Material of the month – Leather

Materials World magazine
,
4 Jan 2017

Anna Ploszajski delves into the history of one of mankind’s longest used natural resources. 

Leather is surely the man-made material with the longest history of any covered in this series. So long, in fact, that it could be argued that leather has played a role in the evolution of our species. Since Homo sapiens lost their fur to become the naked apes, only those with the ingenuity to don the fur and skins of the animals they hunted would survive the changing seasons and ice ages in the subsequent millennia.

But the advances required for even this seemingly simple technological step cannot be underestimated. Notwithstanding the dexterity and skill need to separate animal hides from their hosts, once our ancestors had obtained the raw materials, the skins quickly began to rot and smell. Eventually, by trial and error, our predecessors discovered that this could be eliminated by drying the hides in the sun, hanging them out above the fire, applying animal fats or salts, or even soaking them in pond water. This allowed people to use the skins of the animals they ate as building materials, clothes, shoes and tools. Nowadays, we recognise these actions as the tanning process. 

While we have still found no better substitute for leather in footwear and, some would argue, trousers, the other applications have become a great deal more varied. The Ancient Egyptians used leather for sandals, buckets and clothes, the Romans used it for military equipment and harnesses. In the Middle Ages it was used as horse tack and furniture. Leather played a key role in the Industrial Revolution, as a strong, durable and flexible material well suited for the belting to drive machinery, prior to more suitable synthetic materials being developed.

Remarkably, since these basic origins, the theory behind leather treatment and tools used to produce it has remained unchanged, although the techniques have become more advanced and the process whittled down from up to a year to just a few days. The secret of the pond-soaking method is in the dissolved tannins that derive from plant matter. Today, we call leather treated in this way ‘vegetable-tanned’. When boiled in water, it became rigid enough that it was used historically as armour.

Tanning, crusting and coating

The theory behind smoked leather is in the formaldehyde vapours released by burning wood. The modern incarnation of this technique uses glutaradlehyde or oxazolidine compounds, or aldehydes, which come from natural forms such as in brain-tanned leather, which uses emulsified oils from animal brains, or in chamois leather using oils from cod. If brains make you squeamish, you can stick to synthetic-tanned leathers, which use aromatic polymers for tanning instead. The technique was developed during the Second World War, when vegetable tannins were in short supply.

The vegetable tanning process is slow and painstaking, involving repeated soaking and drying, which can take months. In the mid-19th Century, a much faster process was developed involving chromium salts. The end product is suppler than vegetable-tanned varieties, and takes only a few days from start to finish. The large majority of leather made today is tanned in this way.

Modern leather processing involves a great many stages to achieve the best possible material. First, the raw hide is subject to unhairing, degreasing, soaking in an alkaline solution in a process known as liming to remove keratin proteins, and pickling to control pH. Next comes the all-important tanning process, stabilising the proteins so that the dried material remains flexible.

After tanning comes the ‘crusting’ process, with can involve further tanning steps, controlling water content, adjusting the pH, shaving and dyeing. Finally, surface coatings are applied by brush, roller or spraying and the surface buffed and polished as desired to achieve a material that is strong, breathable, flexible and attractive.

Unpatented leather

Patent leather, developed in 1818 by American inventor Seth Boyden, involves applying a high-gloss plastic coating as the final step. Strictly speaking, Boyden wasn’t the first to make patent leather, since he in fact reverse-engineered a sample sourced from Germany, and high-gloss examples could be found all over England at least 20 years prior to Boyden. Nevertheless, his formula, based on a series of treatments using linseed oil-based coatings, was the first to take patent leather into commercial production. Ironically, Boyden didn’t ever patent the process.

When we think of leather, it is usually the cow that comes to mind. But actually a wide range of animals such as deer, goat, pig, alligator, ostrich, snake, kangaroo and even fish such as salmon, eel, shark and stingray are used.

Many people choose to avoid animal-derived products like leather for ethical reasons, and there are religious implications surrounding the use of pigskin or cattle hide, too. Fortunately, many synthetic alternatives made from polymers such as polyvinyl chloride, polyurethane and polyamide are available and modern techniques arguably render them indistinguishable from the real thing, at least to the untrained eye.

The debate surrounding the environmental implications of leather versus polymer imitation materials is a complex one. Religious or ethical feelings aside, it is widely accepted that the meat, dairy and leather industries are enormous emitters of greenhouses gases, principally from methane emissions, as well as consuming valuable land and water resources. Leather production also puts a strain on wastewater treatment facilities as tanning agents, solid waste and biological enzymes must be separated from the outflow. This being said, the synthetic alternatives are derived from petroleum and of course consume energy in production. However, leather goods are usually more durable, so require less frequent replacement than the synthetic materials in the long run.

The test of time

Despite its ancient roots, the leather industry in the UK has precipitated a surprising and novel spin-off technology in the modern day. Early in the 1950s, a collection of ancient iron artefacts was discovered in York, UK. Buried in the waterlogged ground of the city’s Hungate area were iron nails and knives in a remarkably good state of preservation, some dating back 2,000 years. The high density of leather artefacts found in the area and concentration of tannins in the soil led excavators to believe that the site had once been a local centre of leather industry. Investigations into the remarkable lack of corrosion of the iron objects concluded that it was the high concentration of tannins in the soil that was producing the protective effect, and this led to the development of so-called rust converters. These water-based primers contain tannic acid and an organic polymer. The tannic acid reacts with iron oxide to form iron tannate, forming a passivation layer of a dark, stable reaction product. Today, they are frequently used to protect underground structures from corrosion, and even the cast-iron roof over Big Ben in London has been given this protective treatment.

From our primitive ancestors to heavy metal rockers, leather really has stood the test of time. Although its cultural significance has shifted over the millennia, the fact that we haven’t let go of leather for its synthetic alternatives suggests a primordial attachment to donning the hides of other animals. Could it be the connotations with domination over the natural world that keeps us enamoured with this material? Or is it simply that, at least for now, our best efforts in materials processing are still no match for nature?