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Klarinet Archive - Posting 000225.txt from 1993/12

From: Cary Karp <nrm-karp@-----.SE>
Subj: Re: Clarinet Cracks
Date: Tue, 14 Dec 1993 11:04:02 -0500

On Tue, 14 Dec 1993, David Lawrence Shea wrote:

> What is the best way to cope with cracks on an instruments. As far as
> I know, there are two schools of thought on how to repair a crack
> (which doesn't involve tone holes).
>
> 1. Take the instrument to a repairperson and have him or her pin
> the crack.
>
> 2. Simply apply super glue to the crack and forget about it.

Actually, there are two more standard processes. The one is called "flush
banding" and involves shrinking a metal ring into place around the
instrument in the middle of the crack. This method is currently frowned
upon because it induces compression shrinkage in the wood causing the
ring ultimately to loosen. (If we're going to keep up with these technical
discussion I guess the wood technology "lecture" may be unavoidable.)

The final method is quite popular at present and may be see as a hybrid
of the other three. It involves the use of thread soaked in
cyanoacrylate glue ("super glue"). The thread, which is often carbon
fiber, is either bound around the joint where a flush band otherwise
would have been placed, or is "sewn" through the holes through which the
pins otherwise would have been screwed. Since the fibers are glued to the
surface of the instrument, the wood can't shrink away from them. (Buffet's
Elite model clarinets have glued carbon fiber rings instead of
conventional metal ones.)

> My reasoning for using option 2 is as follows. If a crack occurs on a
> clarinet it is due to a certain stress on the wood. When the wood
> cracks that stress is relieved. By super gluing the crack, the crack
> is held together and the stress will not be present. By pinning a
> crack, it seems to me that one just puts the stress back into the
> wood. This can lead to other cracks. A student here at IU had a crack
> pinned recently and since has had several other cracks occur in other
> areas of the instrument.

This reasoning is fundamentally sound but there are two kinds of
stress-induced cracking. In the first, the crack relieves internal
stresses when it opens. Forcing it closed again returns the stress to the
wood. Since, however, the weakest spot in the wood has just been
reinforced, the "next weakest spot" might be able to withstand this
stress. If it does, fine; if it doesn't, there will be new cracking. In
the second case, the wood cracks when subjected to extreme external
conditions (mechanical "blows", excessive thermal stress, excessive
moisture induced stress). Here, when the force that causes the crack is
removed, the crack closes. In this case, holding the crack shut will
eliminate the risk of its reopening when subjected to what otherwise
would be normal levels of externally induced stress.

In general, if a crack wants to remain open it shouldn't be forced shut.
If a crack wants to remain closed, it should be helped to do so. In the
latter case, one of the various "permanent" techniques should be applied.
In the former case, the cyanoacrylate method is just fine under one
condition -- the glue should not be allowed to form a barrier to the
movement of the wood. If the crack wants to open, the glue should not
hold it shut with greater strength than the wood had before it cracked
(lest a new crack open instead). If the crack wants to close, the glue
should not prevent it from doing so. The classic material for repair of
this type is grenadilla dust mixed into stick shellac. The shellac keeps
the crack airtight and has a mild adhesive effect. The shellac will also
shatter into a powder if the crack tries to close around it.

Cyanoacrylate seems to be assuming the role of shellac in this regard,
but I haven't gotten around to comparing the two materials. Unless wood
is treated prior to the application of cyanoacrylate, its bonding
strength is not all that great, so I guess it should be okay. I
don't know how it behaves when compressed but again, on the basis
of the popularity of the technique, assume that it squooshes enough
to avoid harm. Otherwise, plain old nail polish will also do the trick.

   
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