I want to thank everyone who has participated in this discussion,
adding valuable personal experience that would have taken forever or
never to discover in the years before the Internet.  I have some
further comments on the recent posts.

Whiting has always been defined for me as calcium carbonate.  However,
I acquired a small amount of titanium dioxide powder from a potter, and
it has lasted me for years.  I've been told that titanium dioxide is a
substitute for lead oxide, which was once used as a base for white
paint, among other things.  (This makes me worry that old pianos are
some kind of toxic waste nightmare, with lead oxide from the key
weights, ivory glue, and maybe even the felt?)

I mix the titanium dioxide right in with my hot hide glue until it
is not translucent.  The adding of a solid substance to the glue does
not seem to weaken it, although there might be some subtle effect.
Solid pigments are added to paints of all sorts, and they seem to work
all the same.  I brush on the hot hide glue, then use the brass ivory
caul and two screw clamps.  The process of tightening down the clamps
seems to almost always slide the ivory out of place; so I have to re-
check it once or twice to make sure that the joint is tight.  I usually
end up with good results.

The titanium dioxide provides a solid white backdrop to the translucent
ivory slip, with no sign of yellowing that would come from aliphatic-
resin glues like Titebond or from the key-lever wood itself showing
through the ivory.  I do not use Titebond for re-gluing ivories.  I
also do not heat up the brass caul, as normally suggested for an
individual ivory replacement.  This just makes the hide glue take
longer to set up, liquefies it further, and makes the ivory slip and
slide all over.

The glue-and-whiting (or titanium dioxide)impregnated linen patches
were intended to be used with cold water and a hot brass caul, but I do
not use these patches ever, even if it means I must do a repair in the
shop and return it.  They are not necessary for a good ivory repair,
and often are more headache than they're worth.   Linen was used to
reinforce the glue joint and prevent it from failing if the wood should
expand or shrink.  I suppose it probably works in some circumstances
(thin soundboards on Flemish harpsichords and clavichords have linen
strips glued to the underside of the glue joints).  Some keyboards had
linen (or cotton) laid down on them before being sawn apart, on the
theory that this would stabilize the ivory and the wood of the key
lever, preventing the wood from splitting the thin ivory slips.  But it
isn't really necessary; most splitting on ivories occurs on the
overhanging front edge and can travel straight up the grain, regardless
of how well it is glued down.

For gluing, I use the electric glue pot available from piano supply
companies, but I mix my glue in a glass jar salvaged from food
products.  Pickle jars are squat and have wide mouths, along with a
metal lid (increasingly hard to find; I hoard them).  I prefer a metal
lid over a plastic lid because of their problems.  Of course I sanitize
the jar in the dishwasher to try to kill any molds.   Still, they can
accumulate.

I place the jar inside the glue pot, with a bit of water between it and
the aluminum wall of the glue pot, in order to transfer heat evenly.  I
leave the lid partially covering the jar to control evaporation, and I
add more water if the glue becomes too thick.  After usage, I screw the
lid down and place the jar in the refrigerator.  Even it the glue is
fairly hot I've never had a glass jar crack on from doing this.
Immediate refrigeration lessens the opportunity for molds to take hold.
If I do get mold growth, I cut out a small section of the congealed
glue with a sharp knife, and it will last for a long time.  I have
sometimes put alcohol in the jar to try and kill the mold.  But if
there is too much, I abandon the glue and the jar and start over.

For hide glue with titanium dioxide mixed in for re-gluing ivories, I
do the same thing, mixing the dioxide right in the glass jar.  This
seems to keep for years, and I've never had a mold problem with hide
glue infused with titanium dioxide.  I use separate natural-bristle
artist brushes for applying ivory glue, because the dioxide never fully
washes out of the brush, and I do not want whitened glue on a normal
glue joint.

Mark Ritzenhein