Patent leather is leather that has been finished with chemicals that give it a shiny, reflective surface. It is usually black, and has long been popular for dress and dancing shoes. Most stages of the preparation of patent leather are the same as for other fine quality leathers. However, it is in the final finishing stage, when it is coated with a lacquer to give it its characteristic glossiness. All leather is derived from animal skins or hides. Most hides are a byproduct of the meat industry. The hides of cattle slaughtered for beef form the bulk of the leather industry. Other common leathers are made from the hides of sheep, goats, and pigs, and so-called novelty leathers are derived from reptile skins, such as alligator and snake, and even from the ostrich. Patent leather is usually light and thin, and usually derived from a calf or a kid. Today, however, patent leather can be made from any kind of hide, and need be of no finer quality than most shoe leathers.
Mammal hides are comprised of three layers: a hairy outer layer, a thick central layer, and fatty inner layer. The process of making leather, called tanning, involves removing the fat and the hair, and working a chemical change on the thick middle layer to preserve and strengthen it while giving it flexibility. A hide removed from a slaughtered animal begins to decompose within just a few hours. So the first step in tanning is to preserve the hide. Throughout history, this was usually done by salting. Then, the preserved hide is treated in any of a number of ways to remove the hair and dissolve the fat. It is then treated with chemicals that work on the collagen, a fibrous protein making up most of the middle layer of the skin. The word tanning derives from tannin, a chemical found in many plants that reacts with collagen to strengthen its molecular bonds. When tanned, the original hide becomes strong, elastic, and durable.
The treatment of animal hides to make leather is an ancient art. The basic technique of tanning leather dates back to prehistoric times, when primitive peoples apparently tanned hides with plant matter. The ancient Egyptians and the Hebrews tanned leather with plant products. The Hebrews used oak bark, and the Egyptians the pod of a plant called babul. The Romans had a thriving tanning industry, using certain tree barks, berries, and wood extracts. Tanning was lost in Europe during the Middle Ages, but the art was kept alive in the Arab world, and reintroduced to Europe later. By the eighteenth century, tanning was widespread in the Old World and the New. Though tanning was a relatively low-technology operation, it still required some specialized tools, such as fleshing knives, scrapers, and soaking vats. Up until the late nineteenth century, all tanning chemicals were plant derivatives, such as hemlock, oak, or sumac bark. Tanners salted hides, soaked them in lime to dehair them, delimed them in an acid solution, usually manure, and then soaked the hides in increasingly strong solutions of vegetable tannin.
At the end of the nineteenth century, chemical tanning became possible. In this method, the tanning agent is chromium sulfate. The process was discovered in 1858, and the first commercial production of chrome tanned leather was in New York in 1884. Though the initial method had some drawbacks, chrome tanning quickly replaced vegetable tanning. As the industry developed in the twentieth century, the tanning process was increasingly mechanized. Large machines made high volume possible. Earlier tanneries were usually situated near a source for vegetable tanning materials, such as the many that grew up in Virginia, Tennessee, and North Carolina in the United States because of the availability of chesnut wood. By the early twentieth century, vegetable tannins were being imported in large amounts from South America, and the ingredients for chrome tanning were not tied to any particular locality. Tanneries thus could be built anywhere, and centered in the Midwestern region of the United States, site of most beef slaughtering. Entering the twenty-first century, the tanning industry in the United States is declining as low labor costs in other parts of the world make imported leathers more economical.
Leather has many uses and comes in many forms, from thick, sturdy cow hide leather for straps and harnesses to soft kid leather for gloves. The most common shoe leather up through the nineteenth century would have been a very heavy sort to make sturdy boots. For practical purposes, both men and women in Europe also wore wooden shoes or iron-soled shoes called pattens to hoist the wearer above the mud and muck. From the time of Louis XIV up through the early nineteenth century, men's shoes were more subject to the whims of fashion than women's, as women's feet were usually covered by voluminous skirts. The exception was dancing shoes. Both sexes of the upper classes craved fancy, fashionable flat shoes for balls and parties. It was for this kind of shoe that patent leather first became popular. The process for making patent leather was invented in 1799 by an Englishman, Edmund Prior. Prior patented a process for painting leather with dyes and boiled oil, and finishing it with an oil varnish. In 1805 another patent was granted, this time to one Mollersten, for a leather finishing technique using linseed oil, whale oil, horse grease, and lamp black. The shiny, black, waterproof surface offered by this patent or "japanned" leather set off a craze for it in England and abroad. Patent leather first appeared commercially in 1822, and remained popular in cyclical fashion through the present day. The earliest patent leathers would have been made from fine leathers, such as calf or kid. The leather was tanned by the usual process for making black shoe leather. From there, the tanner carefully coated the leather with a varnish imbued with dyes and other ingredients. A patent in 1854 described the varnish ingredients as "oil, amber, Prussian blue, litharge, white lead, ochre, whiting, asphalt, and sometimes copal." In practice, many tanners kept their varnish recipe secret, and even the ingredients listed in patent applications may have been falsified in order to throw off competitors. Linseed oil of sufficient purity and the dye known as Prussian blue seem to have been the basis of most patent leather finishes. Starting with a fine, black leather, the tanner built up layers of varnish, applying as many as 15 coats, drying the leather in the sun or in a stove in between. The trick was to get a smooth, hard finish that was also somewhat elastic, so the leather did not crack later. The modern process for producing patent leather is not very different, except in mechanization, from that used in the nineteenth century. The same problem exists of finding a balance between a hard finish and a flexible one, and manufacturers use varying recipes and techniques.
The earliest patent leathers always started with a fine quality leather. Because the varnishes used today work better than the early linseed oil formulas, now almost any quality leather can be given a patent finish. Most patent leather today begins with cattle hide. The finish is a blend of polyurethane and acrylic. These two materials have different characteristics. Polyurethane gives a hard finish, shiny and durable, but acrylic results in a more flexible final product. So leather chemists combine the two for optimum qualities. The actual finish used thus will be different from tannery to tannery, and perhaps from batch to batch. The finishing material is also imbued with black dye. Dye formulas vary widely from plant to plant, as well. Other raw materials are common to leather manufacturing as a whole: salt for curing the hides; disinfectants; lime or other caustic chemicals for dehairing; various acids and salts for deliming the hides and getting them to the proper pH balance for tanning; chromium tanning salts, and water for various stages.
Quality control differs from tannery to tannery, and it depends mostly on for what the customer contracts. Good patent leather should not crack, the finish should be thoroughly dry and hard to the touch, not tacky, and it should not scuff easily. A fully equipped tannery might carry out tests for all these conditions, as well as chemical analyses of the finish. Other tanneries may just visually inspect the end product. Usually, the customer for the finished patent leather must agree with the tannery what tests should be carried out or what standards the leather should meet.
Tanning leather and finishing it into patent leather creates much waste water. And if a spray application of the finish is used, this creates air pollution. In the United States in the 1980s, the Environmental Protection Agency (EPA) stiffened its standards for air emissions from tanneries, and as a result, most now use water-based finish applications. Tanneries must find ways to deal with waste water, which is heavily polluted with chemicals. The water can be cleaned in a wastewater treatment facility. Then the cleaned water can be reused by the tannery. Some leather byproducts can also be reused. Rawhide scraps can be sold as dog chews. The waste hair, fat, and other animal solids can be collected and made into fertilizer. Though tanning is an industry that has a reputation for pollution and unpleasant smells, it is possible for a dedicated plant to recycle its waste for miminal environmental impact.
McDowell, Colin. Shoes: Fashion and Fantasy. New York: Rizzoli, 1989.
Thorstensen, Thomas C. Practical Leather Technology. Huntington, NY: Robert E. Krieger Publishing Co., 1976.
Welsh, Peter C. Tanning in the United States to 1850. Washington, DC: United States National Museum, 1964.
McDowell, John. "Leather Company Creates Alternative to Landfilling." BioCycle (June 1998): 32.
— Angela Woodward