Experimenting with cupric acetate

Many if not most leatherworkers know that a solution of ferric acetate, aka vinegaroon, will turn vegetable-tanned leather dark grey or black.  Thus, while it's not actually a dye in the strict sense (the black color is not the result of black pigments in the solution but of a chemical reaction that takes place in the leather) it can be used as one in a limited capacity, and is cheap to produce using just vinegar and steel wool.

Years ago I saw a post on the myArmoury forums mentioning that if you use copper instead of iron, the result is blue.  I bought some copper wool to try this out, and finally got around to it last week.  After soaking the copper in vinegar for a few hours, I wondered if it might not take a long time to create the acetate, since copper is more resistant to corrosion than iron.

It turns out there's a way to speed the process up enormously.  All that's needed is one more common household chemical, et voila.  The video doesn't show it, but the midpoint of the steeping looks amazing, like a breaking wave filmed underwater.

It was at this point that I began to worry.  Was the blue color just a reference to the liquid itself?  What would it do to leather?

The answer is just a little.  The dry leather stained with cupric acetate is on the lower left.  To the right is the same piece after oiling, next to an unstained, un-oiled piece from the same hide.

The color doesn't seem to wash out easily after drying, so there may be some chemical reaction taking place, but the visual effect is modest - and this is after getting the leather sopping wet with the solution.  It probably contributes to the darkness of the oiled brown leather.  What you see in the photo, though, is the test scrap at its darkest - freshly-oiled leather will lighten with time as the oily and dry areas reach equilibrium.