laminated bridge question

[Delwin D Fandrich]

Stephen,

No adhesive (with the exception of epoxy) is particularly good at gap filling.
Some are less bad than others. Regardless of the theory you sight, my experience
-- and that of countless other woodworkers -- has taught me to expect much
better gap filling qualities from aliphatic resins (Titebond, etc.) than from
animal hide glue. With aliphatic resins this gap filling quality can be enhanced
by adding a bit of wood flour to the glue. I don't know if this would also work
with hide glue. I've never heard of it being tried. As may be, many -- not all,
but probably most -- of the failed animal hide glue joints I've taken the time
to look at over the years were the result of an excessively wide, or thick, glue
line. Not all of these were "old" glue joints.

Contrary to popular belief, aliphatic resin adhesives really don't require all
that much clamping pressure. Especially with soft woods. Our rib-to-soundboard
press uses 45 pounds of pressure. We've never had anything remotely resembling a
glue joint failure. In fact, within ten or fifteen minutes after applying
pressure to one of the cauls it is impossible to separate the rib from the
soundboard without either breaking the rib or tearing off the surface of the
soundboard.

The glue does like a bit more pressure with hard woods, but it doesn't really
require the extreme amounts recommended by Hoadley either. If there is a good
mechanical fit between the two adherands, aliphatic resins do make quite good
"rubbed joint" bonds. Good thing, too, or all of those millions of mortise and
tenon joints that woodworkers the world over have made with the stuff over the
past few decades would have come apart by now. In fact, it is in applications
like this that aliphatic resins really shine. They do tend to withstand the
sheer stress created by the cross-grain wood movement inherent in most mortise
and tenon joints or in a half-lap joint extremely well.

Regards,

Del

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Stephen Birkett wrote:

> Del:
>
> Good to hear from you.
>
> Trouble with English is that colloquially there is no distinction between
> adhesive and cohesive bonding. In this respect other languages like German
> have a separate word (Leim) meaning specifically cohesion...obviously the
> appropriate process for wood joints. Of course titebond and hide glue both
> have excellent cohesive strength.
>
> Titebond is not good at adhesive bonding, where the glue layer is not
> expelled and no molecular links are formed between the two parts glued
> together. That's preceisly why ultra-thin heavily clamped joints are so
> essential for titebond type glues...i.e. extremely high pressures are
> needed...as Hoadley points out considerably more pressure is really
> required than anyone would typically use. Hide glue, as well as being an
> excellent cohesive bonding agent, is also reasonably good adhesively.
> Hence the applications in gluing materials like felt and leather, for
> which yellow glues are useless. Within reasonable limits the adhesive
> strength of hide glue can maintain a poor wood joint where a titebond glue
> would be less adequate....before you come down on this I say reasonable
> limits. Apart from chemical deterioration, the majority of failed old hide
> glue wood joints can be attributed to wood shrinkage and expansion i.e.
> movement after the glue has cured, causing the crystalline structure to
> break.
>
> Stephen
>
> Stephen Birkett Fortepianos
> Authentic Reproductions of 18th and 19th Century Pianos
> 464 Winchester Drive
> Waterloo, Ontario
> Canada N2T 1K5
> tel: 519-885-2228
> email: birketts@wright.aps.uoguelph.ca