Leather Tanning: The Tanning Process Explained
Ever wondered about the steps that go into producing the leather for that product you bought? The production of all leather involves several steps, from drying and curing to tanning and dyeing. We’ll look at the tanning process and discover just why the kind of tanning done on the hide impacts the final result so much.
Why Does Leather Need To Be Tanned
Tanning is necessary for leather because if the raw hide is not tanned, it will rot and putrefy. The tanning process stabilizes the protein of the raw hide or skin, preventing putrefaction, and preparing it for a wide variety of end applications.
The main difference between tanned and untanned dry hides is that untanned hides will putrefy after getting wet, while tanned leather will not. Tanning also changes the appearance of the hide. The change depends on how long it takes and the tanning agents used.
Tanning of hides for leather has been done for thousands of years by ancient civilizations like those in Sumer and India.
Preparation For Tanning
Animal hides go through several stages before they are ready for use in making products like belts, wallets, shoes, furniture and clothing.
Once the hides arrive from the slaughterhouse or farm, the first step is to remove any scraps of flesh from them.
The next step is to cure the hides with salt. Curing prevents putrefaction by preventing bacterial growth on the hide between procurement and processing. It greatly reduces the moisture content of the hide. One way to cure the skins is by wet-salting, in which the hide is heavily salted and then pressed into packs for up to 30 days. Another way is to soak them in a saltwater brine for about 16 hours.
The steps after curing are called Beamhouse operations.
The first step is to remove the salts left over from the curing in a process called soaking. This process involves soaking the skins in water which has certain ingredients added to prevent bacterial and fungal growth.
After soaking, the hides and skins go for liming. This means soaking the hides in a drum or pit filled with milk of lime, an alkaline solution. Liming results in the removal of natural grease and fats as well as keratin and hair. It also causes the swelling and splitting of fibers to the desired extent and prepares the collagen in the hide to a condition that is ideal for tanning.
After liming, the pelt goes through a machine to remove fleshy tissue from the flesh side. This process, fleshing, involves removing connective tissue and meat residue from the flesh side of the skin. It uses rotating scraping rolls.
Unhairing follows fleshing, which sounds exactly like what it is; the removal of hair from the hides. After applying unhairing agents most of the hair is then removed, first with a machine and then by hand using a dull knife.
Next comes deliming, the step in which the alkalinity of the hides is reduced by adding acids to the hides in a drum or pit. The swollen fibers of the hides shrink once more in preparation for the bating process. The deliming process can take about 2 hours for cattle hides.
The hides can then be bated, depending on the intended use of the leather. Bating involves the addition of enzymes to the hides to soften them.
After bating is pickling, which involves treating the hides by salt and then acid. The salt prevents the adverse effects of a sharp increase in acidity from the acids used in pickling. This process is important because it prepares the collagen for maximum penetration of the tanning agents in the tanning stage.
Degreasing comes next. This process uses solvents or water-based systems to remove excess grease before tanning.
Tanning Process
After the preparation stage comes the tanning. There are many ways to tan hides, but the end result is that the hides will not putrefy and are ready for dyeing and other uses such as manufacturing goods.
Chrome tanning is a relatively recent method of tanning, but easily the most dominant. It involves soaking the hides in baths containing acidic salts until they are ready for ttanning.
There are two main methods of tanning, one using a variety of natural products, the other using chemical solutions
The most common form of natural tanning, called vegetable tanning, has been in use since ancient times and involves using the tannins from the bark and leaves of trees and plants. After the preparation phase, the hides are placed in the tanning pit, where there are drums containing tanning solutions of various strengths. Over a period of two to three months, the tanners move the hides from drum to drum, with the solution getting progressively stronger. The tanning agents penetrate the skins and the fermentation and depositions process leaves the hides with a distinct color and appearance. Other natural products can be used to replace the tannins, such as bat guano.
Natural tanning produces a very high quality leather (often called "veg tan leather") that can be used for a huge range of applications. This is the leather we mostly use as it can be carved into and impressed with designs.
The fibers of the animal hide used for the leather are visible in the finished product, lending an authenticity and personality to the leather that makes it more personal. It also has a distinctive sweet, woody fragrance that is normally associated with leather.
Chrome tanning is faster than vegetable tanning and can take up to a day with modern machinery and a cocktail of chemicals. The bath includes chromium tanning agents and the acidity of the bath is decreased until the surface of the hides starts absorbing the chromium agents. The wet hides produced from this process are blue, and known as “wet blue hides.”
After the initial tanning process, the leather is cut into various thicknesses as required by the leather craftsmen. It is then further processed, usually by adding a thin coating finish to the product to give it a more "natural" feed" for making leather goods. Being a highly automated process, this kind of leather is popular and costs less than more labor-intensive tanning methods. This is the leather most commonly used in upholstery.
Brain tanning is a labor-intensive and specialized way of tanning hides. It is usually practiced by hunters to preserve the hides of fresh kills. Animals such as deer, elk and moose have enough acids of the right type to tan their own hides. The process varies from person to person, but the method involves first cleaning the hide thoroughly by scraping off the flesh, fat and membrane covering the inside of the hide. After this, the tanner washes the hide thoroughly, wrung and stretched on a frame.
A tanning solution, made by mixing warm water and the mashed-up animal brain, is applied by rubbing it onto the hide. Once the hide is ready, it is smoked to complete the process.
Tanning skins and hides with animal fats and oils is a very old method that involves fat-rich animal substances such as fish oil, sebum or marrow. Other substances used include soap, claw oil, yak butter or egg yolk.
Synthetic tanning is frequently employed as an alternative to chrome tanning. It is not employed in isolation, but in combination with either vegetable tanning or chrome tanning. It uses artificial tanning agents such as formaldehyde, glutaraldehyde, phenols and acrylates. After tanning, the wet and undyed leather has a bright color which gives it the name wet white. Tanning with aldehydes and oils produce very soft leathers like those used in car seats. This system can be used to produce dry-cleanable and washable fashion leathers as well as chamois leather.
How Tanned Leather is Sold
After the tanning process, the finished leather is sold to traders. The leather industry uses terms and measurements for the various ways that leather is sold. Some of these are a side, hide, skin, split or belly.
As the size of each side, hide, skin, split or belly varies, the size of the product varies as well. In some cases measurements are in square feet or square meters, while thickness is measured in millimeters and also in ounces.
Because the leather is split into various thicknesses having defined weights, conversion from width to weight is straightforward. Scrap leather is usually sold by weight.
Environmental Concerns
The tanning of hides has traditionally been done using methods that involve organic materials like tree bark and leaves. This kind of vegetable tanning had a smaller impact on the environment since it requires only water which is then discharged into the river or lakes. Because the effluent is organic, it eventually breaks down in the water and does not pollute.
Similarly, brain tanning uses the acids and fats in the animal’s brain to tan the leather, with the only impact being water use. Again, the biodegradable material is not a threat to the environment.
On the other hand, chrome tanning uses several chemicals which can be harmful to health and often end up polluting the air, soil and water. Effluent treatment is mandatory in many countries, but controls are often ax in developing countries, leading to high levels of pollution around tanneries.
Tanneries are usually associated with a very strong smell due to the processes emitting a distinct stench. Historically, this meant tanneries were located some distance away from settlements and towns.