Bridge caps/hide glue failure

[rhohf@eagle.idcnet.com]

Stephen Birkett:

>This explanation doesn't seem to be consistent with the many extant
>pianos, e.g. Grafs from 1820, that have bridges which look exactly as
>they did when installed 170 years ago. The laminations are perfectly
>intact in these Grafs. Glue-joint failure in hide-glues is related to
>moisture and only a very humid atmosphere can affect this...I don't
>recall ever seeing a bridge cap on an early piano...this may be a
>20th C habit. Or perhaps North American pianos are different in this
>respect (I am not as familiar with these).

I confess to being no scholar and am basing my comments on personal
observations and my own (perhaps far-fetched) reasoning.  I had always assumed
that bentwood bridges emerged at the same time as bentwood panels in rims, but
your comments indicate that bridges occurred earlier.  I would be very
interested  in knowing more about the development of wood bending in keyboard
construction. The earliest caps I have seen  on bent bridges have been on
American pianos from the 1870s.

When I mentioned hide glue failure due to exposure to atmosphere, I should
have said atmospheric >moisture<.  We have plenty of hide glue joint failure
here in Wisconsin (is your climate that different than ours?) and would stand
by my implication that the more joints that get atmospher(ic moisture)
exposure, the more failures will occur.  This in not to say that an optimally
made hide glue joint cannot survive just about any conditions or that every
joint, optimal or not, is doomed to failure;  I am continually amazed by the
effectiveness of hide glue.

In my own experience with hide glue (as a fortepiano builder you must have
plenty of your own hide glue stories) it is clear that making an "optimal"
joint requires controlling the gel speed of the glue (mine is ground and very
light, I assume high grade).  To my knowledge there are only two ways to slow
down hide glue:  with heat and moisture, and with chemicals.  I would be very
interested in any other methods of slowing hide glue (I have a strong
prejudice against "liquid" hide glue.) I frequently toy with hide glue wood
joints when my pot is on for other reasons, but do not use it in "real"
woodworking.  It is impractical for me to set up a system for heating the work
and the chemical retarders I know of are more toxic than I want to have
around.  Hammer-veneering with hide glue and without pressure cauls has always
interested me, but my attempts on even small projects (a standard square Mason
& Hamlin leg) have failed without retarding.  I have since learned to make one
of a kind pressure cauls quickly and cheaply, and do all my veneering with
(wet) Titebond.

One might divide all hide glue joints into two groups: optimal and ain't
optimal.
Certainly the level of skill which produced your perfect 170-year-old bridges
is to be emulated.  However, there are still plenty of bridges in the "ain't"
group that have pretty darn good joints and are, by no means, doomed to
failure.  I would still contend that there are many ain't optimal bridges
which have functioned without glue failure because they were capped, whether
or not this was the original intent in the capping.

Bob Hohf