Oak sole leather, by reason of its tough character, and its close, fibrous texture, resists water and will wear well down before cracking. It is by many considered better than other leather for flexible-sole shoes, requiring waterproof qualities.
Sole leather is divided into three classes according to the tanning—oak, hemlock, and union.
Tanning Process
Showing the rotating drums. See [page 24].
Oak tanning is as follows: the hides are hung in pits containing weak or nearly spent liquors from a previous tanning, and agitated so as to take up tannin evenly. Strong liquor would harden the surface so as to prevent thorough penetration into the interior of the hides. After ten or twelve days, the hides are taken out and laid away in fresh tan and stronger liquor. This process is repeated as often as necessary for eight to ten months. At the end of this time the hide has absorbed all of the tannin which it will take up.
Hemlock tanning is similar to the oak tanning in process. The hemlock tan is a red shade. Hemlock produces a very hard and inflexible leather. It is modified by use of bleaching materials which are applied to the leather after being tanned. It is sold in sides without being trimmed, while the oak is sold in backs, with belly and head trimmed off.
Hemlock leather is used extensively and almost principally for men’s and boys’ stiff-soled, heavy shoes, where no flexibility is required or expected. Its principal desirable quality is its resistance to trituration, or being ground to a powder, and its use in men’s and boys’ pegged, nailed, or standard screw shoes is not in any way objectionable to the wearer. In fact, for this class of shoes, it is probably the best leather that can be used. But when hemlock is used in men’s and boys’ Goodyear welt shoes, where a flexible bottom is expected and required, it generally does not give good results. It cannot satisfactorily resist the constant flexing to which it is subjected, and after the sole is worn half through, the constant bending causes it to crack crosswise. On this account it becomes like a sieve, and has no power of resistance in water, and therefore it is not at all suited to flexible-bottomed shoes.
In “union-tanned” hides, both oak and hemlock are used and the result is a compromise in both color and quality. This tan was first used about fifty years ago. Twenty-five years ago the union leather tanners began to experiment with bleaching materials to avoid the use of oak bark, which was becoming scarce and high priced, and eventually developed a system of tanning union leather with hemlock or kindred tanning agents, excluding oak. The red color and the hard texture were modified by bleaching the leather to the desired color and texture. This produces leather which has not the fine, close tannage of genuine oak leather and at the same time lacks the compact, hard character of hemlock leather. Union leather produced in this manner is a sort of mongrel or hybrid leather, being neither oak nor hemlock. On account of its economy in cutting qualities, however, it is largely used in the manufacture of medium-priced shoes where a certain degree of flexibility is required in the sole. This is particularly true of women’s shoes.
Union leather is sold largely in backs and trimmed the same as oak, though not so closely.