What Is Leather and How Is It Made?
Posted: 22nd January 2018 in Uncategorised
Leather is a versatile material made from animal hides. Cow leather is the most common type, but it can be made from a wide range of skin from animals like sheep, deer and even kangaroos. Depending on how it’s made, leather can have any number of properties. It could be thick or thin, soft or rough, stiff or flexible – the list goes on. These diverse qualities have made it a staple of human cultures around the world throughout history.
The process for making leather is complex and varies depending on the properties that the finished product needs to have. Creating a comprehensive guide to the leather-making process is impossible, but we’ve broken it down into the main steps (including the crucial tanning stage) and the key things that almost always happen at each point.
Curing fresh skins
The best leather is made when the process starts with the animal skin is as fresh as possible. However, the skin is immediately at risk of putrefaction and decay. To slow down this process, the hide is cured.
Curing is achieved with salt, some amount of water (which can vary significantly) and often antiseptics. The process removes water from the organic tissue, allowing it to be stored for much longer without the risk of decay. When dry, cured hides can progress to the next stage of the leather-making process whenever they’re required.
The ‘beamhouse operations’
Collectively known as ‘beamhouse operations’ thanks to their traditional location in a tannery (where leather is made), these next steps between curing and tanning prepare the hide for the rigorous chemical treatment it’s about to receive.
While it might seem odd to soak the hides after they’ve been salted in the curing phase, adding water to the skin allows it to be cleaned thoroughly. It’s common for different types of biocides to be used during soaking to prevent the hide from putrefying.
Liming and de-hairing
Next, a lime-based chemical agent (normally ‘milk of lime’) is applied to the hide. This substance removes hair from the epidermis (the outer layer of skin) and removes the fat and grease found in the subcutaneous tissue (the lowest layer of skin that connected it to muscle tissue). Depending on how effective the lime is and the finish that’s desired, the leather might also be scraped to help the process along.
Any subcutaneous tissue that still remains after liming is then removed mechanically. All that remains of the original hide is the dermis (the formerly living layer of skin containing roots and nerve endings) and the newly de-haired epidermis.
Deliming and bating
Without us getting bogged down in chemical processes, the liming stage involves raising the pH of the hide (making it more alkaline), while the crucial tanning stage lowers the pH (making it more acidic). The deliming part of the beamhouse operations in between these steps starts to gently lower the pH in preparation for tanning, which prevents damage to the hide that a sharper drop could cause.
Around the same time, some hides will also go through ‘bating.’ This involves putting the leather in a bath full of enzymes (proteins involved in digestion), which starts to soften the material. If the finished leather is required to be very soft and supple, bating is an important step.
The final beamhouse operation is pickling. The standard process involves salting the hide and acidifying it (normally with sulphuric acid). This process prepares it for tanning and helps preserve the hide if it needs to be stored in the interim.
How the tanning process works
Tanning is the most well-known stage of the leather-making process. It involves permanently changing the structure of the hide to significantly slow down or stop decay and make it more durable in general. The word itself is derived from tannin, a German term for the plant-based chemical mixture that was originally used in the process. Leather-making facilities are called tanneries because of this word.
Tanning in the modern-day
Since the Industrial Revolution, most tanning uses a chemical compound called chromium (III) sulphate. This acidic compound replaced natural tannin as the de facto tanning agent.
In most tanneries, tanning is achieved by loading the hides into large drums along with the chromium compound. They soak in the chemicals as the drum slowly rotates, which helps distribute the compound evenly throughout the hide.
Workers will periodically take a small cutting of hide out of the drum to see if the compound has been evenly distributed.
When the tannery workers are happy with the distribution of the compound, the pH level of the whole mix is raised (making it more alkaline) which causes the chromium (III) sulphate to bond to the hide, which is now leather. Because of the compound’s natural colour, this new leather is normally a light shade of blue.
Other ways of tanning
While tanning with chromium (III) sulphate is the most common way to tan, ‘vegetable’ tanning processes also exist, which hark back to the process’s pre-industrial methods. Natural tannin is derived from trees such as oak and fir.
The leather created with vegetable tanning is normally less flexible than leather tanned with chromium compounds. While this makes it less suitable for applications like clothing, it’s still widely used in products like furniture and luggage.
Some other chemical compounds are used in additional alternative methods, but none are anywhere near as widespread as chromium tanning or even vegetable tanning.
Crusting and dressing
The newly made leather isn’t finished after the tanning process is complete. Depending on what the leather’s being made for and the qualities that it needs, it can go through a wide range of finishing stages.
Collectively, these options are a range of chemical and mechanical processes that change the way the leather looks and feels. Colour is almost always added, whether it’s classic black or something bold and bright. The thickness will very likely be altered as well. Other processes do things like make the leather softer, increase its flexibility and remove flaws.
Drying and finishing
At long last, the leather is almost ready to be delivered to the huge variety of companies that will turn it into all manner of products. But before that, it needs to be thoroughly dried.
As a final step, a coating is sometimes added to the product which can change its appearance slightly or add a few other qualities. When this is all over, the leather is finally ready for use.