The tinker wrote:
 
"My understanding is that hide glue never goes bad (as in loses effectiveness).
It will get moldy. Then it stinks to high heaven. Refrigeration stops mold
growth."
It does very definitely go bad, very bad indeed for some uses, though in other
uses you might not notice the problems.
Hot glue comes in a range of "bloom value," made by taking sequential boilings
of hides and bones etc. and then blending them to get the desired properties.
Very high-bloom glue (close to pure gelatine) has an extremely short open time
but very great strength and a clear color; it absorbs a lot of water and so can
be brushed on in a very thin layer---not always a good thing. As glue is kept
hot and reheated, the molecules break down into shorter ones and the bloom of
the glue drops. The glue becomes weaker and more brittle, much more viscous so
you can't brush on a thin layer, it darkens very noticeably, and it becomes
extremely acid. If you let the glue actually boil, if you even let it heat above
140 degrees or so, everything speeds up.

That's apart from the mold growth, which will accelerate the breakdown of the
molecules at well as leaving the glue full of the mold (the bit you can brush
off the surface is not the body of the mold, it is just the reproductive
organs.) Refrigerating glue overnight will slow down mold, about as effectively
as it will slow down mold on jello (which is essentially what hot glue is). But
refrigeration won't remove established (invisible) mold from the depths of the
glue, and it won't kill it. Remelting the glue every day will generally allow
you to keep it indefinitely, at the cost of having your batch go down steadily
in quality with each reheating.

Research at the Government Printing Office in the 1920s established "best
practice" guidelines of having each binder mix up entirely fresh glue at the
beginning of the week, then discarding anything left over on Friday and mixing
an entirely fresh batch next Monday. This replaced the old practice of having
each binder dip his glue from an enormous whole-shop kettle, which was never
cleaned or entirely emptied---fresh glue was added from time to time to top it
up. The clean-each-week method improved economy as well as quality, probably by
reducing the thickness of the glue layer (imagine brushing on root beer as
opposed to honey).

The actual bloom value that is best for glue varies according to the job you are
doing. For hand binding and conservation it is best very high, say 400 or even
more. Machine binding, quite a bit less. For general woodworking it is maybe in
the 140-250 range. For violinmaking the preference is very short-moleculed, low-
bloom "rabbit skin glue" (nothing to do with happy hoppy bunnies, just a name)
of, IIRC, under 100, for precisely the characteristics that are not wanted in
other uses: by using a weak, brittle glue you make it easier and safer to
disassemble an instrument without injuring the wood. Artists' suppliers sizing
and gessoing canvas for oils also prefer rabbit skin glue, though I have never
been able to figure out just why.
Hot glue is inconvenient, cranky, and unforgiving in many ways; in others, it is
very forgiving indeed, and very gratifying to use. It is perhaps the best all-
around adhesive for serious furniture making, but it requires a degree of
devotion to get the best out of it. And, contrary to advertising claims, Old
Brown Glue is not hot hide glue, any more than jello is. The working properties
and manner of use are different. This is because while the materials and many
properties are the same, the method of manufacture is different in many details.
Probably the aging properties are different. That is not to say that OBG is a
bad adhesive; I haven't used it but it sounds perfect for many uses. But it is
different, and clearly demands different treatment.
Tom ConroyBerkeley