The following letter is a comment from an associate, Terry Connors, who
was kind enough to offer his comments on glues and adhesives.

 [ Ref 990621 MMD, "Hot Hide Glue", by Craig Brougher ]

 - - -

The following paragraph is mostly correct, although I would take
exception to the remarks about the wood itself breaking at "vastly"
different loads; I suppose we might allow this as mere exaggeration on
the author's part.  As far as the last sentence, "In every bond some
parts of the glue break and some parts of the wood break...," it is
probable that there are problems with the joints being examined if the
author is observing a significant amount of adhesive failure.  I would
agree that some amount of adhesive failure is common, but in most
successful standard test joints this is a small percentage of the
surface area under test -- certainly less than 20%, and it should be
much less than that.

> The strength of glue is due to its adhesion and it own inherent shear
> strength.  Since this is theoretically greater than the surface of
> wood, anyway, what does it matter how much stronger hot hide glue is,
> than say TiteBond? And granted, even though adhesion is half the
> battle, some woods allow much less adhesion than others.  And you can
> lay 3 dozen bonds out on a table and wait for them all to set up.  When
> they are fully hard and dry, you can test these bonds and you will find
> that the wood itself breaks at vastly different loads, anyway.  In
> every bond, some parts of the glue break, and some parts of the wood
> break, often (but not always, though).

PVA is not an adhesive intended for butt-joint application as it is not
a gap-filling adhesive (even with end-sizing), and it will flow right
up the wood fibers through the cut fiber ends exposed on the end-grain
on unsized wood.  Hide glue might be more successful bridging the gaps
in this poor type of joint.

> I have, however, tested TiteBond, Elmer's, and hot hide glue in similar
> fashion.  I butt-glued after sizing each piece first, I also lap-glued
> perhaps a dozen sample 1" wide strips 1" overlaps.  What I was
> delighted to find was this: After half-a-week had probably passed (10
> -15 years ago, now), I removed the clamps from the PVA samples, but I
> did not clamp the hot hide glue samples.  The hide glue, which at the
> time was Hudson Industries' 165 gm chrome glue beat all the PVA
> samples, although, if I recall, I had several really good and strong
> PVA samples -- equal to the  hide glue.
>
> So overall, hide glue won.  But I was amazed at the differences between
> the joints of the PVA samples.  They varied all over the map, even
> though I really do know how to glue things well, and did them all the
> same exact way.  I decided the differences were caused by the wood
> itself, and the way that wood soaked up the water, allowing the PVA to
> set.  None of the PVA butt-glued samples held at all -- even after
> pre-sizing them.  But the hot hide glue butt joints were just fine!

What were the results for the lap-joint tests?  If both sets of
adhesive were applied and set correctly, and if the moisture contents
and species glued were the same, I would expect that similar amounts of
wood failure would have been obtained and that the joint strengths
would have been similar.  The only variation would have come from the
wood strength variability, if the tests were accurately controlled.

He does admit that anything that breaks the wood is strong enough, and
mentions the reparability aspects appropriately I think.  Personally I
don't think he pays enough attention to the biodegradability of the
hide glue adhesive -- face it, any protein adhesive is going to be a
bug attractant and will be susceptible to moisture damage -- but that's
his own business.

"Erudite glue strength tests ... are worthless in a shop, overall." ???
How does he think we solve problems for industry?  Perhaps "erudite
glue strength tests" are of less value in an environment where the
user pays only casual attention (if any) to those factors that create
good adhesive bonds.  Some adhesives are more forgiving than others.
I think that this fellow knows his materials, and his systems work for
him, but I don't think that he should be held up as a model for
everyone else just on that basis.

If you are interested, there are any number of labs available that
can perform standard adhesive tests on hard maple according to ASTM
(American Society for Testing and Materials) tests.  I've done a number
of those myself, could always arrange to do a few more for someone who
is interested.

Terry Conners
conners@fpl.msstate.edu