A lot of good replies to the question so far.  I would just like
to stress the need for clamping ivory when using a water based glue.
Water will cause ivory to curl and if not secured flat, then the ivory
will buckle.

The best glue to use, as mentioned, is hot hide glue.  Mix titanium
oxide (show white) with the dry glue, 1:5 if the glue wafer is still
intact on the key.  Brass clamping plates are heated to the temperature
of the glue pot and when clamped squeeze out the excess water.
Unfortunately the ivory also skates around and needs to be secured on
the sides as well.

However, when the ivory is to be applied to bare wood a wafer should
be used.  The wafer adds the white background for the translucent ivory
but more importantly, it is a buffer between the wood and the ivory
allowing each to expand and contract at different rates with seasonal
changes.  The wafer is nothing more than a piece of linen with the 1:5
glue mix applied and allowed to dry.  When it comes time to glue, the
wafer is dipped into water and the heated caul reactivates the glue.

On a related note, I'm sure you've seen new plastic keys which have
split.  This is caused by the rigid adhesive used to secure the keytop.
The wood expands while the plastic does not.  A more suitable glue for
plastic keytops is PVC-E glue.  It allows movement while offering a
secure bond (that's also what makes it a good choice for lapped bellows
joints).  Contact cement is more time consuming and has the ability to
etch the keytop surface.

A method I've used a few times to secure slightly loose head fronts
is to apply a little water to the front edge and allow it to wick in.
Apply a heated caul and clamp, this should reactivate the glue in the
wafer.  Or better yet, apply a thinned glue mix to insure a bond.

Regards,

Jon Page,  piano technician
Harwich Port, Cape Cod, Mass.