John Keats wrote: ‘A thing of beauty is a joy for ever: its loveliness increases; it will never pass into nothingness.’ While leather may be a ‘thing of beauty’, unfortunately, in the current climate of environmental control, ‘never passing into nothingness’ is not an ideal situation. And one of the major concerns is the amount of chrome tanners put into the environment.

The original concept of the ‘wet-white’ process centred around the further development of chrome-saving processes. In their wet-white system, Schill + Seilacher used a formulated product based on glutaraldehyde and a polymer. This improved the penetration, fixation and exhaustion of the chrome, thereby reducing chrome in the effluent considerably, but did nothing toward solving the problem of disposing of the chrome-containing shavings. An important step towards addressing this problem was a system which would allow the hide or skin to be shaved in a largely untanned state. This would leave the shavings in a condition to biodegrade.

Reversibly masked

Leather from the original chrome-saving process was tanned after pretanning and then shaved in the wet-blue. Due to the high rate of reactivity with collagen, the glutaraldehyde in the formulation was able to penetrate only a small distance into the pelt, encouraging the penetration of the chrome but doing little in terms of raising the overall shrinkage temperature to facilitate shaving. To achieve this rise in Ts, the aldehyde needed to be masked.

After much experimentation, a system was developed based on formulations of glutaraldehyde with certain alcohols. A commercial product was created and successfully patented in many countries of the world. The formulations act in a very reproducible way without the need for any special controls. They were designed so that the tanning reaction should take place throughout the cross-section of the hide after penetration and after reversing the masking action in situ. It was thought possible to crosslink approximately 10% of the available tanning active sites of the collagen without influencing the final leather character.

The concept of biodegradablity of waste and the concept of achieving a substrate allowing any sort of tanning system to be employed after shaving was believed to be possible with a 10% crosslinking.

The reversal of the induced masking effect is partly a function of time and partly of dilution. The controls on the processing are not unlike those required for chrome tanning, that is the simple control of the pH and the degree of pickling. The aim is to produce a pickled substrate, with a pH of 3.0-3.2 throughout the cross-section, allowing the penetration of the product in about 2 hours, then raising the pH, with alkali addition, to about pH4. These conditions allow the achievement of a shrinkage temperature of 70-75°C which is adequate for shaving. A recipe is shown in Table 1.

Managing wet-white technology

From this basis, of a substrate with approximately 10% crosslinking of the tanning functions, to achieving a commercially interesting leather, there were many hurdles to be crossed. Initially, a number of problems presented themselves:

* Yellowness of the wet-white

The yellow colour is caused mainly by a reaction between glutaraldehyde and ammonium salts. The answer is to use ammonium-free deliming or employ better washing at the end of deliming.

* Difficulty of achieving a level finished substance

The difficulties of substance control are mainly a function of the de-watering of wet-white at sammying, as pure wet-white does not release water freely. This problem is solved by the small addition of a syntan at the end of the pretanning stage. Another important factor is at the beginning of the final tannage when a salt is necessary to prevent acid swelling of the pretanned material.

* Uses for biodegradable shavings

In 1991 and 1992, a leading agricultural research institute in Karslruhe in Germany, the Staatliche Landwirtschaftliche Untersuchungs- und Forschungsanstalt, Augustenberg (LUFA), was specifically commissioned to examine the use of the shavings as a fertiliser. Their conclusion was that the wet-white shavings were comparable in effect to an inorganic nitrogenous fertiliser.

The optimum recorded quantity of shavings for crop plants with a low-to-medium nitrogen nutritional requirement was 2.5g shavings per litre soil. High nitrogen requirements could be met with 4g shavings/litre soil, and for flowering plants, 10g shavings/litre soil.

Interestingly, the flowering period of the plants treated with shavings was extended. Furthermore, it was shown that the shavings could be composted with other organic materials to great benefit.

* Glutaraldehyde: safety of use

The ‘Worksafe Australia’ glutaraldehyde assessment, a 177 page in-depth report on the examination of glutaraldehyde, carried out in the mid-nineties, concluded that: ‘Risks may be satisfactorily controlled in a cost effective manner by a judicious combination of training, ventilation, good work practices and personal protection.’

The cause for concern in this case was the use of glutaraldehyde in hospitals in the operating theatres where the material was splashed around without any regard to possible health hazards.

The recent reclassification by the EU of glutaraldehyde as toxic is based on an old study whereby rats exposed to glutaraldehyde vapour died. Rats, however, are only able to breath through their noses and their death was prompted by inflammation of the tissue in their nasal passages which closed the air-way, leading to death by asphyxiation. In most countries of the world, including the USA, the material is not classified as toxic.

The environmental fate of glutaraldehyde is by metabolism to glutaric acid and, finally, to carbon dioxide. Incidentally, the formation of formaldehyde from glutaraldehyde is impossible.

* The anti chrome-free or pro chrome lobby

There is now a chrome-free lobby and the principle of chrome-free as an alternative tanning method is accepted.

Wet-white pretanning is interesting and economically viable also for the production of chrome tanned leather, but must be costed properly. For example, if wet-white is shaved to give a specific substance, it weighs 25% less than the equivalent wet-blue but the retanning and fatliquoring may be carried out with the same percentage offers of materials. This has been shown to produce exactly the same leather type as in the normal chrome leathers, but with a big saving in chemicals. The chrome salt offer on the basis of shaved wet-white for the tannage must be adjusted upwards, in percentage terms, to give the required Cr2O3 content, as the chrome offer is normally based on the considerably higher limed weight.

* Consumer acceptability

When people speak of wet-white today, it is coupled with the production of what is loosely called ‘chrome-free’, ‘metal-free’, ‘bio-leather’, ‘eco leather’ etc and often tanners are confronted by customers who simply do not understand the aim of a wet-white leather.

This must be to produce a leather which is acceptable to the consumer which could be best described as follows:

‘A piece of leather or an article made from leather which in no way would be expected to have a harmful effect on the end-user either through contact with his skin or by the leather emitting volatile substances and which could be disposed of without consideration of the environmental effects, while allowing him to enjoy the comfort and aesthetic beauty of leather.’

In the near future, conformity to the required automotive standards will give rise to a material as described.

Automotive sector

At the moment, the automotive producer is the biggest consumer of chrome-free leather, and at a recent scientific meeting in Würzburg in south Germany, a representative from Audi AG (Dr B Angemeier) declared: ‘Leather represents exclusivity in all (Audi) car models, 51% of cars are fitted out with leather. There will be no return to chrome-leather – chrome-free leather has no heavy metal contamination as determined by the EU recycling concept and has higher dimensional stability.’

Many of the requirements, stated as specifications, are consumer-driven, often arising from litigation. Fogging and formaldehyde emissions are examples of this. What is most interesting, is to deal with an industry that specifies exactly what it needs. From the beginning of chrome-free automotive leather, the automotive industry has increased, almost daily, the technical specification of the leather, to above that ever required of chrome-based leathers at the beginning of the development. There are 40 single tests to fulfil the standards for a piece of leather to be used by Audi in their vehicles.

The processing of wet-white to a chrome-free leather involves using a blend of vegetable and synthetic tanning materials and fatliquoring agents, each tailor-made to fit the requirements of the consuming industry. A recipe is shown in Table 2. Naturally, this area is one demanding daily reassessments and apart from the raising of the leather specifications, the improvement of the efficiency of the process is of paramount importance. Leathers tanned with an anionic system have a lower affinity towards the standard anionic dyestuffs and fatliquors, often necessitating a re-engineering of the process and process chemicals.

Analytical methods today, for example ‘head-space testing’, allow anyone to detect anything volatile in leather. The smell of leather and the new car smell are caused by volatiles, each component of which can be identified. The fogging effect is caused by volatiles, and this area is very much under control today. Formaldehyde emissions which arise mainly from syntans in the leather, but can also emanate from preserving agents in other auxiliaries, are also coming under control and reduced to extremely low levels. Single digit ppm levels are achievable with new products from the chemical industry.


It can be seen that the development of chrome-free automotive leathers has been instrumental in pushing the development of many chemical products used in the leather industry as a whole by reducing volatile substances or free unwanted substances. To misquote John Keats: ‘Leather, a thing of beauty, is a joy for ever; its loveliness increases; the waste will pass into nothingness.’

Schill + Seilacher products relating to the recipes are:

1) Silastol R687

2) Lipsol SQS

3) Derugan 2080/3080

4) Ukatan GM/GMF

5) Lipsol SQ/SQS

6) Ukatan AG

7) Derugan ND