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Technical Information:

CUIR BOUILLI: TECHNIQUE

If wet leather is heated there is a point when it begins to change radically. This is the leather's shrinkage temperature and for vegetable tanned leather this occurs between 75 and 85 degrees Celsius. At this temperature the collagen fibers, the major structural component, literally shrink. The amount of shrinkage can range from 5% to 40% depending on the characteristics of the leather. While the leather is hot and wet it has an elastic quality, if stretched it springs back like a rubber band in slow motion. As it cools and dries it gets hard and when fully dry is very difficult to re-wet. The longer the leather is heated and the hotter the water the more the leather changes. It becomes increasingly brittle and eventually quite unstable.

For leather to have sufficient strength and hardness to make it useful as a structural element in furniture it must be taken to shrinkage temperature and most of the fibers need to be fully shrunk, but be kept at this temperature as short a time as possible to avoid the brittleness caused by longer exposure to water heated above shrinkage temperature. This is simple in principle and complex in practice.

To work successfully the following things need to be understood and controlled:

1)
Leather Tannage - determines shrinkage temperature
Weight (thickness) - the thinner the hide the faster the rate of change and the less stable the material after shrinkage.
Part of the hide - the hide density varies greatly from one area to another. The more dense the hide the less the actual shrinking but the more quickly the hide changes.
Animal - determines the fiber structure, thickness, and density.

2)
Water Temperature - when over shrinkage temperature, the hotter the water the faster the leather will change.
Volume - the volume of water needs to be sufficient to maintain a constant temperature.

3)
Boiling Time - The longer the leather is at shrinkage temperature or above. the more it will change. If you heat it long enough it will dissolve.

4)
Working Time - To manipulate the leather after shrinkage temperature it must be hot and wet. As it cools it hardens which makes stretching difficult and increases the potential for cracking or tearing.

5)
Control of 'Boiled' Leather
I use two different approaches:
a) I boil the leather first then stretch it over a form, or
b) I secure the leather around a form or forms, shrink it, and stretch it out again until I have the shape I want. This second method is the one I use to make the structural elements in the tables.