Pray, how is this operation performed?
MRS. B.
Various methods are employed for this purpose, which all consist in exposing skin to the action of tannin, or of substances containing this principle, in sufficient quantities, and disposed to yield it to the skin. The most usual way is to infuse coarsely powdered oak bark in water, and to keep the skin immersed in this infusion for a certain length of time. During this process, which is slow and gradual, the skin is found to have increased in weight, and to have acquired a considerable tenacity and impermeability to water. This effect may be much accelerated by using strong saturations of the tanning principle (which can be extracted from bark), instead of employing the bark itself. But this quick mode of preparation does not appear to make equally good leather.
Tannin is contained in a great variety of astringent vegetable substances, as galls, the rose-tree, and wine; but it is nowhere so plentiful as in bark. All these substances yield it to water, from which it may be precipitated by a solution of isinglass, or glue, with which it strongly unites and forms an insoluble compound. Hence its valuable property of combining with skin (which consists chiefly of glue), and of enabling it to resist the action of water.
EMILY.
Might we not see that effect by pouring a little melted isinglass into a glass of wine, which you say contains tannin?
MRS. B.
Yes. I have prepared a solution of isinglass for that very purpose.—Do you observe the thick muddy precipitate?—That is the tannin combined with the isinglass.
CAROLINE.
This precipitate must then be of the same nature as leather?