Processing of Leather
Raw hides (skins)
- Contain high amount (about 70%) of water and become hard and inflexible after drying.
- Can be easily attacked by bacteria and rot. They have to be kept refrigerated or the water removed by some means (not very useful for making into upholstery!)
- Are dirty with blood, dung etc sticking to it and contain hair and other materials which are not needed.
For making into product, the leather should be
- Dry and flexible
- Stable
- Should have color, softness and other properties for the specific use
Tannery operations achieve the above changes.
Tanning makes the leather fiber resistant to bacterial attack and is the most important processing in a tannery. Nowadays, chrome tanning is the most popular method.
The modern leather processing can be divided into three parts:
- Preparation of wet blue (chrome tanned leather)
- Modification of the properties as needed in the final leather.
- Finishing on the surface.
Most of the processes in 1 and 2 are done inside drums (or other vessels) as batch operations in water and a few operations are carried out in special machines on individual hides/skins
Preparation of wet blue (chrome tanned leather)
The first job in the tannery is to give the hides a good wash, with water, some special detergents and some other chemicals. This is called Soaking.
Any loose flesh (remember, hides are taken from animals killed for meat!) sticking to the flesh side is scraped off by a knife (using a specially designed machine). This is called Fleshing.
Unhairing removes the hair on the skin using certain chemicals, which can eat away the hair but not the leather making fiber.
The chemicals used to tan the hides and modify the color, softness and other properties should be able to penetrate into the fiber network. The fiber network should be opened up and most other non-fibrous material removed. This is done in Liming. It is a special soaking which uses alkali (lime) to increase the water entering the fiber network. Natural grease is also removed in this alkaline solution.
Thicker hides can be split in to two layers in a machine using flat knife at this stage. The primary purpose of this operation is to reduce the thickness to the required level and obtain an uniformly thick layer of high quality and varyingly thick layer of inferior quality.
Originally the hide is thickest in the neck portion of the animal and thinnest at the belly portion. The top layer, which is stronger and of uniform thickness, is used for making high valued leathers. The bottom layer is of non-uniform thickness and processed for different uses.
Then a series of adjustments in the acid/alkali balance (pH) of the hide is done before applying the chrome tanning salts. These operations are called Deliming and Pickling.
In case of sheep skins or other skins containing high amount of natural fat a separate degreasing is carried out to remove the fat.
Chrome tanning salts penetrate the hide and react chemically with the molecules, making the fiber network resistant to bacteria (Chrome tanning).
Modification of the properties as needed in the final leather.
The main properties modified here are:
- Thickness
- Color
- Softness
A number of other characters can be changed depending on the type of the leather being made by using specific chemicals.
Thick hides, which are not split before, are split into two layers in a splitting machine to get the uniformly thick grain layer and non-uniform flesh layer
Thickness is further reduced by Shaving machine, which shaves some thickness off from the back of the hide to make it more uniform in thickness.
After adjusting the pH balance, further chemical treatments are carried out. Retanning uses special plant material or synthetic material to modify a range of properties. Dyes used in dyeing color the fibers in the leather. Specially prepared oils are used in fatliquoring to make the leather soft (the oils lubricate the fibers). Special requirements such as water proofing, flame proofing, solvent resistance etc. can be fulfilled at this stage.
The leathers are stretched and flattened in setting machine and dried. After drying, the leather is subjected to mechanical operations to make it soft, flat and tight. Either side can be modified by emery paper on a buffing machine.
Finishing on the surface.
A wide variety of finishing operations is carried out on the leather surface. The most common type is coating with colored mixes. Also worth mentioning is buffing the leather surface with emery (sandpaper) to get suede or nubuck surface (velvet like surface) and application of oils and waxes to get pull up effect.
You can see the different types of finishing done here.
Finishing modifies the surface color, gloss, and the final appearance of the leather. It also provides a protection to the surface from water, wear and abrasion.