What limits should be applied for testing purposes? These are all questions to which many companies need an answer in order to maintain their reputation. Many companies have put restricted substance lists in place, which is a positive measure, and is becoming essential in order to maintain the integrity of the brand. However, without the correct policing of such documents, they lack value. In the process of looking at reducing limits of the possible harmful substances, the issue of chrome VI in leather is sometimes either overlooked or mistaken for other forms of chromium.
Why use chrome tanned leather?
Chrome tanned leather is tanned using chromium sulfate and other salts of chromium. The active ingredient in this tanning agent is trivalent chrome (III), used because of its properties which allow leather to be supple and pliable. Chrome also increases the propensity of the leather to take up dyestuffs. Chrome tanned leather is used in most leather products for these reasons. There are alternatives to chrome, but they have different properties. The chrome tanning process is relatively cheap and has the unique characteristic of a shrinkage temperature greater than 100°C, allowing it to ‘withstand the boil’ for a period of time (one to three minutes). Chrome is the only tanning agent which gives a highly versatile leather suited to most industries.
Chrome (III) or chrome (VI)
Chromium is used in the leather industry in its trivalent form for tanning. It exists in many valency (oxidation) states of which the following are most relevant to leathermaking:

  • Trivalent chromium (Cr III) – These compounds occur naturally in the environment. They are found in rocks, soil, plants and volcanic emissions. Chromium salts are present in foodstuffs and are a necessary nutrient for the human body.

It is required for the normal metabolism of fats and sugars. Nutritional supplements are currently on sale containing chromium picolinate. Chromium (III) sulfate is safe to use and is non-hazardous. It is used in the leather industry for tanning.

  • Hexavalent chromium (Cr VI) – This is the hazardous form of this element. It can be formed when trivalent chromium is oxidised. This usually occurs in the presence of oxygen combined with other factors, such as extremes in pH. The salts have a characteristic yellow colour and are classified as carcinogens as they can easily penetrate through skin and adhere to tissue cells.

Chrome III is used in the tanning process for chrome tanned leather and if the tanning processes are carried out correctly then the resulting leather will be completely safe to use. 
In leather production, however, there are three ways in which chrome III could oxidise into chrome VI:

  • During the tanning process – If the temperature or pH is too high then chrome III has the propensity to oxidise to chrome VI.
  • Chemicals used – If chrome powder is sourced from a non-reputable supply then there is a possibility that it may contain chrome VI.
  • Finished leather – After the leather has been made, there is a small chance of chrome III oxidising into chrome VI at high temperatures. This is why chrome tanned leather should never be incinerated except under very strictly controlled conditions.

The test method to detect chrome VI has recently been revised and published as EN ISO 17075:2007. This method has a detection limit of 3ppm and for a leather to pass; no chrome VI must be detected. 
End of life issues
BLC has carried out a life cycle analysis (LCA) to evaluate
tanning chemicals. This compared chrome tanning with vegetable and aldehyde based processes. It should be noted that when considering the cradle-to-grave issues, there were no significant differences found between these technologies. 
However, there is one area where chromium may have a disadvantage; it is environmentally persistent. It cannot be destroyed and it will always be present in some form within the environment. Incineration, composting and gasification will not eliminate chromium. Another consideration, in terms of end-of-life leather or management of chrome tanned leather waste, is the possibility of the valency state changing from the benign Cr III to the carcinogenic Cr VI.
BLC are currently looking into composting all types of leather to determine the length of time needed for decomposition. Following the initial stage, analysis of the composting material will be carried out to determine any harmful substances which may be generated during the process. For more information about this process contact BLC.
Chrome tanned leather is an extremely useful and valuable commodity because of its variable nature. The naturally occurring chrome III, which is used as the tanning agent, is perfectly safe if the leather is produced under the correct conditions and must not be confused with chrome VI. There are many advantages of using chrome tanned leather and this is why it is preferred for most leather products. The leather industry as a whole works extensively with different tanning processes, such as vegetable and aldehyde tannages, but so far has failed to produce a material which is as versatile as chrome tanned leather. 
‘Environmentally friendly’ is a term which most suppliers would like to use to describe their products. This does not mean, as is sometimes misunderstood, that chrome tanned leather may not be used. The term actually applies to the processes used to produce the leather, namely tannery and manufacturing procedures. This means that audits of these
factories would make sure that the processes are carried out correctly, and that no byproducts affect the surrounding
environment. BLC have recently launched a new ‘Environmen-tally approved’ Leathermark, which will encompass this whole process. More details can be obtained from BLC upon request.
For further information contact BLC Leather Technology Centre Ltd on info@blcleathertech.com or +44 (0) 1604 679999.