- 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.
- 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.
- 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.
- 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.
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.