Practical suggestions on tanning leather

Scientific American 19, 8.5.1869

By C. Gilpin.

(Continued from page 259)

Uniform temperature of liquors and disintegration of bark

It is generally conceded that the process of tanning is much more rapid during the summer months than through the colder seasons of the year. In other words, the tannin penetrates the hide much more readily during the months of May, June, July, August, and September, than during the other seven months of the year. Those who have given this subject any attention, have doubtless observed that during the summer months the liquor ranged from 70° to 80° Fahrenheit, and fell as low as 35° and 40° during the colder seasons in this climate, and perhaps lower in more northerly latitudes. What the difference in time required, to tan out a stock handled regularly in the liquors of the temperatures named, might be, I regret that I have no exact means of presenting to the trade, at this time, but shall endeavor to possess the information at no distant day, when I will make it public. One fact all are aware of, that it takes considerable less bark to maintain the liquors up to a certain degree of strength in cold than it does in warm weather; the tanning principle not being so rapidly absorbed by the hide, less bark is ground during the winter months.

I can well recollect when it was the custom among tanners to lay away their stock in very heavy layers during the month of October, after sending off all that was ready for market, and letting it lie until the frost was entirely out of the ground, when it would be drawn, and, as a consequence, was found to be but little advanced during that period.

It may be asked upon what principle can this be accounted for, and what is proposed as a remedy. The causes appear obvious, upon the wellsettled principle that all matter animate, as well as inanimate, is influenced by the elements, heat and cold, throughout all nature, to a greater or less degree, contracting and expanding the most solid metals, as well as the most delicate animal and vegetable formations; hence, when the liquor are below the expanding temperature, the hide contracts and the pores necessarily become more or less closed, and, when at that point, cause the hide to expand and the pores to open, just in proportion as the temperature is raised, until the whole mass may be disintegrated and a chemical change produced, which alters the entire organic structure of the material operated upon; thus it is clear that when the temperature of the liquors is allowed to fall below the expanding degree, the capillaries, by contraction, present an obstacle to the free passage of the tannin, and it cannot be taken up so rapidly; hence a much longer time is required to accomplish the object.

Having endeavored to define the cause, we will suggest a remedy, which will be to introduce, into every department of the yard, a regular temperature of liquor; and the facility with which the liquors can always be kept at a certain ten, perature, is the best reason why the manufacturer should lose no time in adopting it, particularly when, as a general thing, they have the facilities for supplying themselves with all the heat that may be necessary for this purpose; and, as a further auxiliary, all the operations of the tannery should be housed in good comfortable buildings, with steam pipes running through them, that would keep them at summer heat, or as warm as experience found necessary, then the junk through which all the liquor passes, should, by means of steam pipes, have sufficient heat thrown into it to insure the desired temperature throughout the entire yard at all times.

Thus, the liquor, passing from the vats to the junk, thence upon the leaches, with the entire establishment kept at a mild temperature, would, it is believed, accomplish all that is deemed necessary to overcome the loss sustained by the inaction produced by cold and freezing, weather upon the operations of tanning.

The expense of putting in pipes for the purpose of generating heat sufficient to warm the building, should be no obstacle, when we consider the cost as compared with the advantages gained. Mr. James Calley, of Pittsburgh, informed me that the cost of putting in twelve hundred feet of threequarter pipe, was two hundred dollars; this included all expenses; and from information received through wellinformed sources, it is believed that the whole cost of fitting up a firstclass tannery, with the necessary apparatus in the way of pipes, to accomplish our object, would not exceed twelve to fifteen hundred dollars, while the actual amount saved in time and labor in handling the stock, would, it is believed, fully equal one-half of the cost annually, to say nothing of making the workmen more comfortable, and thereby saving time that is always lost in warming feet and hands, in cold, frosty weather, which is the necessary consequence in all yards exposed to the frosts of winter, particularly in northern latitudes.

In communicating with the manufacturers upon the subject of having the bark thoroughly disintegrated before leaching, they pronounced, with one accord, that this was a great desideratum, if not the greatest want, in the operations of tanning. Now, that we have a system of leaching upon the upward hydraulic pressure, by which the liquid can be forced through any density that bark might acquire by being ground never so fine; believing, as most of them do, that under the present imperfect systems of preparing the bark, a considerable portion of the tanning principle is not, and cannot be extracted, under the methods of leaching in general use, without the application of steam and boiling, or very hot liquids, which, it has been repeatedly shown, are generators of impurities, that have an injurious influence upon the leather, what we want is a machine that will thoroughly separate the bark, and discharge it into the leach in the form of sawdust or shreds, allowing no part of it to escape without being brought to this condition. The best prepared bark that ever came under my notice, was prepared with stones arranged upon the plan of mill stones. It was thoroughly pulverized, or, perhaps, I might more properly say, ground into shreds. Under this method of grinding, no lumps or thick pieces could escape, as is the case now with the most approved mills in use. It would appear, from the past history of the world, in all ages and in all professions, that human nature has been averse to changing old methods for new ones, and the most useful as well as the most profitable improvements, have, too frequently, lingered longer in the murky and almost impenetrable atmosphere of prejudice, and those inveterate old fogies, custom and habit, than the advancement and interests of the great industrial pursuits of life would seem to demand. Yet, however much we dislike change in our systems of doing business, it is among the inevitable consequences, that necessity moves more individuals to adopt improvements than a desire to promote and encourage progress.