The process of hides and skins into leather is a fascinating procedure that requires a precise combination of many chemical and mechanical operations. Below you will find cutting instructions for your leather, along with a step-by-step summary of these processing operations as provided by our suppliers.
Follow this step-by-step process to see how tanneries convert a natural resource into leather hides.
Deterioration begins immediately when a cow is killed. After the hides are removed from the carcass, they are salted through and through at the slaughterhouses to prevent decay. After they are salted, 55% of the water in the hide is removed, and the hides are dried for 3 to 6 days. The rawhides are then sold to tanneries.
In order for the tanning process to work properly, the dry-salted hides must be washed free of the salt. This is done by soaking the hides in water to which chemical wetting agents (similar to household detergents) and disinfectants are usually added for 8 to 20 hours, depending on the thickness of the hides. This soaking procedure rehydrates the hides to their original flaccid condition and removes the dirt.
The hair must now be removed from the hides. This is done by soaking the hides in chemicals (depilatory agents), which (a) destroy the hair by attacking the hair root so it will release freely from the hides, (b) loosen the epidermis and (c) remove certain soluble skin proteins that lie within the hide substance without destroying the desirable collagen of the hides.
Excess flesh, fat and muscle must now be removed from the hides. This is done with a fleshing machine.
All of the depilatory chemicals must now be removed from the hides. This is done by washing the hides in ammonium sulfate or ammonium chloride and then clear water in big drums. These chemicals not only clean the depilatory chemicals from the hides, they also adjust the acid-alkaline conditions (pH) to the proper point for receiving the bate — enzymes similar to those found in the digestive system of animals. When bates are applied, they attack and destroy most of the remaining
The hides must be placed in an acid environment (low pH) so they will be ready to accept the tanning materials, because chrome-tanning agents are not soluble under alkaline conditions. This is accomplished by adding salt and acid to the hides. This is a preserving process in itself, and hides can be kept in this state for extended periods of time without any deterioration.
The raw collagen fibers of the hides must be converted into a stable product that is no longer susceptible to rotting. This is done by adding chrome-tanning agents to the hides in a revolving drum. These tanning agents also significantly improve the hide’s dimensional stability, abrasion resistance, resistance to chemicals and to heat, ability to flex innumerable times without breaking, and ability to endure repeated cycles of wetting and drying.
The excess moisture must be removed from the hides. This is done by placing each hide through two large rollers similar to those on a clothes wringer.
The hides must now be split into the desired thickness. Unsplit hides average 5 mm thick. The thickness for upholstery leathers ranges from 0.9 mm to 2.0 mm. The hides are put through a splitting machine that is set to split the hides to the desired thickness. The machine cuts the top-grain off first. Another layer and sometimes two are cut. These layers are called splits.
The thickness of the hides must be made uniform all over the hide. This is done with a shaving machine through which the hides are run. The helical-shaped cutting blades level the overall thickness to exact specifications and open the fiber structure to better receive subsequent chemical processing.
This process is done to impart special end-use properties with other tanning chemicals. The substances used add solidity and body to chrome leather and help minimize variations in the character of the leather that may still exist between different parts of the hide.
As soon as the re-tanning process is completed, aniline dyes, derived primarily from petroleum and added to very hot water, are added to rotating drums to penetrate the hides for desired color.
This is the last of the wet-chemical operations to which the leather will be subjected. Fat-liquoring has the most pronounced effect on how soft a leather will be and it contributes greatly to its tensile strength. The more fat-liquors that are added, the softer the hides will be.
14 SETTING OUT
This operation smoothes and stretches the hide, while compressing and squeezing out the excess moisture. This puts hides in the proper condition for drying.
The hides are stretched across a perforated frame and held in place with clips called toggles. One hide is clipped to each side of the frame. The frames are then slid into channels in drying ovens.
Leather is staked to make it pliable. In combination with the correct fat-liquoring treatment, staking governs the final firmness or softness of the leather.
17 DRY MILLING
The hides are placed in a large dry drum and tumbled until the desired softness is obtained.
This process improves the final appearance of the hides by lightly sanding the surface to remove some of the natural imperfections such as scratches, healed scars, etc. It provides the hide with better cutting yield.
This process applies film-forming materials on the surface of the hide. Here is where layers of pigments are added, if required. This process also adds the protective sealant to the surface.
20 PLATING (EMBOSSING)
This is the final step in the leather process. During this process, heat presses a chosen grain into the surface of the hides.