Friday, June 26, 2009

Leather Tanning Process Outlined

At Advanced Leather Solutions we are constantly working on leather repair and restoration projects. For us to do our job right, we must have a deep understanding of leather. In that context, knowledge of the tanning process is useful. What you will learn is that not all leather is the same. The differences are broad and varied. It's the specific processes the hide is goes through that determines its characteristics.

The following details are posted for those readers interested in a deeper understanding of how leather is created. For most consumers or even leather technicians, this post will induce deep yawns and sleepy eyes. You can quickly skip through it picking up on the high points or dive into the details. As such it might be useful reading for an insomniac. With that warning, this outlines a complex process that moves a skin from the point of harvest to a useful material we know as leather.

TANNING
The Basics.... Leather is a natural product. It comes from animal skins which have been chemically processed to preserve them. The chemical procedure used to prepare raw animal hides for use as upholstery, shoes or other applications is called tanning. A properly tanned hide or skin creates strong, flexible leather, resistant to decay.

The majority of today’s leather comes from tanned cow hides, though many types of hides can be used.

TANNING STAGES
Harvested skins enter the tannery facing a multi-step chemical and mechanical procedure. The type of hide and the desired end-product determines what specific procedures are employed for the desired end result.

CURING
Hides are cured first through salting and/or drying the hide after it has been harvested. As hides are normally a by-product of the meat processing industry this step often takes place inside a meat-packing facility. It’s important to do this fairly quickly in order to arrest the natural decaying process.

Hides can be cured in one of two ways:

Brine-curing is the preferred method as it’s quicker and easier. The hides are put in large tubs or vats and infused with salts and disinfectant. After about 12 hours, the skins are completely cured, ready for the next stage.

A more primitive method is Wet-Salting. It’s accomplished by salting the hide, then placing many skins on top of each other making a damp clump of skins. Left to cure for a month, the salt is completely absorbed into the skin, preserving the hide.

SOAKING
Once cured, the hides are soaked and rinsed with water. This rids the skins of salt residue, dirt, and other unwanted materials. Did I just detect a yawn? If so, skip to vegetable tanning section.

FLESH and HAIR REMOVAL
After soaking, the cured hides are processed on a machine that removes all remaining flesh. The hides are then immersed in a mixture of lime and water, loosening the hair from the skin. After about a week, the hair is taken off the hide by machine.


SCUDDING
Scudding is done by humans. Stray hairs, etc., missed by the machine are removed from the hide by hand tools.

DELIMING
Hides are then de-limed with acid. After the lime is removed, hides are exposed to enzymes. This process evens the grain of the leather. The resulting product is soft and flexible leather.

VEGETABLE TANNING
Vegetable tanning agents create some flexibility in the leather, but its primary characteristic is to develop tough, durable leathers. Example application would be luggage, leashes, belts, straps, saddles, and harnesses.

Vegetable tanning is accomplished in large vats where the hides are covered with tannin. Tannin comes from the bark, wood, leaves and fruits of chestnut, oak and hemlock trees. From beginning to end, the hides are exposed to increasingly stronger tannin solutions. As with all tanning, vegetable tanning stops the decaying process.

CHROME or MINERAL TANNING
Mineral or chrome tanning produces softer leathers with more stretch, such as those found in fine purses, upholstery, gloves, jackets, etc.

Initially, the hides are pickled with acid and salt, then, soaked in a chromium-sulfate solution. This process is much faster than vegetable tanning, typically a 1-day project.

SPLITTING
This is kind of important to know. Most harvested cow hides are 1/4 to 3/8 inch thick --- too thick for most purposes. To create the appropriate dimension the tanned hides are split laterally using a machine that's like a horizontal band saw. This process renders an upper and lower image of the hide. The process creates a grain side (top-grain or epidermis) of somewhat consistent thickness. The top grain side is the outer surface and maintains the natural grain. This is also the dominant component of the leather's durability and tensile strength.

The operation also produces an inner portion of the hide known as a "split." It initially appears as suede. Splits have no grain pattern and were historically considered a waste byproduct. Some if it was used for suede garments, book binding and other applications where durability and tensile strength are not required. However some clever chemical engineers developed a process where splits are coated with an epoxy/resin mixture forcing the suede fibers to bind together rendering a smooth surface. Then a machine stamps a mechanical grain pattern into the coated split. This once waste product is applied to furniture and sold as leather. Unfortunately for the unsuspecting consumer, this material lacks the durability generally associated with leather. It's clearly NOT the real deal.

RETANNING
Hides are often reprocessed through the tanning cycle to improve and attain a specific physical characteristic. In many cases a combination is used to develop leathers that have the positive characteristics of each tanning process.

CRUST
A hide that has been fully tanned, but not colored in any way is called a crust. Chrome tanned leather is a light grey color belying the notion that many people have about the “natural” color of leather. Vegetable Tanned leather is the classic tawny color that is most indicative of "natural" leather color.

DYEING
The hides then are dyed. Typically the dies used are aniline dyes. This is accomplished in vats of heated dye solution where the dye permeates the entire cellular structure of the hide. This process may also add moisture back into the skin. Vegetable tanned hides may be soaked with oil, grease and waxes to make them more pliable and give them different use characteristics.

FINISHING
Depending upon its intended application, this step may include covering the grain surface with a chemical compound, then brushing, buffing and sanding the surface. Leathers which are sanded for long periods of time become brushed or Nubuck. The sanding step may also reduce the amount of imperfections in the hide. However, it erodes the epidermis, which is the most important part of the hide, contributing the dominant component of strength. Waxes, glazes, oils, and other solutions may also be added.

PIGMENT COATING
Most upholstery grade leather goes through a final phase. One, or in many cases several flexible, pigment coatings are applied to the leather which determines the color presented on the outside of the hide, then a clear-coat is applied, which determines the sheen. These are protective coatings that enhance the wear, fade and stain resistance of the leather.

Wow, you made it through without falling asleep. The most important thing to take away from this is that not all leather is the same. The process can be very sophisticated producing leather with an expected 30 year or more useful life. It can also produce low quality leather with a life expectancy of a few years at best. To learn more visit our web-site at advleather.com.

Copyright 2009, Kevin Gillan

1 comment:

  1. Hey Kevin,

    I am restoring an old chair for a client. It has a deer hide seat. The hide is structurally strong. I would love to restore it, but someone put MANY coats of yellow paint over it along the way. Do you have any suggestions of what I can do to remove the paint without destroying the hide? This is my first experience with hides, so all advice will be appreciated. KJ

    ReplyDelete