From whale oil to plastic coating
Reviewing the invention and modern production of patent leather, which has long been popular on a variety of footwear styles.
Image © fortton | iStockphoto.com
One of the earliest references to ‘patent leather’ with its high-gloss finish can be found in a British periodical called The Bee, or Literary Weekly Intelligencer. In an article published in 1793, one Mr Hand, working in the English city of Birmingham, was described as having obtained a legal patent for a process said to be ‘for preparing flexible leather having a glaze and polish that renders it impervious to water, and need only be wiped with a sponge to restore it to its original lustre’. The fact that the process was recognised by the patent office was what reportedly gave the material its name.
The news of patent leather’s creation caught the attention of inventors around the country. Six years later, Edmund Prior of London received a legal patent for a method of painting and colouring all kinds of leather with dyes and boiled oil, and finishing it with an oil varnish. In 1805, another legal patent was awarded to London-based inventor Charles Mollersten for ‘the application of a chemical composition in the preparation of hides, skins and leather to give a beautiful gloss’. Mollersten’s leather finishing technique apparently used linseed oil, whale oil, horse grease and lamp black. Demand for this shiny, black, water-resistant surface of patent (or ‘japanned’) leather quickly grew in England – and then further afield.
Patent leather was introduced to the USA by Seth Boyden of New Jersey in 1818. He had obtained a piece of patent leather made in Germany and ‘reverse engineered’ this specimen to determine a way to produce his own product. Boyden set out to improve the process and, like Mollersten before him, chose layers of a linseed oil-based lacquer coating to add the glossy finish. Commercial manufacture of his material began in the USA in 1819, but Boyden did not lodge a legal patent to claim any new invention.
Yet another legal patent was awarded in 1854. This listed the varnish ingredients as including oil, amber, Prussian blue (Fe7(CN)18 and described as the first modern synthetic pigment), litharge (a natural mineral form of lead (II) oxide), white lead, ochre, whiting (powdered and washed white chalk), asphalt and copal (resin from the copal tree). During these early days of patent leather production, many tanners kept the finer details of their coatings a closely guarded secret, so even the substances that were listed in patent applications may well have been falsified in order to mislead and confuse their business competitors.
For many decades, linseed oil and Prussian blue dye seem to have been the basis of most patent leather finishes. Using fine, black leather, the tanner applied as many as 15 coats of varnish, allowing the substrate to dry each time in the sun or in gentle heat from an oven. The aim was to achieve a finish that was smooth, hard, and also somewhat elastic, to avoid the leather cracking when in use.
A 1906 European method of manufacture involved a foundation coat of lamp black mixed with linseed oil being laid on the flesh side of the leather. Successive coats of this mixture were built up, with the leather substrate being allowed to dry and the surface rubbed down with pumice stone between coats. After this process, the leather was once again blackened with a lamp black/turpentine mixture, and hung up to dry. The substrates were then laid in a pile for at least a month, after which they were tacked onto a frame and brushed with another coat of varnish. They were then baked in a moderate heat for three days and exposed to the sun for ten hours to complete the process.
The arrival of synthetic coatings
The development of plastics from the mid-19th century onwards raised the opportunity of patent leathers being made more cheaply. Polyurethane (PU) was invented in the 1930s by Professor Otto Bayer and this substance, sometimes blended with other resins, further simplified the process and cut production costs, allowing for the mass production of patent leather. Today, the majority of patent leathers are manufactured with polyurethane coatings.
Most modern patent leather is based on cattle hide. One example of a blend used is that of PU and acrylic – the former to give a hard, glossy and durable finish, with the latter included to produce a flexible final product. When blending patent coatings, leather chemists combine the base substances to create the optimum quality to suit the required finish. As a result, the actual finish produced may vary between tanneries – perhaps even from batch to batch. Patent leather is sometimes confused with poromeric materials, which may have a similar glossy appearance but are actually synthetics used as leather substitutes.
The sale of patent leather shoes grew during the 1950s and 1960s, when the material was particularly used on formal shoes for young girls. Glossy black (and, on occasions, white) patent leather shoes were often worn for special occasions.
While black is still a very popular colour for items made from patent leather, it is by no means the only one in which such products are available. Even in the ‘swinging sixties’ – a period when patent leather shoes were particularly in fashion – they were also on sale in blue, green, ‘hot pink’, orange, red, white and yellow.
Spray application of the PU or resin blend was once a common finishing technique. However, spraying is an inefficient method of laying down the relatively thick coatings required to achieve the depth of gloss on patent leathers. ‘Curtain coating’ is by far the more efficient process and gives the best smoothness and depth of gloss on the patent surface. However, modern roller coating techniques are also claimed to produce similar gloss levels. Both solvent-based and water-based PU resins can be used in all processes.
In the curtain coating process, a tank is loaded with the liquid polyurethane or blend, and the leather substrates pass beneath the tank on a conveyor belt. A ‘waterfall’ of the liquid overflowing from the tank hits the substrates as they travel along, coated them with the finish. In the next step of the process, the substrates pass through a heated tunnel to dry. The first coat of finish is formulated to ensure that it completely penetrates the leather. After drying, the substrates travel through the machine once again, where they receive a coat of pigmented finish. Then, having once again been dried, the substrates pass through for a final clear top coat that dries hard, glossy and water-resistant.
Low-cost patent leathers produced by transfer coating pre-formed PU coatings onto leather splits are popular for school shoes. High-cost curtain-coated patent leathers must be produced using grain leathers to achieve the required smooth, mirror-like finish.
Toward the end of the 20th century, dress became more casual, and patent leather shoes lost much of their ‘essential’ appeal. Nevertheless, the inclusion of formal patent leather shoes in a footwear producer’s portfolio is often based on prevailing fashion, which regularly cycles around every few years. Many women’s court shoes (pumps) feature this finish, and men’s and women's patent leather formal dress shoes and dance shoes have long been popular. As an example of a modern twist on classic styling, a number of shoemakers have released patent leather trainers (sneakers) in recent years. In addition – as in past decades – girls’ black patent leather school shoes are always in demand, so the use of this mirror-like upper material appears to have a positive future.
How can we help?
Please contact SATRA’s footwear team (firstname.lastname@example.org) to discuss the assessment of patent leathers.
This article was originally published on page 38 of the July/August 2022 issue of SATRA Bulletin.