I had a melodeon keyboard that was just about orange from being in
the dark for years.  It took about 1-1/2 years in the window, but it
turned bone white eventually.  I think it would work faster if you are
somewhere very sunny.  I'm here in Iowa, and I sent some out to my
brother in California and it worked much faster out there.

Much of the ivory on piano keyboards is really from the mastodons or
wooly mammoths or whatever they were called.  Somewhere around here
I have an old article called "You and You're Ivories" and it mentions
that much of the piano ivory was being "mined" in Siberia and at that
time (around 1910) they saw no end to that source for piano keyboards.

In short, they were making the point that your piano keyboard could
very likely be 20,000 years old and you should take care of it!  It
also says that an elephant tusk could lay on the jungle floor for 300
years and not show hardly any signs of decomposition.

Oh, and I read somewhere else that the pre-1910 or so keyboards have
ivory that's thicker and so those keyboards don't usually come up with
loose ivories, like the later keyboards often do.  They learned how to
slice the Ivory thinner about then and, well, you know the end result
there.

I met a guy back in the 1980s who was involved with ivory some way
and he also mentioned that if you have a keyboard that does yellow from
keeping the key cover down, that the ivory came from an animal that
was found dead or the tusks were found on the jungle floor or whatever.
The ivory that does not yellow was taken from an animal that was
harvested (or shot, I guess) for the purpose of taking its ivory.
Some keyboards don't seem to yellow, and that may be the reason.

It's an interesting subject.  Good luck with your old keyboards.

Earl Hennagir