There is much
more to glue than just holding the individual wooden parts
together. In fact, glue has a profound impact on the acoustical
properties of a musical instrument. It can make the difference
between a ‘dead wooden box’ and a great sounding musical
instrument.
A traditional 48
key hexagonal concertina consists of about 172 wooden parts,
divided over 2 ends each with 86 individual parts each. The primary
function of glue is to hold these wooden parts together, but
preferably without obstructing soundwaves.
Soundwaves
travel primarily by air through a concertina. After the
soundwaves are produced by the chopping action of the reeds,
they bounce off walls in the reed chamber, travel through the
air hole into the action cavity where they are further filtered
and/or amplified by bouncing off the walls and action, before
leaving the instrument through the fretwork.
The shape and materials of the cavities have an effect on the
sound quality. This process of filtration and amplification of
specific frequencies is called sound reflection.
In a
well-constructed concertina there is a second way soundwaves are
projected; through the body of the instrument. Sound travels
about 13 times faster through wood than air.
This secondary sound projection adds considerable depth to the
sound of the instrument. How much depends on the type and
quality of the wood, and most of all, the uniformity of the
wooden body.
One large piece of wood will project sound much better than many
smaller pieces. The 87 glue joints of individual parts per side
create potential obstacles. In order for this process to work,
sound waves need to able to travel through the glue joints
with as little obstruction as possible.
This is why the type of glue used in a musical instrument is so
important. It makes the difference between one uniform body, as
a result of ‘acoustically invisible’ glue joints, or a
collection of 86 individual parts per side, separated by
insulating glue joints.
Modern glues
Unfortunately, not all glues are equal. There are 2 popular
types of glue used for wood: the white PVA (Poly Vinyl Acetate)
glue, and the ‘carpenter’s wood glue which is yellow.
Unfortunately, both are used extensively in concertinas. The
white PVA glue is not waterproof and not as strong as the yellow
glue, which is waterproof after it has dried.
They are both easy to work with, available at every hardware
store, dry very fast, and create very strong joints, much
stronger than needed in a concertina. However, they both have
one characteristic that makes them unsuitable for use in musical
instruments: they both
expand
and
remain rubbery after they dry, and actually create
insulating glue joints, preventing soundwaves to pass through.
Protein glues
The only type of glue that does not create insulating glue
joints is protein glue. Luthiers have been using protein glues
such as bone, hide and fish glues for centuries. These glues are
‘acoustically invisible’. When protein glues dry they contract,
become rock hard and leave very little residue between the
parts, allowing soundwaves to travel thru the glue joint with
minimal obstruction.
Hyde and bone glues are
the most common protein glues used in musical instruments. They
come in granular and powder form and need to be mixed with water
and slowly heated ‘au bain marie’ to around 145F. The shelf
life is about 1 week.
Protein glues are not as
easy to use as modern glues. You need to control the temperature
of the wood to be glued, the thickness of the glue. The
‘open time’, the time between applying the glue and the moment
you cannot move it anymore, is very short. This means you need
to plan every glue joint carefully.
The consistency of the
glue is determined by the type of wood and glue joint that needs
to be made. Adjusting the glue for the particular joint requires
knowledge and experience. Because protein glues contract when
they cure, there is no need for excessive clamping.
Another important
characteristic of protein glues is that a glue joint can easily
be reversed. Just add moisture and heat, and the glue becomes
liquid again. Old glue can also be activated again by adding
moisture and heat. This allows for replacement or re-gluing of
individual parts without damage to the rest of the instrument.
All vintage concertinas
were originally built with protein (bone) glues. Nowadays, only
manufacturers of high end musical instruments use protein glues
in their instruments, because of their far superior quality.
Glue damage in Vintage concertinas
Overtime, both high and low humidity can weaken the original
protein glue joints in vintage concertinas, especially when
mahogany wood is used for the frames or action boards. Failing
glue joints can cause damage to faceplates and action boards due
to loss of support.
All this is
easily repairable by reactivating the original glue, or
replacing it with new protein glue.
Unfortunately, quite often the loose parts are re-glued with a
PVA or yellow wood glue, creating an insulating glue joint.
This can become a serious problem when many parts are re-glued
with wood glue.
Even worse, some people ‘repair’ a soundboard crack, which is
common in vintage concertinas, by inserting wood glue in the
crack, instead of making a sycamore/maple shim and gluing it in
place with hide glue.
We’ve seen many
vintage concertinas reduced to ‘dead wooden boxes’ during the 25
years of restoring. If the instrument has potential, it is
always worth it to take it apart, remove all the wood glue and
build it up again with the correct glue. It is amazing how much
better it sounds when acoustic unity is restored.
You can test the acoustical properties of a concertina by
placing a tuning fork on different places on the body and listen
to the amplification it produces. Or, if you don’t have trained
hearing, place a contact microphone on the other side of the end
and check the harmonic spectrum.
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2001-2018, Concertina Connection Inc8, All rights reserved.