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قراءة كتاب Animal Proteins

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Animal Proteins

Animal Proteins

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دار النشر: Project Gutenberg
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14]"/> Sterilizing the hides has been frequently suggested, but no method has yet been advocated which does not interfere either with the tanning processes or with the quality of the finished leather.

Hides from the European Continent, usually wet salted and well flayed, exhibit much the same variable quality as the home supply, those from highland districts tending to be thick, yet even, well grown, tight textured and smooth grained, whilst those from lowland regions are less satisfactory. Thus hides from the Swiss Alps and Scandinavia have ranked high, whilst the spready Dutch cows are typical of a lowland hide. In the hides which once came from Germany the same features appear. Bavarian highland hides had an excellent reputation, whilst those from Berlin, Cologne, etc., tended to be long in shank and not well grown. French hides are often ill flayed, and Spanish and Portuguese are often subject to scratches. Italian hides have a very good name, being small but stout in butt.

The American supply is important. South America yields an excellent class of hide, salted or dry-salted. They are from an excellent breed of animals, slaughtered and flayed with every care, and efficiently cured. A most serious defect in this class of hide is the "brand," which is both deep and large and in the most valuable part of the hide. One side, however, is usually unbranded, so that each hide yields one good "bend." These hides, e.g. "Frigorifics," have recently been much more extensively tanned in Britain because of the shortage in the home supply of market hides caused by the European War. South America also yields good horse hides. North American hides are usually wet-salted (e.g. packer hides). They are usually good. Central America yields mostly dried hides exhibiting usual defects.

The Asiatic supply comprises the frozen China hides, which are clean but small, with flaying of uncertain quality. There are the buffalo hides from Asia and East Europe, which are suitable for cheap and sole and strap leather, and also the dry-salted "E.I. kips," obtained from a small breed of Indian cattle, and extensively made into upper leather. The Asiatic humped cattle also provide a limited supply. The African supply is of increasing importance. The tropical parts yield dried hides of uncertain quality, but the more temperate parts of South Africa yield a growing supply of good quality.



REFERENCES.
"The Manufacture of Leather" (Bennett), pp. 27-37.
"Principles of Leather Manufacture" (Procter), pp. 33-56.
"The Ox Warble or Bot Fly" (E. Ormerod).
"The Making of Leather" (Procter), pp. 2-22.

Section II.—THE PREPARATION OF PELT

Before hides are tanned it is necessary for them to pass through a series of preparatory processes. The object of these processes is to obtain from the hide the true hide substance in a pure and suitable condition. Each class of leather has its own appropriate processes, the adjustment of which largely determines the quality of the finished article. So prominent is the influence of these preparatory methods that the paradox "good leather is made before tanning" is in trade circles almost a platitude. These processes, sometimes lumped together under the general name of "Wetwork," comprise soaking, liming, beam house work and deliming. These will be discussed in turn.

The term applied to the hide after these processes, but before tannage, is "pelt."

Soaking has for its object the cleansing and softening of the hides, chiefly by means of water. It aims at the removal of dirt, blood, dung, and curing materials by washing. The process is usually simple, and is much the same for all classes of leather. The ideal to be aimed at is to restore the hide to its condition when it left the animal's back. Cleanliness in leather manufacture is as essential at the commencement as anywhere, for the hide is in its most putrescible state. The soluble proteids (blood, lymph, part of dung, etc.) which always adhere to hides encourage the rapid growth of putrefactive bacteria, and cannot be washed away too soon. Dung is often difficult to remove, being caked on the butt end amongst the hair. Soaking only softens it, and mechanical removal is usually necessary. If such substances are not removed, they go forward with the goods into the lime liquors, causing stains, loss of hide substance, and counteracting plumping.

The detailed method and time of soaking are determined mainly by the nature of the cure. One of the purposes of the soak liquors is to dissolve the salt used in curing hides and to rehydrate the hide and make it again soft and pliable. As a 10-per-cent. salt solution exerts a solvent effect on hide substance, it is necessary soon to change the first soak liquor of salted goods.

Market hides, which are uncured, require the least soaking, the cleansing effect being most needed. The hides are inserted into pits ("water dykes") of water for a few hours, and the water changed once or twice. The soaking should not be prolonged as the hides are so putrescible, and where it is customary to leave the goods in a soak liquor overnight, it is advantageous to add a little slaked lime to the water before inserting the goods. This not only softens hard water, but is mildly antiseptic and plumping, and forms a suitable introduction to the liming proper. Each pit contains a "pack" of 30-50 hides, according to its capacity, which varies in different tanneries from 1000 to 2000 gallons. Tainted goods, which are indicated by a characteristic white colour on the flesh side and by loose hair, need a preliminary washing either in a "drum," "tumbler" or in a "paddle." This ensures a rapid change of liquor and the removal of most of the putrefactive agencies. Bad cases may need the application of antiseptics, such as immersion in 0.1 per cent. carbolic acid; but if possible these should be avoided, as they lengthen the time required for liming. After drumming or paddling, tainted goods should be placed directly into a lime liquor.

Salted hides need very similar treatment to uncured hides, but the soaking is longer, because of the dehydration caused by salting. Hence they receive also a greater number of changes of water, three or four usually, but often more. As much loose salt as possible should be shaken from the hides before insertion into any liquor. The employment of drum or paddle before pit soaking is extremely useful to effect the rapid removal of superficial salt, and is also useful after pit soaking to remove the last traces.

Dried and dry-salted goods need a soaking still more prolonged, up to one week if water alone be used. With the assistance of caustic soda, however, the process can be shortened to about two days. The first soak liquor should consist of a 0.1 per cent. solution of caustic soda, and after the goods have been inserted twenty-four hours, they will be materially improved by a few hours' drumming or paddling. Another caustic soda soak will complete the process. Sodium sulphide crystals may replace caustic soda, but about three times the weight will be needed. Carbonate of soda and caustic lime also are a convenient commercial substitute for caustic soda. For 10 lbs. caustic soda, use 36 lbs. carbonate and 7 lbs. lime. Extra lime should be added in all cases when the water is hard. Acid liquors will also soften dried and dry-salted goods, but such processes do not fit in so well with the subsequent liming. The use of putrid soaks and stocks may be now considered out of date.

Liming follows soaking, and consists essentially in immersing the hides for 7-10 days in milk of lime. The chief object in view is to loosen the hair and prepare for its mechanical removal. Liming takes place in pits,

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