Darkening

 MYTHS:

1. If you store leather in the dark, it will not darken.
   Well, it is not light, or sunlight, or UV light that darkens leather.  
   It is air - oxygen - it is the tanins in the leather that oxidizes with its contact to air, that turns a darker color.
2. Oil makes leather darker.
   No, oil might accelerate the oxidation process.
   When all the tannins have oxidized, you can add as much oil as you want, the leather will not go darker.
3. Leave leather in the sun to make it go darker.
   Well, it might just accelerate the oxidation a bit, but it is still oxidation that darkens the leather.
4. Saddle-Lac, NeatLac, Resolene will prevent the darkening of vegtan.
   No, all it might do is temporarily seal off the leather fibers from the air around it.  As the finish wears through and the fibers get exposed again, they will oxidize once more.

Here is an example of two sample sheets made about two years apart -  you can see how much the leather oxidized under Resolene.  The newer one will oxidize too, but nobody can say how much - that will depend on the recipe used to tan the leather - the more tannins, the darker it will oxidize.

Long ago in a guild we experimented with a complete UV light blocking finish we got from France.   It was a bit slower, but the leather with that finish on, still turned darker.

Then one day I saw a kit that had been hanging at the very back of a peg on the wall at a Tandy store.   The leather of the kit was the normal light color you expect in a vegtan.

Except for a very small round spot that had darkened - it was right next to s small hole in the plastic.   The leather darkened only in that one little spot.
This made me think that it was not light that had any effect - that whole piece of leather was exposed to the exact same amount of light.   The only difference was the exposure to air.

I asked the question on the forum of the Leather Chemists of America, and they confirmed for me that it is oxygen that oxidizes the tannins in the leather that makes for the darker shade and not light.

There is an interesting aspect to this:

When leather is exposed to sunlight for too long, it bleaches to a very light unnatural color.

And the moment you touch that leather with water, it immediately gets to its darker oxidized color again.  

You can see that on this sign that hung in a store window for a long time and got afternoon sun every afternoon.   All I did around the buffalo head was to paint it with clean water (and when I took this picture, that water had dried totally).