It thus consisted exclusively of sulphates, with the exception of a trace of chloride. The cures, after ignition, were both neutral to phenolphthalein, but before ignition the Dacca was distinctly alkaline, in consequence probably of the presence of ammonium salts, and both showed considerably larger traces of carbonates before than after.
The most striking feature of these analyses is the absence of more than the smallest traces of chlorides. The cures are thus practically free from common salt, and owe their antiseptic power to the sodium sulphate which they contain, and which indeed forms their principal constituent. Nitrates appear to be entirely absent. Sodium sulphate sometimes forms large crystals in pits used for soaking these kips.
The iron-staining of hides which has been mentioned appears to result only when the hides after cure are exposed for a lengthened period to a moist atmosphere, in which the carbonic acid present probably also plays its part, the iron passing into solution as hydric carbonate.
The analyses show a striking resemblance to those of the soda deposits of Wyoming, given by Dr. Attfield,[17] except that their percentage of sodium carbonate is smaller, which is quite intelligible in the light of Mr. Brunner’s abstract on the ‘Probable origin of natural deposits of sodium carbonate,’[18] which supports the view that the sodium carbonate is derived from sodium sulphate by the reducing and carbonating action of low organisms.
[17] Journ. Soc. Ch. Ind., 1895, p. 4.
[18] Ibid., 1893, p. 116.
It may be noted here that the preservative properties of sodium sulphate are well known, and the anhydrous sulphate has been recommended as a substitute for common salt (see [p. 23]).
Drying is a very common method of preserving hides as well as other putrescible matters. It has no effect in killing bacteria, but putrefaction can only go on in presence of a considerable amount of moisture. As applied to hides, it is, to the tanner, one of the least satisfactory modes of cure, involving very considerable difficulties in bringing hides back to the moist and swollen condition which is necessary at the outset of his operations, but it is the only practical method in districts far from the coast and with primitive modes of transit, both on account of the cost of salt, and the lessened weight of the dried hide. Great differences are found in the ease with which dried hides soften, according to the way in which the drying has been accomplished, the difficulty being greater the higher the temperature which has been used (see [p. 111]). The best mode of drying is to hang in the shade in a good draught of cool air, with the flesh side out. Hides or skins dried in a tropical sun are not only difficult to soften, but are liable to damaged portions, which either refuse to soften, or blister and go to pieces in liming, owing to the structure of the hide being destroyed by heat, the outer surface drying first and forming an impervious layer which hinders evaporation from the inside, so that the moist interior becomes melted, while the outside appears quite sound. Such injuries are often only to be discovered by soaking and liming. Very similar damage may occur from putrefaction of the interior after the outside has become dry, and to get good results, the drying must be gradual, but rapid, especially in hot climates. South American hides are mostly dried in the sun, suspended by head and tail from stakes, with the hair side out.
The risk of injury by putrefaction during drying is diminished by the use of antiseptics. Solutions of arsenic have been frequently used for this purpose, and many of the dried Indian kips are of what are known as “arsenic cures,” although the writer has never been able to detect arsenic in any which he has examined, and its use seems by no means general. The arsenious acid is usually dissolved in soda solutions. Unless used pretty freely it has little antiseptic effect, but is useful in preventing the attacks of insects, which are often very destructive. The larva of a small beetle, Dermestes vulpinus, frequently devours the whole tissue of patches of the hide, leaving only the epidermis.
It may be well here to say a few words about the injuries and defects to which hides and skins are liable, although some of them are not strictly due to the cure. The most serious, and yet preventable injury is that due to butchers’ cuts. As the value of the hide bears only a small proportion to that of the meat, many butchers do their work extremely carelessly, and this is encouraged by the loose classification of “damaged hides” in some markets. There is also an idea that the appearance of the meat is improved by a thin layer of the white skin-tissue being left on it, and for this reason as well as mere carelessness, butchers frequently score the flanks of the hide with shallow cuts which greatly diminish its value. The “packer hides” of the United States, and the products of the large saladeros or slaughtering (“salting”) establishments of South America, such as Liebig’s, show what can be done by skilled work in this respect. In the United States, much of the flaying is done by means of a wooden cleaver, instead of a sharp knife. Another method to some extent in use, and which may be recommended for calf and sheep skins, is to inflate the carcase before skinning, with air from a compressing syringe, which tears the connecting tissue between the skin and the body, and renders flaying much easier.