The causes
These dark ‘ink’ stains are caused by the reaction of metals, usually iron, with tannic acid in the vegetable tanning agent resulting in the deposition of coloured basic metal tannate in the leather. This chemical reaction, considered to be a curse by tanners, has actually proved to be very useful in other industries where the intense and very stable blue/black colour has, in the past, proved invaluable in the production of writing inks and dyes.
The iron responsible for these stains can originate from many sources:
1. Chemical contamination
2. Blood in hides/skins
3. Rust particles from machinery, trucks, horses etc
4. Splitting/shaving machines
5. Nails in pallets
6. Water contamination from rusty storage tanks or pipework
7. Toggle frames
Iron contamination of tan liquors can result in extensive staining. However, the ionic state of the iron and pH of the liquor will dictate if staining will occur. If the iron is in the ferric state, staining will not occur if the pH is below 2.7, but this pH may be undesirable. If the iron is reduced to its ferrous state, staining will not occur if the pH is below 5.7. It is thought that very low levels of iron in tan liquors can result in staining; studies have indicated that between 0.02 and 0.04% iron can cause staining depending on the type of vegetable tanning agent used1,2.
Sometimes iron stains on vegetable tanned leather are easily confused with mould stains; the pattern and colour can be very similar (Figure 2). To determine if the staining is mould or iron, apply a little oxalic acid to the stain. If it disappears, the staining is due to iron, if it remains then mould is likely.
Obviously, the best way to overcome the problem of iron staining is to avoid contact of vegetable tanned leather with iron. Iron is a very common element indeed so its total avoidance is impossible. However, much can be achieved by good housekeeping in the tannery such as keeping iron and steel mechanical parts rust free by regular maintenance. The thorough soaking of hides should ensure that iron contamination from blood is minimised. Replacing iron piping with plastic and using stainless steel trucks and tanning vessels will go a long way to minimising the problem.
Stains can be prevented during the tanning process by introducing substances that have a higher affinity for iron than tannins, eg the sequestering agent ethylene diamine tetra acetic acid (EDTA). As a ‘rule of thumb’ approximately five parts EDTA is required to chelate 1 part of iron. Acidification, with oxalic acid for example, will also remove stains. There are also many syntans on the market that claim to bleach iron and other process stains from vegetable tanned leather.