As a rebuilder of larger instruments than players, reproducers, band
organs, nickelodeons, etc., I cannot understand the use of any casein
based glues.  Let me put this in terms that, as a physicist and a
restorer both, will make the issue a bit more clear to the discerning
collector.

On the electrostatic level there are two forms of attraction:
(1) cohesion, and (2) adhesion.  Cohesion is when a substance likes
to stick to itself.  Adhesion is when two dissimilar substances like
to stick to each other.

Most glues have good adhesive properties but once set have poor
cohesive properties.  This is why you clamp a glue joint for any glue.
You want the glue layer to be as thin as possible because most glue
will separate from itself before it does from the material being glued
together.  To reiterate, glues don't like to stick to themselves as
much as they like to stick to other things.

The best case in point: old dried casein glue does not like to stick
to new casein glue.  So, when you are doing a restoration and you find
casein glue, you must go down far enough to expose open wood pore space
to get a good clear surface.  This usually means removing wood grain to
some extent.

Casein glues take longer to dry, so they soak farther into the wood
than just the surface.  This would be a plus, until releathering comes
along and you must make the pneumatic smaller by some dimension until
open pores are found for the new glue.  For example, on Wurlitzer pipe
organs some of the pneumatics for pedal offsets are just the right size
to begin with, _until_ you sand or plane them down to new wood.  Then
the size is too small to allow the wind pressure to operate the
pneumatic and it won't open the pipe valve.

You end up making new parts of the correct size, or reducing the force
of the original return spring, which slows the repetition speed.  On
repeated releatherings through the ages you can imagine that casein
glued parts must get smaller and smaller with each "restoration."
Where white PVC-E type glues have been used I have seen "restorers"
advocating leaving the pneumatics in a bucket suspended above acetone,
methyl-ethyl ketone, or some other brain-cell-destroying solvent in
order to remove the glue.

On the other hand, hot glue lets you remove the original leather in a
microwave oven without solvents.  The long chain proteins of hot glue
are not adverse to linking up with newly applied hide glue.  Sanding
may be kept to a minimum so the dimensions of the pneumatics are
retained.

Water is the only substance you need to work with when using hot hide
glue.  Most water has a low toxicity level.  Therefore, the dimensions
of the gluing surfaces need not be reduced as the wood grain is still
accessible to the sticky proteins in the glue and any slight
application of water will further raise the grain.

Recently an organ club rebuilt a large reservoir.  They did everything
correctly but missed one small critical point.  The old casein glues in
all of the leather/wood connections would not allow new glue to adhere
with strength.  When wind was applied, the leather lifted cleanly off
of the wood in several areas and the gussets let go.  All of their work
was wasted, as was the leather used.  They now have to get into the
reservoir again by taking it out, which necessitates removal of wind
lines, pipes, tremulants, and an offset chest.  They have a few
unprintable words to say about white glues.

Casein glues may allow for easier work now but the next restorer-
rebuilder will undoubtedly have a much harder time doing a good job.
Some argument might be made for using these glues on surfaces that will
never ever have to be separated, such as case joints and wooden pipes,
but, again, when you go to put a finish on the joint or wooden pipe, if
any casein glue has crossed the wood grain, your finish won't sink in.
Hot glue can be sanded on a surface to where a finish will sink through.
You have to sand much more to get the same open grain with the casein
smear before a finish will go into the wood.

Just my 2 cents...

Al Sefl, Ph.D.
Sniffing hide glue for 50+ years with no ill effects.