw mineral 37.09 per cent. 
NaOII soluble S0 2 in mineral after 
roasting' , . . ♦ , . 36.96 „ 
The roasted mineral was readily soluble in lukewarm 5 per 
cent. NaOII solution, whilst the raw mineral required several 
minutes on a water oven to complete solution. 
The roasted mineral was only very slowly attacked by water, 
a portion of it dissolving and at the same time producing a volu- 
minous precipitate of aluminium hydrates. A sample of commer- 
cial potash alum ignited at the same temperature proved to he 
fairly readily soluble in warm water and only produced a slight 
precipitate. The foregoing suggested that, on roasting al unite at 
the temperatures mentioned, a basic sulphate of aluminium was 
formed, which, on the addition of water, hydrolysed and pre- 
cipitated aluminium hydrate, it was considered possible that the 
precipitate formed may he a basic sulphate of aluminium, but, on 
dissolving it in hydrochloric acid and adding barium chloride solu- 
tion, only traces of sulphates were detected. 
The reactions taking place at this stage may be expressed 
thus : — 
2 [K 2 S0 4 • 3H0A1S0 4 * 3A1 (OH) 3 ] heated— > 
Molecular weight. 1658-24. 
2 K 2 SC) 4 - 6H0A1S0 4 *3A1 2 0 3 -f 9H 2 0 
1496-06 162*18 
On the addition of water to the roasted product the basic sul- 
phate formed dissociates, precipitating aluminium hydrate, leaving 
aluminium sulphate in solution, thus: — 
6HOALSO4 2A1 2 (S0 4 ) 3 -f 2Al(OH) a 
Basie alu- Aluminium Aluminium 
minium sulphate. hydrate, 
sulphate. 
The completed equation for the formation of alum from roasted 
al unite is — 
2K a S0 4 + 2A1 3 (S0 4 ) 3 + 48H..O — > 4TCA1(S0 4 ) 2 - 48H a O 
Potash Alum. 
Alum is not formed by roasting but only by a subsequent series of 
reactions after the addition of water. It therefore appears that 
the effect of heat at the above temperature is to remove the water 
immediately attached to the aluminium, shown in the formula as 
attached to the potentially free alumina. 
