The leather that your bag is made of has been tanned. And no, that doesn’t mean that it has been left in the sun to get a bit of colour.
Basically, tanning is the process that converts animal skin to leather. How this process is carried out has a significant impact on the qualities of the bag you hope to use for years to come. Will it soften and acquire a patina over time? Or will it be supple from day one and keep its looks over the years?
Once an animal skin has been turned into a hide by being degreased and having all its hair removed, the tanning process can begin.
An untreated skin would both harden and putrefy as it is an organic material. So, the aim of the tanning process is to prevent this from happening ― to turn the hide into leather.
This is something human beings have been doing for a very long time. Some evidence suggests that leather tanning was performed as far back as approximately 6,000 BCE in the Indus Valley ― one of the cradles of modern civilisation.
The basic principle has been the same for millennia: To modify the protein called collagen, which the skin is made up of. You can actually get a sense of this protein with the naked eye. Collagen molecules like to first line up and then to twist together into "fibre bundles" that you can easily see if you look closely enough at quality leather.
What tanning does is leave the skin much less susceptible to hydrolysis ― the separating of chemical bonds caused by water ― which would otherwise cause its degradation. Tanning modifies the molecular structure of the skin.
For the vast majority of the past thousands of years, this modification has been performed by soaking the skin in a solution made up of vegetable tannins. These tannins would most often come from trees such as oak, chestnut or mimosa, but hundreds of tree types and other plants have been used.
In fact, the word 'tannin' derives from 'tanna' ― an old German word for 'fir.' So 'tanning' has nothing to do with colouring, as in getting your kit off and letting the sun brown your skin.
A tannin is a molecule that bonds easily with proteins and will draw liquids out. If you are a wine drinker, you may have had heated debates over a wine’s 'tannins' ― the ingredient that makes the wine feel dry in the mouth, sometimes to the extent of making both your tongue and gums feel unpleasantly arid and barren.
Just as the tannins in wine come from the skin of the grape, the tannins in trees are found in the bark.
When tanning hides to make leather, the hides are soaked in a tanning solution. The tannin molecules will enter the hide and displace the water that is bound in the collagen.
The water is drawn out, but as the tannins take the place of the removed water, the leather does not grow inflexible as fully dehydrated leather otherwise would.
It may sound easy, but it isn’t. The process is complex and the skins require multiple treatments over a period of up to two months in order for the water molecules to be fully extracted and let the tannin molecules take their places in just the right way.
A lot of work from skilled craftsmen is involved too.
Mineral (chromium) tanning
The complexity, expense and time involved in tanning with vegetable tannins led, in 1858, to the development of chrome tanning using mineral agents — initially in laboratories, but later adopted as a commercial process by some tanneries.
The basic principle is the same, removing water molecules from the collagen and replacing them, but the process is much quicker using chrome, which is the most popular mineral tanning agent today.
The whole process can be automated and finished in a day, and the chrome ions which displace the water and bind with the collagen are much smaller than vegetable-tanning molecules. This generally makes chrome-tanned leather thinner and softer than vegetable-tanned leather.
The process, however, is less natural than when using vegetable tannins. It involves first placing the hides in acidic salts to better make the chrome fit in between the collagen molecules – and then returning the hides to a normal pH level.
This requires the use of acids and other chemicals as well as the chromium sulphates themselves. If not properly managed, these will have a negative environmental impact, and the industry continues to be under pressure to "clean up" as more regulations are introduced.
Leather and the environment
Vegetable-tanned leathers are generally biodegradable, but the tanning process requires the use of more water and more tanning agents than when chrome tanning.
Chrome-tanned leathers, on the other hand, cannot be recycled as such, but some companies now extract the chrome out of unused leather and resell the chrome to tanneries where it can be used again.
Today, no less than about 90% of the world’s leathers are chrome-tanned ― partly due to the lower cost, but also because chrome-tanned leathers are softer and less susceptible to water stains, making them preferable for clothing, such as jackets and gloves, and upholstery for, say, car seats.
Vegetable-tanned leather continues to be associated with tradition and craft, but relatively few tanneries today have the capability to produce vegetable-tanned leather.
The time and skill involved in its production makes it an expensive material, reducing its demand. It is a thick and malleable leather, making it ideal for goods like sturdy bags and belts.
TELL THE DIFFERENCE
Look, feel and smell
The cost and environmental impact of chrome and vegetable-tanned leathers are not the only differences between the two. There are more visible distinctions too:
▪ Because of the way it is tanned, the colours of vegetable-tanned leather are usually rich and deep in natural earthy tones.
▪ Being an entirely organic material, vegetable-tanned leather will change over time. It will grow softer and darker, and will acquire a patina depending on its uses.
▪ Has great durability and strength, and can ― if well cared for ― potentially last several lifetimes.
▪ Scratches fairly easily, but shallow scratches can be buffed out.
▪ Has a distinctive fragrance – the sweet, earthy fragrance you probably associate with leather is the smell of vegetable-tanned leather.
▪ Prolonged exposure to heat can cause it to dry out and/or crack.
▪ Is not water resistant. Water can cause splotches or marks – eventually becoming part of the patina.
▪ Is available in a huge variety of colours.
▪ Its colour will remain uniform and the leather will not develop a patina as fast or to the same degree as vegetable-tanned leather.
▪ Is softer and suppler than vegetable-tanned leather.
▪ Is usually thinner than vegetable-tanned leather.
▪ Is fairly resistant to water, stains and heat.
▪ The fibres of chrome-tanned leather are not able to show through in the same way they do on vegetable-tanned leather.