FIG. 206.—Sections of a side of sole leather
SHOE Soles From “Bend” of Hides Most Durable
Our bill of about $1,500,000,000 for over 300,000,000 pairs of boots and shoes each year makes the quality and wear of shoe soles a matter of real economic importance, both individually and collectively. Most of our shoe soles are made of leather. As the result of various factors, however, leather soles are not all alike in either composition or wear.
Quite naturally the wear of a pair of soles depends largely upon their thickness. In general, thick soles are made from the best heavy steer hides, as distinct from the lighter-weight cowhides, from which many thin soles are obtained. Thick soles contain more leather substance, generally of a better fiber, and not only last longer but afford more protection to the feet against the weather and against injury from pebbles and rough surfaces.
In the leather trade the thickness of soles is measured by a unit known as the “iron,” which is one-forty-eighth of an inch. Consequently one-quarter of an inch is 12 irons. The United States Army specifies outsole leather of part of hide used at least 9 irons for soldiers’ shoes.
Another important factor in the wear of soles is the part of the hide from which the leather is cut. Experimental work done in the Bureau of Chemistry indicates that this is more vital than the kind of leather, so far as the present-day tannages, such as oak, union, or hemlock, are concerned. An animal’s hide varies widely in texture and fiber. Consequently some sections of it make much better leather than others. The sections into which sole-leather hides and “sides” are divided are the head, shoulder, bend, and belly, as shown in Figure 207, which is an outline of a side, or one-half of a hide, obtained by splitting the hide down the backbone line. Hides are usually split this way before tanning. The bend is about 48 per cent, or very nearly one-half, of the side; the belly is about one- fourth; and the shoulder is about one-fifth.
The best-wearing soles are cut from the bend, approximately a rectangle of leather extending 50 to 55 inches from the root of the animal’s tail toward the head and about 25 inches from the backbone line toward the belly. The exact size of a bend is determined by the "breaks,” or soft spots, at the fore and hind flanks. The cut that separates the bend from the belly is nearly parallel with the backbone edge and passes through the top of the two “breaks.” The cut that divides the bend from the shoulder meets the belly cut at the “break” at the fore flank.
Bends of sole leather can often be seen in shoe repair shops. The leather in the bend is dense, firm, and thick fibered; that in the belly is flabby and more open fibered. Wear tests conducted by the Bureau of Chemistry have shown that soles from the bend wear about twice as long as soles from the belly and nearly one and one-half times as long as those from the shoulder. Often when one sole of a pair of shoes wears out very much faster than the other it is because the poorer sole was cut from a poorer section of the hide.
A third important factor in the wear of soles is the kind of leather. The sole leather that the public knows best is vegetable tanned. It is made by treating hides with infusions and extracts of barks, woods, nuts, and leaves, all products of the vegetable kingdom, which is responsible for the name, “vegetable-tanned leather.” Its natural color is tan, varying in shades from fawn to reddish brown, depending upon the materials used and the treatment.
Among other tanning processes is one known as mineral tannage, in which products of mineral origin are used. The most important and most widely used mineral-tanned leather is called “chrome leather,” which is tanned with chromium chemicals. Although chrome leather has been made for 20 years or more, it is extremely modern as compared with vegetable-tanned leather.
Most shoe upper leather of to-day is chrome tanned. Practically all such leather is dyed, but often the natural and very characteristic pale blue to green color of chrome leather can be seen by closely examining an exposed edge.
At present the quantity of chrome sole leather made is relatively small. Natural or unwaxed chrome sole leather is used to some extent on gymnasium and other athletic shoes for indoor wear. Unwaxed chrome soles are very porous and readily absorb moisture, which makes them unsuitable for outdoor use except in dry regions. To increase its water resistance and thus make it more suitable for general wear, unwaxed chrome sole leather is filled with waxes and oils, producing what is known as “waxed” chrome, which is generally dark green or nearly black. Such leather is used to some extent in men’s and boys’ shoes and in work shoes.
Recent wear experiments conducted by the Bureau of Chemistry with soldiers and civilians show that unwaxed chrome sole leather is the longest wearing sole leather made. Waxing to impart water resistance sacrifices some of the wear, but even then the resulting product wears longer than vegetable-tanned sole leather.
Figure 208 graphically summarizes the results from a set of experiments conducted with soldiers. The days’ wear for a standard thickness of 9 irons is directly proportional to the length of the heavy black line. For these experiments all soles were cut from bends. Unwaxed chrome soles showed an average wear of 126 days and waxed chrome soles one of 102 days. Vegetable-tanned sole leathers, of oak bark, hemlock bark, and chestnut wood tannages, showed from 78 to 80 days’ wear. These experiments showed also that loading such leathers with glucose and epsom salts adds nothing to their wearing quality.
Chrome sole leather presents some difficulty to the shoe manufacturer, often requiring special handling through certain factory operations. It is sometimes slippery and has a tendency to spread, producing an uneven and slightly frayed edge. Although at first it is rather stiff, especially if heavily waxed, the stiffness usually disappears after a little wear. Chiefly for such reasons chrome sole leather has not been more generally adopted by the trade. Chrome soles are particularly serviceable when wear and not extreme refinement in appearance is the first consideration, as, for example, in men’s outing and work shoes and in boys’ shoes.
Recently efforts have been made to combine the desirable properties of vegetable-tanned leather and of chrome-tanned sole leather in a product known as chrome retan leather, In this process the hides are first tanned with chromium and then retanned with vegetable materials. Experiments are now under way to determine the relative merits of such a leather for shoe soles.