Black Oak Bark In Tannin.

Scientific American 25, 17.12.1864

The black oak (quercus nigra of botanists) grows spontaneously in the northern American States, and is used in the art of dyeing for producing colors on cotton called "bark greens, bark yellows, bark browns, and olives." The name by which it is commonly known is "quercitron bark," and constitutes the inner hark of the tree. The color which it produces in a simple aqueous solution is yellow. Its coloring properties were discovered by Dr. Bancroft, of London, in 1794. He discovered it while on visit to America in search of new dyewoods, and the British Parliament granted him a patent for its exclusive use for twenty years. It was the principle substance employed in Britain for coloring yellow on cotton from the date of the Doctor's patent until about the year 1820, when the bichromate of potash was introduced, — which has now almost superseded it.

The bark of this tree, when used for tanning, makes leather of as good quality as white oak bark, but because its color is a light yellow, it will not bring the some price in the market as hemlock and white oak tanned leather: — The prejudice against it on account of the color is wrong, and is founded on ignorance, but tanners cannot afford to wait until this public prejudice is cured. Many of them, therefore, knowing the quality of the yellow bark, have consulted us in reference to some method that would enable them to use it in their vats and change its color, and make the leather tanned by it resemble the reddish hemlock, or the buff of white oak.

We will give them some information relating to substances which act as reagents on the color of the bark, and then they can make experiments for themselves, and no doubt they will discover a method of giving the leather the desired color, although, with us the yellow leather would meet with the most favor.

Decoctions of this bark should always be made very strong, as it then deposits a portion of its coloring matter on cooling. It contains a great quantity of tannin and quercitrine — the coloring matter. Much of this coloring matter disappears if the decoction is allowed to stand until it becomes stale, a hint which may be of use to tanners. Lime water gives a yellowish red precipitate with a decoction of this bark; the muriate of tin a yellow precipitate; alum a yellow precipitate; the sulphate of copper, a greenish yellow precipitate; the sulphate of iron (copperas) a dark olive green. In dyeing cotton a brown color with this bark, the goods are first dyed yellow with it, then redwood and logwood liquors are given on the top of the yellow. It has been observed by dyers that the yellow forming the base of the brown color will disappear, as it were, by long handling of the goods afterwards in a redwood or logwood liquor. Tanners may take advantage of this property of quercitrine and use its decoctions, in the earlier stages of tanning, and then finish off with hemlock bark liquors. They may also get the proper Oades of leather desired, by using the bark with hemlock in the same vat, or with catechu.

We have no doubt but this bark will yet come into more extensive use, and that the leather tanned by it will come up to a useful value, which does not lie in the color of it.

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