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INDUSTRIAL A N D ENGINEERING CHEMISTRY
and controllable variable factors and the prospects are bright that the ultimate consumer will find a steady improvement in quality of leather. When used for footwear, leather must have great strength and yet it must stretch sufficiently t o permit the shoe t o conform perfectly to the shape of the foot. It has been shown that the greatest variable factor iduencing strength and stretch is the extent to which the thickness of the leather is reduced by splitting. When a sample of chrome calf leather was split into two layers of equal thickness, the grain layer was found to be only 26 and the flesh layer only 16 per cent as strong as the unsplit leather, per unit width. The resistance of the leather to stretch was found to vary directly with the strength. Increasing the oil content of leather caused a n increase in strength with a decrease in resistance to stretch. Increasing the water content, which occurs naturally with increasing relative humidity of the atmosphere, has a similar effect. There is a very great difference in both strength and stretch over the area of a skin. Quantitative relations have been plotted for both chrome- and vegetabletanned calf leathers, but the task of utilizing the information for the benefit of the consumer is difficult. I n the first place changes made to alter strength and stretch values also change other important properties of the leather. Secondly, while increasing ease of stretch enables the shoe more readily to conform t o the shape of the foot, it also causes the shoe t o lose its shape and fineness of appearance more quickly. This is very marked in comparing shoes of calf leather with those of kid leather, which stretches much more easily. Another important property of leather is its power to ventilate the foot. I n this respect it is a very remarkable material. It can be made water-repellent from the outside while still retaining the power to pass water from the foot to the outside air. Tests have shown that a good shoe-upper leather will transmit water from a moist to a dry atmosphere about 70 per cent as fast as though the two atmospheres had direct contact over the same area. This power is decreased in proportion to the kind and amount of finishing material, such as casein, wax, lacquer, etc., applied to the surface. A small amount of finishing material very greatly increases the resistance of the leather to wetting. With the first application of finishing material, the water-repellence of the leather is increased out of all proportion to the decrease in ventilating power. Quantitative studies of these relations have
Quantitative Relations of the Countercurrent Washing Process
Vol. 21, No. 2
made it possible to increase the serviceability of a shoe very greatly. The comfort of a shoe is also largely determined by the temper, elasticity, flexibility, and resilience of the leather. These properties are greatly influenced not only by the amount of oils incorporated in the leather, but also by their distribution throughout the thickness of the leather. Investigations in this field are complicated by the necessity of analyzing the leather a t different depths and a t different locations over the area of the skins. In the early part of this lecture a property was mentioned which greatly affects the wearer of shoes-namely, the tendency for the leather to suffer dimensional changes with the relative humidity of the atmosphere. Many people have attributed to their toes the power to foretell changes in the weather, little realizing that the pain was merely a n indication that the leather in their shoes was shrinking. In going from a dry to a moist atmosphere, chrome leathers increase in area by a n average of about 18 per cent. They shrink in area correspondingly when the relative humidity falls. Vegetable-tanned leather, on the other hand, undergoes changes in area only one-third as great as this. For reasons that had nothing to do with the comfort and happiness of the ultimate consumer, it became desirable for the tanner and shoe manufacturer to have about 95 per cent of all the sole leather vegetable-tanned and about 95 per cent of all the shoe-upper leather chrome-tanned. When this was brought about, no one suspected the differences of shrinkage and expansion of the two kinds of leather. The shoe upper is thinner and tends to reach equilibrium with the air much more quickly than the very thick sole, and so the changing size is much more effective when the upper is chrome-tamed. Nearly everybody wears shoes with chrome-tanned uppers, which are subject t o these great size changes with changing atmospheric conditions. For this reason most people suffer unnecessary discomfort. The tanner will use any method of tanning which the consumer demands, but, like most people, is slow t o make a change until the demand is urgent. The interesting scientific fact is that the kind of tannage so greatly influences the power of leather to take up water and t o change in size. It has not been possible, in this lecture, t o do more than give just a glimpse of leather chemistry as a whole and t o show what the leather chemist is doing t o make the footsteps of his fellow man a little less weary.
ANfl - A [AA'+l - 1 1 weight of solute recovered weight of solute fed number of tanks werght liquor transferred as liquid weight liquor transferred adhering t o solid
=
where L =
W = N = A =
Editor of Industrial and Engineering Chemistry: In the article by Ludwik Silberstein under this title, IND. At the end of his paper the author gives an illustrative example, ENG.CHEM.,20, 899 (1928), the author's mathematical analysis which will now be solved by the above equation. For the case leads to equations which are correct, but the work is needlessly where N = 4 tanks, we have
involved. The consideration of limits gone through to find conditions in the final steady state is unnecessary, as these relations can be derived very easily by simple material balances. Moreover, the author has apparently entirely overlooked certain similarities in the equations he develops for systems of different numbers of tanks, and has therefore failed t o discover that a single equation can be written for a system of any number of tanks. This has led him t o an enormous amount of work in developing a new equation for each system, as the process is quite laborious for systems of three or more tanks. Some time ago the writer had occasion to attack this problem, and developed and proved an equation applicable to a system of N tanks.
700 = 1.868 375 W = (0.40) (375) = 150 pounds L = 150 [1.8686 = 144 pounds recovered 1.86S5 - 1 150 - 144 = 6 pounds lost -(6) = ('O0) 1.6 per cent concentration, agreeing with the 375 author's figure. SMITH D. TURNER
A
=
-
~
HUMBLE OIL & REFINING COMPANY BAYTOWN, TEXAS October 13, 1928
