Wednesday, 8 October 2014

Tanning Tales

Over the coming weeks, the first redisplay of objects on the Museum’s first floor will appear. This case will display objects chosen to illustrate leatherworking and will show some of the wide applications for leather from clothing, headwear, shoes and bags to drinking vessels, buckets, saddles and writing materials, plus some of the specialist tools used in their manufacture.

Before preparing objects for the new display, our project conservator Andrew attended a week-long course ‘Understanding Leather - From Tannery to Collection’ hosted by the Leather Conservation Centre (LCC) and Northampton University‘s Institute of Creative Leather Technologies (ICLT):

"The course gave me a hands-on opportunity to follow the vegetable tanning process of a hide - transforming a salted animal skin into a beautiful finished piece of leather. Seeing the processes first-hand, and smelling and touching the hides, helped me to appreciate the skill of the tanner - how many steps are involved in the process and how many chances there are for things to go wrong along the way.

Partially-tanned deer and calf skin © Pitt Rivers Museum
"Very simply, tanning is a chemical process whereby skins are treated with chemicals known as tanning agents. These bind to the collagen proteins in the skin and prevent the skin from putrefying.
Tanning agents come in many types – Vegetable (or Bark), Mineral and Aldehyde. Different tanning agents produce different qualities in the leather – softer or harder, better tensile strength, different colours, etc.
Mineral tanning using chrome-based compounds takes just a few hours whilst vegetable tanning with oak bark traditionally takes 12 months and 1 day!
At the ICLT I followed a (much shorter) process of vegetable tanning using a number of different tanning agents:  
Some raw materials used in vegetable tanning © Pitt Rivers Museum
Traditionally, people of different areas of the world used tanning agents local to them and in good supply – in the UK this was oak bark; in Italy, the ground-up heartwood of the chestnut tree. 

Although the steps involved in tanning have probably changed little since ancient times, the machinery used today certainly has! In medieval England, tanning was carried out in pits where skins were hung or layered in a liquor containing ground up oak bark. Modern tanning is carried out in drums, something like an industrial washing machine:

Tanning drum with goatskin covered in vegetable tanning 
liquor © Pitt Rivers Museum

Today, skins are removed of excess flesh and fat by machine, de-haired by placing them in a drum in an alkaline mixture of lime, and then treated with enzymes in a process called 'bating'. This removes unwanted proteins and opens up the structure of the skin to allow the penetration of the tanning agent.

I was prepared for some bad smells along the way, but other than the smell of the salted skins and maybe a hint of ammonia, the tannery was a very clean place. Tight restrictions on waste disposal means that today's tanneries produce very little waste. Surprisingly, the ‘waste’ produced from the de-hairing contains many proteins useful in the cosmetic industry.

Kangaroo tail © Pitt Rivers Museum
The modern tanning process involves the prepared skins being gently turned in the drum along with a mixture of water, a purified extract of the raw tanning agent and added chemicals to produce the optimum acidic pH all at a controlled temperature. As the skins absorb and react with the tannins they are less susceptible to damage. The temperature is gradually increased and the drums turned more vigorously all of which contribute to speeding up the process from weeks and months to a number of hours.

Any animal skin and can be tanned from the common cow, goat and pig to the more unusual such as crocodile, salmon, seal and, on rare grizzly occasions, human.

The LCC has a large collection of different reference samples that are invaluable when trying to identify what an object has been made from.

The patterns of the hair follicles on the surface of mammalian skin can help in identification but often on museum objects they have been worn away, or obscured by surface treatments, paints or dirt, so you have to look for other clues. 

What animal do you think this bag is made from?

© Pitt Rivers Museum

© Pitt Rivers Museum

Yes, eel! Eelskin leather is highly prized. It is very smooth and exceptionally strong. Oddly however, it does not come from eels but the Pacific hagfish, a jawless fish also known as the 'slime eel'.

Today, there is just one remaining tannery in the UK manufacturing traditional oak bark leather - J&FJ Baker, based in Colyton, Devon. Watch this video to find out more about their process:

The new hide and leatherwork display at the Museum will reflect some of the processes and variety outlined above by including, not just calf and goat skins, but also items made from monkey, lizard, and fish, plus differently processed examples from vegetable tanned to brain tanned to alum tawed leather."