Jon, Ivory is tricky material to care for in that it is composed of
both organic and inorganic substance. It is anisotripic, meaning
that , like wood, it absorbs and gives off moisture depending upon
the relative humidity of its environment. It grows in layers much
like the growth rings of a tree. Unfortunately, these rings will
expand and contract at different rates and this leads to
delamination/cracking. The only way to prevent this is to avoid big
swings in relative humidity, especially low humidity. Anything below
30% (as I'm sure it is often in Texas) is especially damaging. You
should also avoid getting it wet for the same reasons. Strong light
and heat should also be avoided. It also absorbs skin oils, makeup,
etc readily and stains. The inorganic framework of ivory is
destroyed by acids.
Once it is cracked there isn't much you can do. The best thing
would be to get it into a more stable environment. If they are
especially valuable, your customers may want to have a special
exhibit case that is sealed (a micro-climate could be created in the
enclosure using silica gel products that can be conditioned to
specific RH levels).
Some curio shops up here put glasses of distilled water in their
cases thinking it will prevent cracking. Since the cases aren't
sealed well, I seriously doubt that it does anything more than make
them feel better.
One thing I wouldn't do is try to fill the cracks or "repair" them.
First, it will greatly affect their value unless done by a
professional conservator. Secondly, if you put them back into the
same climate, the cracking will continue and be assisted by the
"wedges" used to fill the existing cracks.
Again, if they really want them "repaired" consult with a
professional conservator. You can get references from any major
museum or art gallery. I, however, would spend my money on getting
an enclosure made or somehow otherwise improve the environment they
are in and consider the cracks part of the piece's history. The
cracks will no doubt be less evident once they are placed in an
environment with a proper humidity level.
PS.: My "real career" is a museum curator of collection. If you have any
other questions feel free to ask.