[Tanning]From skin to leather
Raw hides are usually pickled in salt to prevent spoilage, except for domestic hides. Cowhides are imported mainly from the United States, horse hides are imported from Scandinavia, Oceania, and other countries. After the blood is washed and the body temperature is lowered, the horse is then brushed with 16-17 kg of salt, but if the timing is wrong, ticks, dirt, and mold can result. After such a process, the leather is imported to Japan.
Water soaking is an important process to remove blood and dirt from the skin, replenish dehydrated water, return the skin to its raw skin state, and facilitate later chemical treatment. Dehydration and coal lime soaking swells the lime milk, loosens the collagen fibers of the skin, and breaks down and removes hair, fat, and epidermal layers.
After dehairing and liming, the skin is divided into right and left halves along the spine in a process called back splitting. A freshening machine is then used to remove pieces of meat and fat adhering to the inner surface of the skin. In some cases, this is done after coal lime pickling, and the thickness of the skin may be adjusted in this process.
First, the remaining pieces of meat and fat from the backing are removed again, and then the skin is divided into two layers, the grain and the flesh side, to a specified thickness using a band knife machine. (In some cases, the skin is split after tanning.)
Remove hair follicles, etc. that could not be removed in the epilating and incision pickling stages, and clean the grain. Remove the plaque using a scutting machine or similar device.
Re-immersed in coal lime milk to loosen the collagen fibers of the skin due to the alkali. Remove the lime remaining in the hides by dehairing, lime-soaking, and re-soaking. This neutralizes the lime-soaked naked hides and facilitates the penetration of the tanning agent in the tanning process. Fermentation (basting) uses proteolytic enzymes to break down and remove unwanted proteins and smooth the grain. Next comes acid maceration. The chemicals used in the tanning process must be acidic to dissolve, so the hides are immersed in an acidic solution prior to the tanning process to make them suitable to absorb the tanning agent.
Chromium-tanning agent (trivalent) is infiltrated into the hide and bonded to the collagen fibers to give it durability, including heat resistance. This produces the wet blue color shown in the photo. After that, excess water in the leather is squeezed out by a water squeezing machine. The inner surface of the leather is then shaved by a shaving machine to a certain thickness.
The acid in the leather is neutralized by alkali and adjusted to ensure uniform penetration of dyeing and fatting agents. The leather is then dyed to the desired color using dyes. The basic requirements of dyes are color, solubility and dyeability to leather, and dyeing is the process of coloring materials using dyes with these requirements.
Drying to allow the dyes and additives in the leather. Natural drying, hot air drying, or vacuum drying. This is an important process that directly affects the feel of the leather.
The leather is then moistened appropriately to make it easier to knead. The leather is then wrung out by a staking machine to give it flexibility and elasticity. It is then nailed to a tension board or toggled on a mesh board, dried flat, and the flavor (moisture) is removed.
The grain is painted so as to emphasize the beauty of the appearance with color and luster and to obtain the durability of the leather.
Surface glazing by machine or ironing. Also do stamping and firring if it's necessary.