Ed Straus, age 11, of Birmingham, Ala., for his question:
HOW IS LEATHER TANNED?
Leather is animal skin from which all flesh and hair has been removed. The skin then goes through a manufacturing process that makes it soft and flexible and prevents it from rotting. This process is called tanning.
Two tanning methods are used: mineral tanning and vegetable tanning.
Mineral tanning is also called chrome tanning. It is chiefly used to treat leather that is used in the upper parts of shoes and other footwear as well as handbags. Grease or wax soaked chrome leather is used to make such machinery parts as pump gaskets and oil seals.
In chrome tanning, the hides are soaked or pickled in a solution of acid and common salt for four to eight hours. The pickling draws moisture out of the hides. The skins are then taken out of the tanks and piled flat to drain for one or two days.
With a two bath chrome tanning method, the picked hides are put into a barrel like drum that contains a solution of common salt, bichromate of soda and acid. After an hour or more, workers add sodium thiosulfate and the hides are stirred for an additional three hours until the solution changes in color from orange to greenish blue.
Tanners then test the leather by boiling it. The leather is completely tanned if boiling does not damage it. The hides are then taken from the drums and stacked so the tanning solution can drain away.
The one bath method begins with a greenish blue solution. Workers put the leather into tanning drums and stir them for five to 10 hours.
Vegetable tanning is used to tan heavy leather such as that used to make shoe soles, cases, straps, harnesses, upholstery, linings and belts for heavy industrial machinery.
A bitter ingredient called tannin or tannic acid is used in vegetable tanning. Tannin is made from the leaves, nuts, bark and wood of such trees and plants as chestnut, hemlock and oak.
In vegetable tanning, several tannins are mixed and diluted with water and placed in vats. The hides are attached to cradles or rockers in the vats that rock the hides slowly. At first the tanning solution is very weak but later stronger solutions are added.
It may take from 15 to 30 days for hides to pass through all the steps of the tanning process from weakest solution to the strongest. After they do, they are usually struck through. That is, the tannin has soaked through the entire cross section of the hide.
Tanners then place the hides in still stronger tannin liquor. For the next 15 to 100 days, the tannin liquors are strengthened at intervals to make up for the tannin that soaks into the hides.
A number of additional procedures must be followed before the leather is completely tanned. It must be put through a machine that presses out the wrinkles and another that bleaches the leather. Next the leather is put into large rotating drums that are either heated with hot air or steam. Additional pressings follow.
Finally the leather is mulled, or dipped and left to hang for three or four days in a special compartment. When the leather is dry, the grained surface is polished.