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General Leather Knowledge
FULL-GRAIN LEATHER: (versus plated, embossed, buffed or corrected leather)
This is the term for the leather surface without an attempt to hide or conceal the natural markings of the animal which it incurred during its lifetime. Life leaves its traces on cattle in the forms of healed scratches, insect bites, blood veins, growth wrinkles, and variation of grain, all of which constitute the character and charm of "Nature's Signatures". Full-grain leather can be either pure aniline or semi-aniline (or aniline plus). It is sometimes plated to produce a special effect or even to enhance an already natural grain, such as a textured appeal versus a smooth appeal.
PLATED, EMBOSSED, BUFFED OR CORRECTED GRAIN LEATHER: (versus full-grain leather)
When full-grain leather has too many of "Nature's Signatures", it is buffed or sanded much in the same manner as wood is sanded. The sanding or correcting process levels the high spots of healed scratches, etc. and removes some of the natural grain. A grain pattern is then embossed in to the surface to replace what was lost in the sanding process. Plated leather is usually aniline plus (or semi-aniline).
TOP GRAIN LEATHER: (versus split)
The thickness of all hides before tanning can vary quite a bit. To obtain a uniform thickness for upholstery, the hides are fed through a splitting machine with the grain side up, yielding a grain portion called top grain. The underneath or flesh layer that is cut off is called a split.
SPLIT LEATHER: (versus top grain)
This is the flesh on top and bottom and possesses no natural grain. The tissue structure is not as strong as top grain and is generally not recommended for the inside seating area of upholstery due to this lack of tissue strength.
FULL OR PURE ANILINE: (versus semi-aniline or aniline-plus)
It is generally accepted that only five
percent of the world's hide supply is of high enough quality for pure aniline upholstery furniture. Aniline dyeing is the process of soaking the skins in transparent aniline dyes which color or shade the skins without obscuring the natural markings or grain character of the leather. The hides are dyed all the way through with no pigmented topcoat.
SEMI-ANILINE OR ANILINE-PLUS: (versus full or pure aniline)
There are terms that define a supplemental step beyond aniline dyeing which adds a top coating of highly dispersed pigments and dyes to the surface of aniline dyed hides. By dyeing the leather throughout before the final surface coating, a very even coloration can be achieved with only a thin layer of finish, and the leather remains softer.
The Leather Process
The processing of hides and skins into leather is a fascinating procedure that requires a precise combination of many chemical and mechanical operations. Here you will find a short summary of these operations provided by our suppliers.
Deterioration begins immediately when a cow is killed. After the hides are removed from the carcass, they are salted through and through at the slaughter houses to prevent decay. After they are salted, 55% of the water in the hide is removed, and they 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, or depilatory agents, which destroy the hair by attacking the hair root so it will release freely from the hides, loosen the epidermis, and 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 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-which are enzymes similar to those found in the digestive system of animals. When the bates are applied, they attack and destroy most of the remaining undesirable constituents of the hide.
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 which 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, the ability to flex innumerable times without breaking, and the 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 to be 5mm thick. The thickness for upholstery leathers range from .9mm to 2.0mm. The hides are put through a splitting machine that is set to split the hides to the desired thickness. It 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 retanning 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. Fatliquoring has the most pronounced effect on how soft a leather will be and it contributes greatly to its tensile strength. The more fatliquors that are added, the softer the hides will be.
This operation smoothes and stretches the hide, while compressing and squeezing out the excess moisture. This puts the 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 the drying ovens.
Leather is staked to make it pliable. In combination with the correct fatliquoring treatment, staking governs the final firmness or softness of the leather.
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.
This is the final step in the leather process. During this process, heat presses a chosen grain into the surface of the hides.
NATURAL MARKINGS IN LEATHER
Leather is a natural product, and like anything natural it is not always perfect. It is this natural imperfection that attracts the leather aficionado. Leather will have markings that are considered "nature's signatures", and this is what relates to the public that the product is genuine leather. Leather will have several different types of these natural markings.
These are the result of the cattle being bitten by various types of insects. These bites result in tiny scars.
These are the result of the cattle being scratched in one way or another.
Cattle like to scratch their necks on the rails of fencing. This scratching can result in a neck burn if the cattle scratch too hard or very often.
Cattle sometimes get manure on their hide. This is a very irritating substance. If they do not get it off soon enough, it can result in a burn on the hide.
Holes are usually man-made. They are the result of removing the skin to gain access to the meat.
BARBED WIRE OR OPEN WOUND HEALED SCARS:
Sometimes the cattle get their hides caught in barbed wire. This results in a tear in the hide. These healed scars are just as durable as the parts of the hide that aren't scarred.