To wash and dry the precipitated alum, it is convenient to employ a small centrifugal machine. After once machining for a few minutes, a little water being added as usual during the operation, the alum appears white and dry, but still retains a small amount of syrup. It is then mixed up with some cold water, and machined a second time, after which it will be found free from sugar and fit for sale. The advantages of the process are :—(1) The removal of potash and ammonia from syrups without much dilution ; (2) the removal of a great deal of the colouring and albuminous matters; (3) a considerable improvement both in taste and odour ; (4) the alum produced is nearly equal in value to the sulphate of alumina used, so that the expense of the process is net great ; (5) tho plant required is of the simplest description, the cost of labour small, and the entire process is of a continuous and rapid character.
This process has now been in constant operation during twelve months at the sugar refinery of Jas. Duncan, of London, where the syrup from many thousands of tens of sugar has been treated with excellent results, several hundred tons of potash alum of good quality being, during the same time, produced, and sold at a fair market price. The process has lately been adopted by many of the principal sugar refiners of the United Kingdom, and it has also been applied successfully in Holland, a lgium. and the United States of America.
For the successful and economical conduct of the manufacture of alum, it is absolutely necessary that by whatever process or from whatever materials it is obtained, the exact composition of the mineral or minerals employed should be carefully ascertained. This can only be effected 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. The presence of lime in slum ores and shales is most prejudicial, since its affinity for sulphuric acid is greater than that of either alumina or iron. The ores, therefore, which are selected should contain as little as possible of this substance. The presence of magnesia is also objectionable ; but in this case the sulphate of magnesia left in the mother liquor is not entirely wasted, as it may be crystallized and sold as Epsom salts ; it is the custom in some English alum works to employ this method of utilizing it.
The salt of potash used for precipitation of the alumina solution is generally either sulphate or chloride, but most often the latter. It is used in the form of waste liquor from soap-works, saltpetre refineries, and glass-works. As we have already stated, the gas-liquor, or crude sulphate of ammonia from the gas-works, is used as the common source of ammonia for precipitation. Salts of soda are rarely, if ever, used for the production of alum, since the resulting alum is very difficult to crystallize. But there is certainly one advantage which soda-alum possesses ; that is, the cost of
sulphate of soda is trifling compared with sulphate of ammonia ; and as the consumption of this latter material is gradually increasing, owing to its high value as a fertilizer, and as the agriculturist is now beginning to see the great value of these nitrogenous products, and as their value is lost in alum, it may ultimately, now that the practicability of producing soda alum on the commercial scale has been demonstrated, even with all the difficulty of crystallization, be a more economical way of producing this double salt.
UsES.—The chief use of alum is in the processes of dyeing and calico-printing, as a mordant. This application depends upon the great affinity of the alumina contained in the alum for textile fibres, and especially wool and cotton ; it cannot, however, be employed in the case of aniline dyes. When steeped in a solution of alum, a basic salt of alumina is formed which adheres to the fabric so firmly that it is never removed by washing. The fabric is by this means enabled to combine with larger quantities of the colouring material, and to retain it more tenaciously (see Dyeing). It is used to clarify liquors of various kinds, and especially water ; to harden tallow, fats, and gypsum ; in the " tawing" of leather, along with common salt ; in the preparation of paper, and of book binders' paste, which contains one-sixth of alum ; in the preparation of the lakes, and of pyrophorus ; to render wood and paper incombustible; to remove greasiness from printers' blocks and rollers ; to prepare a paper for whitening silver, and silvering brass in the cold ; in the bottling of fruits for preservation, the preparation of butter from milk, and extensively in the adulteration of bread, beer, gin, and artificial port. A novel and curious application is in the lining of Milner's safes, which is a mixture of alum and sulphate of lime ; owing to the large quantity of water which it contains, which moistens the inner chamber of the safe when heated, and thus prevents the contents from being consumed, and also to the non-conducting properties of the mixture, after expulsion of the water, this substance assists materially in protecting the interior from injury by fire. In medicine, alum is used as a tonic and astringent in doses of 5 to 20 grains ; as a gargle (1 drachm to half a pint of water) ; and as a ecllyrium and injection (10 to 15 grains to 6 oz. of water). In lead colic, to 1 drachm of alum dissolved in gum-water, and taken every three or four hours, is said to be infallible. Powdered alum is often applied with a came-hair brush in cases of sore throat, ulceration of the mouth, &c. According to Dr. Meigs, a teaspoonful is one of the best emetics in cases of croup.