Creation of Leather
HIDES AND PROCESSING: The two critical factors
The quality of the finished product is always dependent on the quality of the materials and processes that go before them. In the case of leather, the raw material is the hide taken from beef cattle. To insure that each hide can achieve its quality potential, it must be processed quickly, using special techniques to enhance its natural characteristics.
HIDES: The raw material
The single most important factor determining the quality of leather is the raw material The Hide. It is the same principle that you will find in cooking (the recipe is only as good as the ingredients) and computers (garbage in, garbage out). If the hide is deteriorated or of low quality, there is little that the tanner can do to improve it.
The term hides usually refers to the skin coverings of larger animals, cows, steers, horses, buffaloes, etc. Those of smaller animals such as goats, pigs, sheep, calves, etc are called skins. Both of these items can be used interchangeably when referring to the animals skin. Most hides used in the manufacturing of upholstery are cow and steer hides because of their size. Sofas and sectionals require large pieces of leather to minimize the number of seams.
Just as meat is perishable, so too are large hides. If they are not cleaned and treated quickly the hides will begin to decompose and lose their leather making properties. Processing basically includes two parts, curing and tanning.
Curing is the initial protective treatment administered to the pelts. It only preserves the hides prior to changing them to leather. The methods more commonly used today employ salt (Sodium Chloride) as the principal curing agent. Tanning is a complex process that changes the raw hide into the stable, long lasting material we know as leather.
Less than 5% of all hides harvested each year are suitable for upholstery. This is due to the size of unblemished pieces required to make a leather sofa. The balance of the hides are used for clothing, shoes, bookbindings, luggage and other goods.
Tanneries buy leather hides by the pound from slaughterhouses. They receive the hides while the hair is still on them in a state we call salted hides. Untreated hides rot if kept damp and dry ones harden. Salted hides are packed in containers and shipped to tanneries all over the world.
SELECTION AND LIMING:
A tannery does not know what kind of hides they have purchased until the hair has been removed and the hides are physically sorted by hand. This process of removing the hair is called liming. Liming is accomplished by placing the hides in large narrow drums shaped like wheels. The drums rotate for many hours until all of the hair is removed.
After hair removal, the hides are then sorted by quality before tanning. This process is called selection. The highest quality hides are those with the fewest markings. The least amount of work must be done to these hides to prepare them for finishing.
Tanning preserves the hide and makes its natural characteristics permanent. Chromium tanning is a relatively new process developed at the turn of the century. This is the reason why current day leather is softer and properly tanned leather does not crack, fade or peel. Prior to that time hides were vegetable tanned. This is an age-old process of simply soaking the hides in vegetable oils and letting them air dry in the sun.
Chromium tanning is accomplished by placing the hides back into the large rotating drums for 24 hours. The drums contain alkaline chrome salts. The hides emerge with a slight bluish tone. This state is called the wet blue state.
The majority of cows have natural markings on their hides such as stretch marks, barbwire marks, brands, tick bites and horn cuts. These markings are the natural story of the life of the animal. In a sense, they are a romantic story telling its history: Stretch marks from giving birth, horn cuts from a fight with a predator or fellow bull, barbed wire marks from fencing, etc. Brands are a different kind of marking, but just as important. Understandably cows are branded to identify ownership. However, in disease-ridden areas of the world, such as Africa, cows receive a brand each time they are inoculated. It is not unusual for and African cow to have 30-50 brands on it, thus making it impossible to use for upholstery.
After sorting, the hides are selected for their intended use and go to a splitting machine. This large, long machine cuts the leather into the desired thickness. Thin for clothing, thick for upholstery and thicker yet for special effects on luggage. The thickness is gauged in millimeters and/or by its weight. 1-1.5 mm will equal 9-11 ounces in weight, which is normal for upholstery. The weight of a piece of leather is a factor in determining the ultimate cost per square foot. This is because the tannery sells the products of the split leather for other uses such as shoe interiors or dog bones. This assists them in utilizing more of the leather hide and a greater yield.
The top portion of the hides is called the top grain. The second cut or underside is called the split or crust and the third cut is also called a split. The second cut and sometimes the third cut, if thick enough, is processed as suede and other times is processed with chemicals and corrected grain plating to make it look like the top grain. This is done to offer the consumer a lower price on some leather furniture.
The average thickness of a cowhide is 5mm. Upholstery leather requires a minimum thickness of 0.9-1.1mm. The hides are split in a splitting machine, which uses a fast running wire cutter. This operation divides one hide into two: The grained hide and the split. The strength of the fibers will vary within the hide. The fibers are very strong on the grain side and weaken as we go deeper in the hide. The split has much less resistance and flexibility than the grained hide.
Thick bull hides can be split twice. Then we have a grain-split and a flesh-split. The flesh-split is very poor quality, and not capable of being used in upholstered furniture. Most splits are used for sports shoes, apparel or lower quality leather upholstery, as they will not be near as durable, resistant or as flexible as top grain leathers.
Because leather is fibrous, its thickness doesnt have much to do with its strength. Leather is one of the strongest natural covering materials known to man. Leather breathes because of its fibrous structure. This makes it unique to all other man made products. Leather will adjust itself to a persons body temperature within minutes of seating. That is why one does not perspire when seated on leather as you do on vinyl.
The final stage of tanning is the finishing process. Due to recent improvements in the technology of finishing leather many new types and textures of leather are available.
Finishing choices are similar to:
A. Painting your car (pigment finish)
B. Dying your clothes (Aniline dyed translucent finish)
In other words an opaque finish versus a translucent finish.
At the beginning of the finishing process all hides are placed back into the wheel drums to soften, remove excess water and apply a dye coat.
The size of these drums becomes important when trying to understand the difficulty of special ordering one leather sofa. The average drum holds 40 hides or approximately 2000 square feet of leather. One leather sofa can require anywhere from 110 to 200 square feet of leather, depending on its size and styling. All of the hides placed into the drum must become the same base color, so the minimum order a tannery will accept is 2000 square feet per color. As leather is quite expensive the investment required to inventory colors, which do not sell quickly, is cost prohibitive. As in any coloring process, dye lots will vary over a period of time. Inventory from one month will be difficult to match with inventory 6 months later.
CORRECTION BUFFING PLATING:
During the finishing process hides that are heavily scarred receive correction. The correction is done by first buffing or lightly sanding the hide and then embossing a grain onto it. This is accomplished through heat and pressure, in which grain is stamped onto the leather. A hot plate is used which has the grain pattern etched into it. This hot plate then permanently stamps the grain pattern onto the leather. Lesser quality hides receive more buffing and plating. With more plating the hide becomes firmer and less soft to the touch. Grains can also be decorative. Anything from an alligator pattern to the natural hair cell grain of the original animal can be stamped onto the leather. The worst hides get a heavier embossing pattern and pigmented finish, which also covers some of the scars. These hides are referred to as corrected top grain leather. The finest hides get no plate; they are only translucent Aniline dyed. These are called full grain full Aniline dyed hides. This leather is also very soft to the touch.
Hides, which are just slightly scarred, are called top grain semi-Aniline leather because a mild plate is used to cover some scars. This leather has a combination of the two finishes. It is also extremely soft and in most cases it takes an expert to tell them apart.
The lower quality hides get a heavier plate and greater concentration combination of pigment. These are called standard leather, still a top grain but not as soft to the touch. One can distinguish full Aniline and semi-Aniline leather by looking at the back or side of the hide. In full grain full Aniline leather, the back will be the same color as the front, dyed through and through. In semi-Aniline leather the back will be almost the same color as the front, the side will sometimes have a different color core. In standard leather the back will be the color of the base coat necessary to achieve that color.
After the initial finishing process, better quality hides are milled or tumbled in drums for several hours to give the hides added softness.