Tanning materials may be divided into two great classes, viz.:  Physiological and pathological.

PHYSIOLOGICAL.

The first class includes those tannins which are the results of perfectly natural or normal growth, and a growth necessary to the development of vegetation, for instance, bark, sumac, etc., whereas the second class contains those which are the results of abnormal growth, caused by diseases, stings of insects, etc.  An example of this is the gall.  Both of these classes are used to a great extent in Europe, while only the first division is in general use in the United States.  We will first consider the physiological tannins.

Oak Bark.—­This material was, is, and will be for some time to come the main tanning material in use here in Europe.  The advantages of the oak tannage are as fully appreciated here as in the United States.  The European oak gives a light colored, firm leather, with good weight results, is comparatively cheap and of an excellent quality.  The varieties are numerous, each country having its own kind.  Those in most general use are: 

Spiegel Rinde (mirror bark).—­This bark is well distributed throughout Europe, and is peeled when the tree has attained a growth of from 12 to 24 years.  It is marketed in three grades.

Reitel Rinde—­Is obtained from the same tree as the spiegel rinde, but after the tree has attained a growth of from 25 to 40 years.

Alte Pische (old oak).—­Obtained from the aged tree.  It is not as valuable as the younger bark, and consequently brings a much lower price.

Spiegel rinde may be judged by small warts which appear on the shining surface of the bark.  The presence of a great number of these, as a rule, indicates a high tannin percentage.

Bosnia has fine oak trees, the bark containing 10 to 11 per cent. tannin.

Bohemia has the trauben eiche (grape oak).

France uses the kirmess oak, which grows in the south of that country and in northern Africa.  Two grades are made, viz., root and trunk.

Tyrol has the evergreen oak—­12 to 13 per cent. tannin.

Sardinia possesses a cork oak, which yields 13 to 14 per cent.

White oak is found throughout Europe, yielding 10 per cent.  The price of oak bark varies a great deal.  The assortment is much more strict than in the United States.  In Austria it brings 4 to 5 fl., equal to $1.60 to $2 per kilo. (224 lb.); in Germany, 11 to 16 marks per 100 kilos.[1]

[Footnote 1:  In the principal districts in America, removed from the cities, the price of oak bark is about $4 to $6 per cord or per ton of 2,240 lb.  The hemlock bark, which gives a sole leather just as thoroughly tanned, but of a darker and reddish color, costs the larger tanners from $3 to $4 a cord.]

The above mentioned varieties are all used for both upper and sole leather.  In Germany a great deal of upper leather is pure oak tannage, but one seldom finds a pure oak tanned sole leather; it is almost always in combination with other tannics.