By whatever process, or from whatever materials alum is obtained, it is absolutely necessary for the successful and economical conduct of its manufacture, that the precise composition of the mineral or minerals employed should be exactly known. This can only be determined by actual analysis, which should be extended to several parts of the same bed, and particularly to the upper and lower strata, which frequently differ in composition from each other, and thus require different treatment, or may be most advantageously employed in combinations with each other. The necessity of this will be seen by reference to the composition of the following minerals, of which the top contains a larger proportion of iron-pyrites than the bottom, and the two require to be mixed, to equally diffuse the sulphuric acid generated by the calcination, &c., to which they are subjected.
The following is the per-centage composition of certain alum shales:—
| Whitby, Yorkshire. (Richardson.) | ||
| Top rock. | Bottom rock. | |
| Sulphide of iron (pyrites) | 4·20 | 8·50 |
| Silica | 52·25 | 15·16 |
| Protoxide of iron | 8·49 | 6·11 |
| Alumina | 18·75 | 18·30 |
| Lime | 1·25 | 2·15 |
| Magnesia | ·91 | ·90 |
| Oxide of manganese | traces | traces |
| Sulphuric acid (SO3) | 1·37 | 2·50 |
| Potassa | ·13 | traces |
| Soda | ·20 | traces |
| Chlorine | traces | traces |
| Coal | 4·97 | 8·29 |
| Water | 2·88 | ·00 |
| Loss | 4·60 | (?) |
| ——— | ——— | |
| 100· | 100· | |
| Campsie, near Glasgow. (Ronalds.) | |||
| Top rock. | Top rock. | Bottom rock. | |
| Sulphide of iron (pyrites) | 40·52 | 38·48 | 9·63(?) |
| Silica | 15·40 | 15·41 | 20·47(?) |
| Protoxide of iron | ... | ... | 2·18 |
| Alumina | 11·35 | 11·64 | 18·91(?) |
| Lime | 1·40 | 2·22 | ·40 |
| Magnesia | ·50 | ·32 | 2·17 |
| Oxide of manganese | ·15 | ... | ·55 |
| Sulphuric acid | ... | ... | ·05 |
| Potassa | ·90 | ... | 1·26 |
| Soda | ... | ... | ·21 |
| Carbon or bituminous matter | 27·65(?) | 28·80 | (?) |
| Coal | ... | ... | 8·51 |
| Water | ... | ... | 8·54 |
| Loss | 2·13(?) | 3·13 | 1·59(?) |
| ——— | ——— | ——— | |
| 100· | 100· | 100· | |
Alum-rock, or alum-stone, is a species of impure alunite, and is not of very common occurrence. That of Tolfa, near Civita Vecchia, according to Klaproth, consists of—
| Silica | 56·5 |
| Alumina | 19· |
| Sulphuric acid (SO3) | 16·5 |
| Potassa | 4· |
| Water | 3· |
| Loss | 1· |
| ——— | |
| 100· |
which exhibits an excess of about 3% of sulphuric acid, and about 14% of alumina, more than are requisite to form alum with the 4% of potassa; proportions which, therefore, require to be supplemented with a potassium salt during the process of manufacture. The alum-stone of Mont d’Or contains, according to Cordier, 1·4% of oxide of iron.
The presence of lime in alum-ore is most prejudicial, owing to its affinity for sulphuric acid being greater than that of either alumina or iron. Ores containing it in any quantity are, therefore, unfitted for the manufacture of alum. Magnesia is also prejudicial; but in this case the sulphate of magnesia left in the mother-liquors is not wholly valueless, as it may be crystallised and sold as
‘Epsom-salt,’—a thing which is actually done in some English alum-works.
The potash-salt employed by the alum-makers is either the sulphate or the chloride—chiefly the latter; its sources being the waste liquor of soap-works, saltpetre refineries, and glass-houses. Wood-ashes, although rich in potash, do not answer well unless freed by lixiviation from the large amount of carbonate of lime which is always present in them.