I would here remark that I changed the process from liming to sweating in the sole leather in 1836—the only change I have made in tanning for twenty years; and for sole leather, it has been proved to be quite as good as liming, if not better, and somewhat cheaper; besides yielding a greater gain of weight, and when well tanned, making leather more impervious to water. Liming and “bating,” however, for light leather, is preferable. Salted hides do not require more than two-thirds the time to soak; but generally rather longer to sweat. After the hides are prepared for tanning, the next process is what is commonly called “handling,” which should be performed two or three times a day in a weak “ooze” until the grain is colored. New liquors, or a mixture of new and old, are preferable for Spanish or dry hides—old liquors for slaughter. They are then, after a fortnight, laid away in bark, and changed once in two or four weeks, until tanned. Much care and judgment are necessary in proportioning the continually increasing strength of the liquors to the requirements of the leather in the different stages of this process. The liquors should be kept as cool as possible, within certain limits, but ought never to exceed a temperature of eighty degrees; in fact, a much lower temperature is the maximum point, if the liquor is very strong; too high a heat, with a liquor too strongly charged with the tanning principle, being invariably injurious to the life and color of the leather. From this it would seem that time is an essential element in the process of tanning, and that we cannot make up for the want of it by increasing the strength of the liquor, or raising the temperature at which the process is conducted, any more than we can fatten an ox or horse by giving him more than he can eat.
Hides that are treated with liquor below the proper strength become much relaxed in their texture, and lose a portion of their gelatine. The leather necessarily loses in weight and compactness, and is much more porous and pervious to water. The warmer these weak solutions are applied, the greater this loss of gelatine. To ascertain whether a portion of weak liquor contains any gelatine in solution, it is only necessary to strain a little of it in a glass, and then add a small quantity of a stronger liquor. The excess of tannin in the strong, seizing upon the dissolved gelatine in the weak liquor, will combine with it, and be precipitated in flakes, of a dark, curdled appearance, to the bottom. At the Prattsville Tannery, the greatest strength of liquor used for “handling,” as indicated by Pike’s bark-ometer, (an instrument to test only FRESHLY made liquors,) is sixteen degrees; of that employed in laying away, the greatest strength varies from thirty-five to forty-five degrees.
After the leather has been thoroughly tanned and rinsed, or scrubbed by a brush-machine or broom, it will tend very much to improve its color and pliability to stack it up in piles, and allow it to sweat until it becomes a little slippery from a kind of mucus that collects upon its surface. A little oil added at this stage of the process, or just before rolling, is found to be very useful.
Great caution is necessary in the admission of air, in drying when first hung up to dry. No more air than is sufficient to keep the sides from moulding, should be allowed. Too much air, or in other words, if dried too rapidly, in a current of air, will injure the color, giving a darker hue and rendering the leather harsh and brittle. * * * *
The average time of tanning in 1842, was five months and seventeen days; of 1843, five months and twenty-two days; 1844, six months; 1845, six months and eleven days; average of the whole time, five months and twenty-seven days. The average weight of the leather was over eighteen and one-half pounds per side. This, according to the best authorities we have, is considerably below the time employed in England. There, it is no uncommon thing for eight or ten months to be employed in tanning a stock of leather, and some of the heaviest leather, it is said, takes fourteen and eighteen months. Such deliberation undoubtedly insures a fine quality of leather, but it may be questioned whether there is not a great loss of weight—a loss of interest on capital, and in consequence an unnecessary enhancement of price, which does not suit the American market.
The tanning of leather, more than almost any other manufacture, is a chemical process, the success of which depends almost wholly upon the skill and judgment with which its complicated manipulations are conducted. To attain the requisite skill in the laboratory of the chemist, is evidently impossible; it can only be acquired in tanning itself, by long and careful attention and observation.
The labor in the loft and peeling bark during the above five years was 8820 days. One man will work through the beam-house, in one year, 6260 sides. One man will tan and finish 2228 sides. One cord of bark tans 196 pounds.
The question has been frequently asked me, how long does it take to tan sole leather. I answer, from four to six months, according to the strength of the liquor and the number of sides in the vats; and the quicker tanned the better.
If the hides are fresh, they are capable of being properly softened, and, if so, the process of tanning may be completed much sooner than in the case of old and hard hides, that cannot be softened with the same facility.
If the hides have sufficient room in the vats, so as not to lay crowded, they will tan much faster.