The hardness of water, and the dissolved carbonic acid which it contains, are, together with its temperature, the principal factors which determine whether a hide will plump or fall in it. Almost the only accurate investigation of this point has been made by W. Eitner.[66] He placed pieces of hide, unhaired by sweating, and quite flat and fallen, in water for four days at a temperature of 46° F. (8° C.), with the following results:—
| 1. | In | distilled water | Scarcely at all plumped. | |||
| 2. | „ | water | saturated with CO2 | Well plumped. | ||
| 3. | „ | „ | with | lime bicarbonate, 20° German scale of hardness | Tolerably | plump. |
| 4. | „ | „ | „ | magnesia bicarbonate, 20° do. | „ | „ |
| 5. | „ | „ | „ | lime sulphate 20° do. | Well plumped. | |
| 6. | „ | „ | „ | magnesia sulphate, 20° do | Best plumped. | |
| 7. | „ | „ | „ | magnesium chloride, 20° do. | Not at all | plumped. |
| 8. | „ | „ | „ | common salt, 20° do. | „ | „ |
| (1 German degree of hardness corresponds to 1 of CaO in 100,000.) | ||||||
[66] Gerber, iii. (1877) p. 183.
The peculiarities which were shown by the hide pieces on removal from the water were maintained throughout the tanning, which was conducted in imitation of the German method, the hide being swollen and coloured through in weak birch-bark liquors, made with distilled water and acidified in each case with equal quantities of lactic acid, and finally laid away, till tanned, in a mixture of oak bark and valonia. No. 6, from magnesium sulphate, was the best; then No. 2; No. 3 was less good, but all the pieces from 1 to 6 were firm, close and of good substance and texture, No. 1 having swelled well in the sour liquor. On the other hand, 7 and 8 scarcely swelled in liquor, but remained flat throughout, and were looser, thinner and of finer fibre. From this experiment it is clear that while sulphates and carbonates exert a favourable influence on plumping, chlorides do the reverse, as they themselves not only do not plump, but they place the hides in an unfavourable condition for the plumping action of acids in the liquors. These experiments are quite borne out by the writer’s experience in practice. The water at the Lowlights Tannery, which in dry weather was mostly obtained from beds of what was originally sea-sand, and which consequently contained a very abnormal proportion of chlorides (up to 68 pts. NaCl per 100,000), required special and very careful management to make thick leather, notwithstanding the fact that it contained a considerable quantity of calcium and magnesium sulphates. These facts also indicate the importance of the thorough removal of salt from hides intended for sole-leather. Plumping is not a desirable thing in leather intended for dressing purposes, and it is possible that the use of a small percentage of salt in the liquors or wash waters might in some cases enable bating to be dispensed with. Like a bate, salt would dissolve a small proportion of hide substance (see [p. 65]). There is no practicable means of removing chlorides from water, but Eitner suggests the addition of a small quantity of sulphuric acid to water containing much temporary hardness (bicarbonates), in order to convert it into permanent hardness (sulphates), which, as stated above, plumps better. The amount required may be calculated from an acidimetric determination of temporary hardness (see L.I.L.B., p. 19). A simple but not very accurate guide, is to add enough acid to purple, but not to redden litmus paper even after moving the latter about in the water for some minutes. In practice the acid must of course be very thoroughly mixed with the water by stirring and plunging. It must be borne in mind that Eitner’s experiment was on sweated hides, and that with limed hide, which is kept plump by the dissolved lime retained in the hide, different results as regards carbonic acid and bicarbonates would be obtained. Both these would convert the lime in the hide into chalk, which is insoluble and inert, and the hide would fall, at any rate when the lime was completely carbonated, while hides would remain plumpest in waters most free from substances capable of neutralising lime. From this we may conclude, what may be a priori expected, that the purer the water, the plumper limed hides remain in it. In soft but peaty waters, hides fall rapidly, from the neutralisation of the lime by the weak organic acids of the peat. Such waters are dangerous for domestic use from their solvent action on lead, but this danger can be entirely removed by storing the water in limestone reservoirs, or allowing it to flow slowly through a limestone culvert before use. In some towns in the north of England a small quantity of lime is added so as to neutralise the water as it leaves the reservoir and before it enters the mains.
Wherever the conditions of putrefaction or decaying organic matter are present, as in a bate, hides fall rapidly, and in extreme cases even the presence of the stronger acids will not maintain plumpness. Eitner mentions the case of a stream at Vissoko in Bosnia, which was in special repute among the tanners from its power of pulling down hides rapidly, and which took its rise in a common on which the pigs of the town were pastured. The causes of this action are no doubt due to the products of putrefaction, but are somewhat obscure. Bacteria present in water are a frequent source of injury in the soaks, and probably in other stages of the tanning process.
Rain water and the water of streams in mountain districts of hard igneous rock are generally nearly free from mineral constituents. This is the case with the Glasgow water from Loch Katrine, and the Thirlmere water which supplies Manchester. Such water, if cold enough, and free from mud and organic impurity, is the best for almost every purpose in the tannery. Most river-water contains material quantities of mineral matter, though it is usually softer than that of springs or wells.
For further details as to the chemical examination of water, and the methods of determining the amounts of its different constituents, see L.I.L.B., pp. 18 et seq.
CHAPTER XI.
SOAKING AND SOFTENING OF HIDES AND SKINS.
As has been explained in the last chapter, hides and skins come into the hands of the tanner either uncured (“green”), as they are taken off the animal, preserved with salt or some other antiseptic, dried, or “drysalted” in which both methods are combined. His object in each case is to remove blood and dirt, and to restore the hide to its soft and natural condition; but the treatment required varies much with the state of the hides.