Fresh hides merely require cleansing from blood and dirt. This is necessary because the blood causes bad colour, and both blood, lymph and adhering dung are sources of putrefaction, which ultimately attacks the grain and fibrous structure of the hide. Hence washed hides keep better than unwashed. Cold water is most desirable, as checking putrefaction. If the water is much over 10° C., or if it is charged with organic matter and ferment-germs; or if, as is too generally the case, the hides are in a partially putrid state when received, the time of soaking must be reduced as much as possible, and it may be necessary to sterilise the water with carbolic acid or creolin ([pp. 26], [28]). In such cases the use of a wash-wheel, or tumbler, is very desirable, rapidly cleansing the hides and removing adhering dung, which interferes with the liming, and is a serious cause of damaged grain. The American pattern of wash-wheel shown in [Fig. 21] is very suitable for the purpose. In no case is it desirable to allow green hides to lie for more than a few hours in water; and unwise treatment at this time is the cause of many troubles, which are only detected at later stages, and which are very difficult to trace to their source. “Weak grain,” in which the hyaline layer ([p. 50]) is destroyed, and which tans a whitish colour; “pricking,” or perforation of the grain with small pinholes, which may go on to “pitting” with larger holes, and a general weakening of fibre, with softening and needless loss of weight, are among these results. An instructive instance may be quoted. A large tanner found that his curried leather was affected with small spots and rings of darker colour, which rendered it quite unfit for staining, and which reappeared even when the leather was buffed. When finished as black grain, these spots had a tendency to “spue,” or rise as little pimples of resinous matter. Before the leather was stuffed no defect was noticeable to the eye, but either then, or on stripping the grease by a solvent, they could be seen under the microscope as lighter patches of open and porous grain which absorbed more than their share of fat. During the tanning process they could hardly be detected, but in the first colouring they appeared for a few hours as blackish specks almost exactly like those caused by particles of iron or iron-rust. By careful observation they were traced back to the limes; specimens of the limed hide were submitted to Director Eitner, who identified the defect as “Stippen,” caused by a species of bacteria, which cannot subsist in limes, and which therefore must have been in the soaks. These, which had been somewhat neglected from pressure of work, were cleaned out and sterilised with creolin solution, and the mischief ceased. It is worth noting that the tanner dated the beginning of the trouble from the soaking of some “Spanish” horse-hides, which may have introduced the infection. Several very similar cases have come under the writer’s notice.

It is not absolutely necessary to soak fresh hides or skins at all before liming, and where the water is scarce or unfavourable, or the skins tainted or “slipping” hair, it is best to pass straight into a weak lime. In this case the limes must be worked in shifts (see [p. 131]) and the whole of the oldest liquor run away and the hides rapidly changed into a fresh lime, or the limes will become so charged with organic matter and bacteria that the hides will cease to plump, and may even putrefy.

Salted hides and skins require more soaking and more thorough washing than fresh ones, as it is not only necessary to remove the salt, but to soften and plump the fibre which has been dehydrated and contracted by salting. If goods with salt in them are taken into limes, they will not plump properly,[67] and creases and wrinkles (drawn grain) are formed which no after-treatment will remove. This is especially important in sole leather. In deciding on a method, we must bear in mind that salt is easily soluble, and diffuses rapidly into water or weaker solutions, and that weak salt solutions tend to prevent the plumping of the fibre, while those of about 10 per cent. have considerable power of dissolving the cementing matter of the fibres ([p. 65]) and so lessening weight and firmness. It may also be noted that though salt is not a true disinfectant ([p. 22]), salted hides are much less prone to putrefaction than fresh ones, and therefore a longer soaking may be safely given.

[67] Experiments mentioned on [p. 89] throw some doubt on the power of salt to prevent plumping in the limes, though the opinion in the text is generally held by tanners.

These conditions point to the desirability of free exposure to water, attained by suspending, handling frequently, or tumbling, and repeated changes to remove the salt. The degree of removal of salt is easily determined by the estimation of Cl in the last wash-water (L.I.L.B., p. 18). American tanners universally soak wet-salted hides three or four days with as many changes of water, and frequently finish by a few minutes in a wash-wheel. Any washing tumbler may be used; but the cheap and simple construction of the American wash-wheel will be easily understood from [Fig. 21]. The sides are open, so that hides can be put in or removed between the spokes. The rim of the wheel is generally perforated, for the escape of water which is supplied by a pipe passing through the axis; and the wheel is often driven by a chain or rope round its circumference. No severe mechanical treatment, such as “stocking,” is necessary or desirable for green or salted hides.

Dry and dry-salted hides require much longer soaking than wet-salted, the amount naturally depending on the thickness of the hide and the character of drying. Even thin skins when strongly dried require considerable time to soften and swell the fibres, although they soon become wet-through and flexible. Many different methods of soaking have been employed. Sometimes hides are suspended in running water; sometimes laid in soaks which may be either renewed, or allowed to putrefy; sometimes in water to which salt, borax or carbolic acid has been added, to prevent putrefaction; and more recently weak solutions of caustic soda, sulphide of sodium or sulphurous acid have been used with much success.

Fig. 21.—American Wash-wheel.

The first of these methods, were it desirable, is rarely possible in these days of River Pollution Acts; of the others, it is difficult to say which is better, since the treatment desirable varies with the hardness of the hide and the temperature at which it has been dried. The great object is to thoroughly soften the hide without allowing putrefaction to injure it. As dried hides are often damaged already from this cause, either before drying, or from becoming moist and heated on shipboard, it is frequently no easy matter to accomplish this. The fresh hide, as has been seen, contains considerable portions of albumin, and if the hide is dried at a high temperature, this may become wholly or partially coagulated and insoluble. The gelatinous fibre and the coriin (if indeed the latter exists ready formed in the fresh hide) do not coagulate by heat, but also become less readily soluble. Gelatin dried at 130° C. can only be redissolved by acids, or water at 120° C. Eitner[68] experimented with pieces of green calf-skin of equal thickness, which were dried at different temperatures, with results given in the following table:—

Sam-
ple.		Tempe-
rature
of
Drying.		Remarks.Time of
Softening
in Water.		Remarks.Coriin
Dissolved
by Salt
Solution.
I.15° C.In vacuo 24hours- Without
mechanical
work		 - 1·68per cent.
II.22° C.In sun2days1·62„
III.35° C.In drying
closet		5„ Twice
worked		 0·15„
IV.60° C.„- Refused to soften
sufficiently
for tanning		 -traces