The issue

It has been reported in recent months that tanners should be aware of the presence of traces of chromium VI in leather.

Chromium III is the basis of over 80% of the world’s production of leather so it is reasonable for tanners to be concerned as to whether there is chromium VI in the leather or not.

The level of interest has been generated because of consumer legislation in Germany where chromium VI is classed as a carcinogen. The current level at which tanners should be concerned is 3 ppm (the current limit of detection as described in the methods for Chromium VI determination (DIN 53314 IUC 18).

Working committees within the leather section of the European Standards Agency (CEN) believe that the current specified detection limit of 3 ppm is below the actual limit of detection and recent interlab trials indicate that for leather the real limit of detection is 10 ppm.

This limit is in accord with that set for the EU Standard for eco-labelling of footwear (Official Journal 17 February 1999 L 57/31 p1)

The cause

Chrome III may be converted to chrome VI by light or heat in the presence of oxidised fats or high pH in the leather or in the extraction procedure.

The chemistry of formation is not yet fully understood. It is also thought that unbound chrome III is more easily converted into chrome VI than bound chrome1, although work at BLC has not been able to confirm this2.

The test

Tanners in today’s environment will wish to demonstrate that their leather complies with legal requirements as a minimum; but also demonstrate product stewardship by ensuring that their product is environmentally compliant.

In order to meet the market requirements a sound test method is required. It is believed, at the current time, that there are limitations to the current test method. It has been reported3,4 that the method is unreliable.

The presence of coloured extracts, the pH of the extraction medium and contaminants in the extract may all interfere with the test method giving rise to false positives. It is virtually impossible to determine whether there is chromium VI in the original leather or not.

When the leather is wetted and the extracted chemicals are exposed to a chemical ‘soup’, this potentially can affect the chromium in solution creating a false positive.

What can the tanner do? The tanner can only effect the performance of his leather product through the choice of chemicals applied during the leather making process and subsequent mechanical operations.

At this stage it is not recommended that chromium III be abandoned as a mainstream tanning material.

Tanners should be aware of the issue rather than panic and transfer their production to chrome free systems for fear of the presence of chrome VI in their leather.

However the tanner should be aware that certain parts of the leather making process can affect the outcome of the analysis for chromium VI and may give rise to positive results when tested.

It is in the best interests of the tanner to be aware of these issues and avoid them.

The recommendations

The following recommendations are given to assist tanners in designing their post tanning processes so that the risk of a positive test for chromium VI is minimised.

To avoid the risk of testing positive for chrome VI:

* Avoid high levels of free chrome by washing well at the end of tannage

* Avoid the use of materials with oxidising potential

* Ensure that the chrome III used in processing is from a reputable source and is free of chrome VI

* Use neutralising agents with reductive capacity

* Avoid neutralising above pH 6.5

* Avoid oils and fats which may oxidise (assessed by a high iodine value)