The Puering, Bating & Drenching of Skins

by Joseph Turney Wood

A culture may be prepared from the sweating bacteria by taking the hair from the skin when it “slips”—that is, when it can be removed by simply pulling it. The root portion of the hairs are then cut off, and about 10 grm. carefully digested in 100 c.c. of water at a temperature of 32° to 35° centigrade for several hours. The liquid portion is then poured off, and used for inoculating 10,000 c.c. of a nutrient medium of the character hereinafter described. The inoculated nutrient medium is then maintained for three days at a temperature of 37° centigrade, and may then be used for the inoculation of a larger quantity of nutrient medium.

Theoretically, a single cell of a bacterium is sufficient to inoculate any quantity of any suitable fluid; but in practice it is found necessary or expedient to employ 5 or 10 per cent. of inoculating matter to the volume of the liquid to be inoculated. The inoculation of the 10,000 c.c. of nutrient medium may be done in a Carlsberg vessel, as described by Hansen in his “Untersuchungen aus der Praxis der Gärungsindustrie.”

It is desirable that the nutrient medium should be without sugars or carbohydrates; and, although its composition may be varied, good results can be obtained by ordinary bacteriological methods, by using a medium composed of gelatin, 50 parts or 20 grm.; potassium or sodium phosphate, 1 part or 0·4 grm.; sodium chloride, 1 part or 0·4 grm.; water, 2500 parts or 1000 c.c. This medium, after inoculation, is maintained at a temperature of 37° centigrade, until the required bacterial action is completed, i.e., until the whole of the nutriment for the bacteria is exhausted, at which time the maximum quantity of the required enzymes has been produced. The time required for this action is not less than three days, nor more than seven days. The exhaustion of the nutriment may be ascertained scientifically by a microscopic examination of the liquid, the bacilli at this period commencing to form spores. To each litre of this liquid there is then added from 2 to 6 grm. of lactic acid, and the acid liquid is afterward neutralized by ammonia, which is added when the liquid is to be used as a bate, the addition of the ammonia forming certain chemical compounds similar to those found in the dung. Instead of lactic acid, acetic or other similar organic acid may be used, but sulphuric or mineral acids must not be used. The addition of the acid to the liquid will immediately stop the growth of the bacteria; but the acid is not added for that particular purpose, but for the formation of the chemical compounds above referred to.

The above-described method is most suitable, when the bate has to be kept for a time before being used, or when it is desired to concentrate it for transit or export; but if the bate is to be used immediately, it may be prepared as follows: 100 parts of gelatin are dissolved in 1000 parts of water, and there is then added 50 parts of commercial lactic acid. The mixture is then heated, in a closed vessel, to 100° centigrade, by which operation the gelatin is partially peptonized, and the subsequent bacterial action is hastened. The acid liquid is afterward neutralized with ammonia, or other alkali, and the whole diluted with water to 20,000 parts. The dilution may conveniently take place in the wheel or paddle in which the bating is to be done, and to the liquid thus obtained there is added from 5 to 10 per cent. of the active culture from the Carlsberg vessel, as above described. The liquid is then allowed to stand for 15 to 20 hours at a temperature of 37° centigrade, and is then ready for use.

For bating skins in a practical manner with the improved bate herein set forth, the method of using the same is varied, according to the skins to be treated and the kind of leather it is required to produce, and the strength of the bate is varied in exactly the same manner as the strength of a dung bate, and such variation is made according to the judgment of the tanner, the improved bate acting in the same manner as a dung bate.

For kid-skins, calf-skins, skivers, and other skins for which dog dung has hitherto been used, the skins, after the liming process, are first well washed in water in the usual way, to remove the bulk of the lime. The skins are then placed in a wheel or paddle, with the bate prepared, as above described, with the acid exactly neutralized, or so nearly neutralized, that the alkaline action of the skins will complete the neutralization, all skins being alkaline when introduced into the bate. The bate is maintained at a temperature of 37° centigrade, and the skins are kept in motion until they have “fallen” considerably, i.e., until the swollen condition of the fibres due to the liming has disappeared. The skins are then removed from the wheel or paddle and scudded by hand or machine, preferably on the grain side, and the skins not sufficiently reduced are returned to the paddle, and the bating continued until the action is completed, this being judged in the usual way by the tanner by the feel of the skins. The strength of the bate, when skins are returned thereto, may be varied from the original strength, if desired, and after the bating is completed the skins are treated in the usual manner.

The strength of the bate into which the skins are placed, is varied according to the amount of liming the skins have undergone—that is to say, for a high-limed skin a strong bate is required. For moderately-limed skins the bate may be of such a strength that each litre as prepared for use contains five grm. of the original gelatin contained in the nutrient medium, which is equal to one part of liquid in which the culture has taken place to three parts of water. When the bate is prepared for immediate use, its strength is varied in the paddle as required. For very hard skins, such as goat-skins, the culture may be used as a bate undiluted or it may be diluted with an equal quantity of water.

For light-dressing hides and similar skins, which have hitherto been treated with pigeon dung, the skins are washed in the usual manner after the liming process, heated to 37° C., and placed in a wheel or paddle containing the improved bate, preferably in an acid condition—that is, before the ammonia or alkali has been added—the strength of the bate being equivalent to five grm. of original gelatin contained in the nutrient medium to each litre of the bate used. When the lime has been removed from the skins by the bate, which will be in about one hour, a quantity of the original culture previously neutralized with ammonia or other alkali and equal to about one-half of the quantity of the original culture first added is placed in the wheel and the bating continued until the process is completed, this being judged by the feel of the skins. If in the above method the chemical reaction of the bate be examined before the second portion of culture be added, it will be found neutral, and the action of the enzymes takes place in a neutral or alkaline solution, as in the first-described method.