The stronger mineral acids have considerable antiseptic power, and are of course especially fatal to such ferments as thrive best in alkaline solutions. The use of sulphuric acid in pickling skivers has already been alluded to, and a very dilute solution applied without salt to raw hides prevents putrefaction, though the principal object in using it is to plump the hides and produce a fictitious weight and substance which disappear on tanning. Such hides of course have a powerful acid reaction to litmus. Sulphuric acid in small quantities has been used with advantage in soaking E.I. kips. A very small excess of hydrochloric acid will sterilise putrid effluents, and no doubt nitric or sulphuric acid would have the same effect. The powerful effect of mineral acids on animal fibre, and their solvent action on cements and iron, preclude however, their general use as antiseptics.
More important is the use of sulphurous acid and sulphur dioxide, which, from their mild acidity and great antiseptic powers, are capable of a variety of useful applications. Considerable doubt has been raised as to the germicide power of sulphur dioxide, and it is certain that the dry gas is less effective on dry objects than when applied in solution, or to moist materials, as is almost invariably the case in the tannery. It may possibly be more efficient in its action on some moulds and putrefaction-ferments than on the pathogenic bacteria which have been most frequently used to test the power of disinfectants; but in practice it is found extremely useful in the brewery and in gelatine manufacture, and there is no reason that it should be less so in the tannery.
The gas is most conveniently produced by burning sulphur, which produces double its weight of sulphur dioxide. If used for “stoving” drying rooms and other places infested with moulds, care must be taken to avoid risk of fire. A shallow cast-iron pot set on bricks or sand is generally the most suitable vessel, and the sulphur may be ignited by a piece of red-hot iron or a rag which has been previously dipped in melted sulphur. It is corrosive to metalwork, and bleaches many colours, but does not produce any marked injurious effect on leather, though the sulphuric acid formed by oxidation may, if not removed, ultimately make it tender.
For many purposes a solution of the gas is required, and this is most easily made by burning the sulphur in a small metal or firebrick stove from which the fumes are sucked through a “scrubber,” which, on a small scale, is conveniently made of large glazed sanitary pipes, packed with coke or broken earthenware, over which water is allowed to trickle. The lowest pipe has an opening for a branch pipe, which is connected with the stove and rests on three bricks in a tub, which collects the acid solution and forms a water-seal to prevent the escape of gas. Above the inlet for the gases is fixed a wooden grating on which the coke rests. The scrubber may be 10-15 feet in height and connected at the top with a chimney or steam ejector to produce the draught. The arrangement is illustrated in [Fig. 5]. Another method is to burn the sulphur in a closed cylinder and to force the products through water with an air-compressor or steam-jet injector.
In place of using a scrubber, the fumes may be blown by a steam ejector direct into a tank. This is a very good arrangement for washing and bleaching hair, etc., but where large quantities of solution are required is inferior to the scrubber. Ejectors of hard lead or regulus metal should be used, and are less acted on by the dry gases than by the very dilute moist exhaust from the scrubber (see [p. 335]).
Fig. 5—Sulphurous acid apparatus.
Bisulphites have also strong antiseptic properties. “Bisulphite of soda” (hydric sodic sulphite) solution may be made by supplying the scrubber with solution of soda-ash or washing soda; bisulphite of lime, by using milk of lime or packing the scrubber with chalk or limestone (free from much iron) in place of the coke. In either case a much stronger solution is obtained than with water alone.
Boakes’ “metabisulphite of soda”[8] is a very convenient source of sulphurous acid when the latter is wanted in small quantities. It is an anhydrosulphite, Na2O.2(SO2), and contains 67·4 per cent. of its weight of SO2. One molecule of the salt (= 190) requires one molecule of H2SO4 (= 98) to set free the whole of the sulphurous acid. For many purposes the sulphate of soda formed may be neglected and the acidified solution used direct.
[8] Patented by Boakes, Ltd., Stratford, London, E.