Patent Leather

views updated May 29 2018

Patent Leather

Background

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.

Raw Materials

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.

The Manufacturing Process

Preparing the hide

1 The hide used is usually cow, and it is produced as a byproduct of the meat industry in most cases. That is, cattle are principally slaughtered for their meat, and then the hide is sold to a tannery. The hide is removed by skilled workers who cut it carefully to preserve its integrity. Any stray cuts or marks can seriously affect the quality of the hide. Within hours after removal, the skin will begin to decay because of the large amount of organisms both on the hair side and the meat side. So the skin is immediately preserved in salt. The hides may be simply laid down, covered with salt on both sides, and the next hide stacked on top. Alternately, in a large commercial slaughterhouse, the hides are taken from the killing floor and sent through a chilling machine. This is a large tumble washer that both cleans off surface dirt and manure, and brings the temperature of the hide down so that the clinging fat solidifies. Next, workers pass the hides through another instrument called a fleshing machine. A pair of workers feed the hides one at a time through the cylinders of the fleshing machine, where the manure is knocked off into one container, and the remaining fat and meat into another separate container. The fat and meat can be sold by the slaughterhouse. The cleaned hides are then loaded into a vat of brine.

At the warehouse

2 After the hides have cured in the brine for at least 24 hours, the slaughterhouse ships them to the tannery. In the United States, most tanneries maintain large warehouses for cured hides, and they could store hides for as long as a year before any further processing. This practice changed around the late 1970s, and now most domestic tanneries work on the "just in time" manufacturing principle, keeping very little hide in stock. So though the cured hides could be kept for quite some time before tanning, in present-day practices, they might proceed directly to the next step.

Soaking, liming, and bating

3 The cured hides undergo several steps at the tannery before they are ready for tanning. These collectively are called the "beamhouse" operations. Total time in the beamhouse takes 12-24 hours. The term beamhouse derives from ancient practice, when the hide was hung over a special curved log or table known as a beam for the dehairing. First tannery workers soak the cured hides in cold water in a vat or drum. This removes the salt from the brine cure. Or if the hides have been cured in dry salt, it rehydrates them. Next lime or another caustic chemical is added to the soak, to loosen the hair. The hides swell up at this stage, becoming blue-white and rubbery. Then, the hides go through a step called bating. Bating gets rid of the hair and fat and other unwanted particles. It also slowly reduces the pH of the hides, from highly alkaline to neutral or slightly acidic. The hides are washed, then placed in a bath of warm water with some calcium salts and an enzyme. More warm water is run into the bath, gradually increasing the temperature. The action of the enzyme lowers the alkalinity of the hide. The fat also breaks down. Gradually, the water temperature is decreased. The hides are washed until all hair, fat, and chemicals have been removed.

Tanning

4 Now the hides are ready for tanning. Workers load them into a huge rotating drum. The drum is filled with the tanning solution, made of chromium salts in water. The hides soak in the tanning solution for eight to 12 hours. The chemical action of the chrome transforms the hide into leather. Dyes in the solution also give the leather its color. For patent leather, this is usually black. Workers remove the leather after the appropriate time in the bath, and send it to a drying area for at least 24 hours.

Finishing

5 All the previous steps apply to any leather. Only in the finishing is the leather transformed into the specific product of patent leather. In the United States, a common finishing technique used to be a spray application of the polyurethene and/or acrylic. But because of air pollution concerns, most patent leather finishing is done by some kind of so-called aqueous dispersion, that is, a liquid application. One common method is to use a machine called a flow coater. Workers load a tank above a conveyor belt with the liquid polyurethane/acrylic. The hides pass beneath the tank on a belt. A waterfall of overflowing liquid hits the traveling hide, and it becomes coated with the finish. Next, the finished hides are stretched on boards and pass through a heated tunnel to dry. Depending on the tannery and the particular application, the drying tunnel may use infrared lights or ultraviolet. The first coat of finish is formulated so it penetrates the leather completely. After drying, the hide is put through the flow coater again, this time for a middle coat that includes dye. Then it is dried as before, and put through for a third and final top coat. This top coat is clear, and dries hard, shiny, and waterproof.

Final steps

6 After the last coat dries, the leather is ready to move on to its buyer, most likely a shoe manufacturer. Despite all it has gone through, the leather is still in its original shape. It has not been cut except perhaps to trim some thick or damaged areas. The shoe manufacturer cuts it into many pieces, with as little waste as possible.

Quality Control

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.

Byproducts/Waste

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.