In the interests of speed and production, most of the tanning of leather hides today is done using mineral tanning. While vegetable tanning is still done to some extent, the time necessary to produce a tanned hide by this method (weeks), make it a method chosen only for higher end products, and not those meant for mass consumption.
Mineral tanning, on the other hand, can be accomplished in one day, which obviously means that production can be sped along considerably as opposed to vegetable tanning. When the raw hide is delivered to the tannery, it must be stripped of hair, fat, grease, any remaining meat, and moisture. At this point, the hide is referred to as being ‘wet blue’. It is now ready for the actual tanning process to begin.
The hides are put into a large drum that will tumble them, along with the necessary chemicals for tanning, for 10 hours or more. Usually, chromium sulfate is used to actually tan the hide, and the reason that it takes as long as it does is because this chemical must penetrate to the center of the hide if it is to be properly preserved. The tumbling action exposes all parts of the leather hide to the action of the chromium.
Unfortunately, some operators, hoping to turn out more hides than would otherwise be possible will shorten the length of time that the hides are in the drum. If the time in the drum is too short, the chromium, and any other preservatives that have been added, will not be able to seep into the center of the hide. Some tanneries will expose the hides to the tanning drum for only 20% of the time actually needed for proper tanning. Alternatively, because the tanning chemicals are very expensive, the tannery will not use the proper amount.
When either of these things occur, the tanning process will only penetrate the outer layers of the hide, leaving the center basically untouched. It is easy to detect whether the leather has been properly tanned just by looking at the cut edge of the leather. A properly tanned piece of leather will be a uniform color throughout. It will look tan or brown from top to bottom. On the other hand, a poorly tanned leather will have a blue center. This is because the tanning chemicals have not been able to penetrate throughout the hide. The blue color is just the remnants of the untanned hide, from the ‘wet blue’ stage.
Needless to say, leather that has not been properly tanned will not last as well as that which has been. At times, the blue center will be concealed with sewn or painted edges. The best way to determine whether the leather piece you are thinking of buying is of the best quality is to make sure you examine the edge – a solid coloration is your best guarantee of quality.
