Steve Sothmann of Real Leather. Stay Different sets out a guide to the different ways that leather Is made in clear, easy to understand terms.

Which approach to leather tanning achieves the best results? Which is the most efficient? Is vegetable tanning really more sustainable than chrome tanning?

Read this leather tanning guide to find out the answer to these questions and others. 

Without tanning, there would be no leather. Tanning stabilises the raw hides and turns them into the supple, hard-wearing material that becomes stylish clothing and accessories, furniture, and car interiors.

Today, most of the world’s leather – around 80% – is tanned using chrome but other options include vegetable tanning and chrome-free tanning. Each method has its pros and cons and is better suited to some applications than others. It all depends on the look and feel the tanner is trying to achieve.

Vegetable tanning

The oldest form of tanning still in use today is vegetable tanning, which was developed by the ancient Greeks.
The vegetable tanning process involves repeatedly soaking skins in natural tanning solutions made from plant extracts, commonly tree bark, leaves, fruits, seeds and roots due to their high tanning concentration.

The resulting leather tends to have an earthy colour and smell. It is also hard-wearing and keeps its shape. This makes it suitable for shoe uppers and soles, wallets, belts, handbags, and watch straps. Another distinguishing feature of vegetable-tanned leather is that it ages beautifully. It tends to darken in colour and develop a unique patina, making it very individual.  

Is vegetable tanning more environmentally friendly?

Because vegetable-tanned leather only uses natural materials, it is often seen as the most sustainable tanning choice. However, it uses more water and several times more tannins than chrome-tanned leathers. The effluent produced also requires more treatment before it can be discharged, so its overall environmental impact is no better than the alternatives. 

The main downside of vegetable-tanned leather is that it takes much longer to produce – usually between 30-40 days, versus the day or two needed for chrome tanning. The process tends to be complex and involves a high level of skill, which means that it is more expensive than chrome leather. Also, the robustness that is its strength can be a drawback, making it too stiff for applications where flexibility or stretchiness is required. So, it is not necessarily the best option for car upholstery or clothing.
 
Chrome tanning

Chrome is by far the most common form of tanning used today, partly due to its efficiency, and depending on its use, it can be a more suitable choice. It first emerged in the mid-nineteenth century and enabled the leather industry to develop at scale.

Is chrome tanning safe?

As the name suggests, chrome tanning involves adding a small amount of chromium (III) in the form of chrome mineral salts – and not the more toxic hexavalent chromium (chromium VI) which has been phased out of production in the current generation of leather products.

Chromium III is considered non-hazardous to health. It is part of our required diet, is taken as a supplement and is naturally occurring in foods such as bread, green peppers and apples. It is also naturally present in many soils and woodlands.

The advantage of chrome tanning is that it is faster and cheaper than other tanning approaches and achieves more consistent, colourfast results. The leather it produces is soft, flexible and blue, called ‘wet blue’ during the wet stage, and can be dyed to a wide range of colours, so it is highly versatile. This versatility extends to its application – chrome-tanned leather lends itself to a wide array of leather goods, from gloves, clothing and handbags to shoes and upholstery. Chrome-tanned leather is also more resistant to water, stains and heat compared to vegetable-tanned leather, which makes it incredibly durable. 

Chromium VI is not used in tanning; the industry is subject to a range of regulations and there are clear guidelines, as set out by the Leather Working Group, to prevent its formation in leather post-tanning.

Chrome-free tanning

A more niche approach to tanning is chrome-free, mainly used in the automotive sector and for children’s shoes.
Chrome-free leathers are typically made using a synthetic product called glutaraldehyde, but other tanning agents include aluminium, zirconium, triazines, aluminium silicates and syntan-vegetable.

The resulting leather is soft and a pale creamy colour, known as “wet white”, which can easily be dyed to pastel colours. It is also said to match, if not surpass, chrome-tanned leather in terms of performance. The fact that it doesn’t contain chrome can be a benefit as, in some circumstances, chrome can be a skin irritant, making it more suitable for allergy sufferers. 

However, chrome-free leather tanning is more expensive than chrome tanning. The actual process is more involved as the leather needs to be processed further with other chemicals such as vegetable extracts, syntans and acrylics to give a final level of finish and performance. Also, in July 2021, glutaraldehyde was added to the Substances of Very High Concern (SVHC) candidate list due to its respiratory sensitising properties. 

According to Eurofins, it may not be an issue for the leather industry right now but it does increase the possibility of greater glutaraldehyde restrictions within the European Union’s  'REACH' regulation in the future.

Tanning innovations 

Newer alternatives to glutaraldehyde tanning already exist, such as Zeology, based on the mineral zeolite, a combination of aluminium, silicon sand and oxygen, said to be abundantly present in the Earth’s crust. The leather it produces is free from chrome and heavy metal as well as being biodegradable and compostable. According to the official information pack, Zeology packs a performance punch too, with unsurpassed “grain tightness, physical leather properties, lightfastness and heat-resistance.”

We are seeing innovation in chrome and vegetable tanning too, as the quest for more sustainable and efficient tanning approaches continues. New developments range from vegetable tanning solutions that produce leather on a par with chrome-tanned leather to green chemicals that reduce the environmental impact of the chrome tanning process.  

Which tanning approach is best?

All things considered, though, which tanning method is best? There is no correct answer. It all comes down to the application. 

As for which approach is the most sustainable, all tanning methods have impacts on the environment, but different ones. This is supported by a Life Cycle Analysis of chromium, synthetic and vegetable-tanned leather ‘chemistries’ carried out by Eurofins, which found that the overall impact of the different approaches was approximately the same.

So, whether a tanner plumps for vegetable tanning or decides on chrome or chrome-free is a matter of choice and preference. Cost and efficiency will be a factor, of course, as will the aesthetic and performance they are looking to achieve.

Steve Sothmann of Real Leather. Stay Different