| crack clamping
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This Article was first
published in
THE STRAD November 2001 |
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Hold on tight |
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Andreas Hudelmayer compares notes with London
restorers on using small clamps for crack repairs |
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Glueing cracks is one of the most common jobs in violin
repair or restoration. Yet to glue a crack perfectly remains one of the
most difficult tasks, with tension, sinking arching and unevenly swelling
wood making things difficult and slightly unpredictable. A traditional
method of glueing involves wide stretcher clamps. However, many London
workshops now use a system of studs and small clamps. This provides
greater control over the pressure on the crack, in terms of both amount of
force and direction, and thus more control over the final result. |
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 figure
1 |
For the
pressure to be controlled at a given point it must be exerted as close as
possible to that point on the crack: the (impossible) ideal is 'x' in
figure1. |
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A stretcher clamp is problematic as the pressure
points are far from the crack at the edge of the instrument (figure
2). |
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figure2 |
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 figure3 |
The next best option is right next to the crack, on the
inside of the plate. Using a wooden stud on either side of the crack, the
sides can be pulled together with a little clamp. Although this will close
the inside of the crack, the outside of it will open (figure 3). |
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 figure4 |
To counteract this, fit a wedge between the two studs,
putting outward pressure on the top of the studs (figure4).
By carefully
balancing the pressure of wedge and clamp you can now control exactly the
angle and pressure with which the two sides meet.
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 photo1 |
The studs
Many cracks, especially on violin fronts, need only
minimal pressure to hold them together. For these, small wooden studs in
the shape of a cube, around 7mm side length will be strong enough.
Sometimes, though, it is necessary to exert more
pressure. To increase the strength of the studs you can increase the size
of the glueing area - but only up to a point. This point is determined by
two factors: the span of your clamp and the distance you dare to have
between the clamping point on the outside of the stud and the crack. The
further away from the crack you go, the more difficult balancing wedge and
clamp pressure will be.
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A horseshoe-shaped stud allows you to increase the
strength of the studs without increasing the distance of the pressure
points. The hollow area allows the clamp to be positioned near the crack,
while the glueing area extends behind it (photos 1 and 4). |
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photo2 horseshoe studs can
be made en masse using a thick piece of quarter sawn and a bandsaw |
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Smaller studs often do not need to be chalk-fitted -
thick hide glue will be sufficient. Hold them on for a few seconds or clamp them, then leave them to dry. For
slightly bigger studs the discrepancy between the flat stud surface and
the shape of the plate might be too big: these will need chalk-fitting. Note
that a clean glueing surface is as important for good adhesion as the fit!
When you glue your studs, leave a small safety gap to avoid glue getting
in the crack.
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 clamp
A |
Clamps
Three types of clamps are commonly used :
Clamp A, a parallel clamp, is able to clamp down
right next to the plate. The most suitable version I have come across,
supplied by Dick GmbH in Germany, spans 28mm. You might want to modify it
slightly by grinding the lower outside edges round, to make sure that the
inside edges are as low on the plate as possible. This clamp might be a
bit heavy if you want to move the plate around.
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 clamp
B |
Clamp B, made by Brian
Hart, is small and easy to use, but the handle can prevent it from
clamping as low as desired and you have to be careful to prevent it
digging into the plate.
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 clamp
C |
Clamp C - another Brian Hart tool - combines
the low pressure points of the parallel clamp with the size and
convenience of clamp B. It spans 20mm. Each restorer has their own
technique of mending a crack, but two methods frame most of the
variations.
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 photo
4 |
Slow and careful (photo 4)
After the crack has been thoroughly cleaned, horseshoe
studs are chalk-fitted and glued. The wedges are then fitted to the studs,
cut not too steeply and with the end of the wedge lightly touching the
plate. Using clamps B or C the crack is clamped dry and the clamp and
wedge pressure is adjusted until the arching continues smoothly across the
crack. The position of each wedge is marked before the clamps and wedges
are removed.After the crack has been warmed the glue is applied and the
clamps and wedges reassembled. Since the hot glue will have swelled the
wood, the set-up needs readjusting by loosening, tightening or
repositioning the clamps and wedges.
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 photo
5 |
Time-efficient method (photo 5)
Simple spruce cubes are glued on either side of the
cleaned crack without prior chalk-fitting. Shorter, steeper wedges are
prepared en masse rather than fitted to the individual pair of studs, the
principle being that the swelling of the wood changes the balance too much
to make it worthwhile fitting them in advance. Clamps B or C are used. The
set-up is balanced during glueing by simply pushing the wedges as far in
between the studs as the crack requires, with- out touching the plate.
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When necessary, the wedges can be
used to help push the plates level. An angle cut on the tip of the wedge
allows it to touch only one side of the crack (figure 5).
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figure 5 angling the wedge can
help push the plates level |
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 photo6 |
Clamping with levelling pressure
In general it is preferable to glue a crack
without any fixtures on the outside of the plate, since they will compress
the ridge that forms along the crack as the wood swells during glueing.
This can cause the same
However,
if the crack wants to move move badly out of position,
it will sometimes be necessary to keep the plates level with pressure from
top and bottom. You can do that with the help of either fitted
counterparts or a plaster
The
method is described below using a plaster cast, although the same
principle applies when counterparts are used (see 'Counterparts', below).
cast of the plate.
1.
Standard method
Simple
spruce studs are chalk-fitted as close as e
possible to either side of the crack and glued with hide glue -
care must be taken that no glue goes into the crack. When the glue is dry
the top of studs are pared level with each other and the two sides of the
crack are brought together with clamps A, B or C. To level the plates a
clamp is secured on top of the studs, with a softly padded clamping block
between clamp and stud to ensure even pressure (photo 6). The spaces
between the pairs of studs are also clamped, with fitted, padded
counterparts on the inside of the plate.
The
cast and plate are then warmed and the crack glued - preferably out of the
cast. The plate is then put back in the cast, possibly with the help of
location pins, and the clamps assembled. If the tension on the crack is
not too big, all clamps can be removed between 1/2 hour and 1 hour after
glueing. Next, a little hot water or thin glue is applied to the outside
of the crack with a cotton bud, to help swell the previously compressed
wood at the crack. Since the levelling clamp and cast (or counterpart)
takes care of the angle at which the plates are glued, a wedge
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 photo
7 |
2. Sliding cracks (photo 7)
For cracks with a
particularly strong tendency to 'slide away' there is a more sophisticated
clamping system. A large spruce stud (approx.l5x20mm) is carefully
chalk-fitted across the crack. This is then cut into two interlocking
parts using a fretsaw and glued to the plate only at the back part of each
side. This double stud will ensure that the plate will be glued in
register, since neither side of the crack can go higher than intended. The
crack can now be glued with the parallel clamp pressing it together and
the levelling clamp going directly on top of the double stud
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 photo8 |
3.
Widely spaced clamps In some cases you might need longer studs to bring a
crack of exceptional tension together, or you might want to fit the studs
further apart to apply a continu- ous fitted counterpart along the crack.
In such cases none of the described clamps will have a sufficient span. An
easy way round the problem is to measure the outside distance of your
studs, draw it on a piece of 6mm plywood and quickly rough out a clamping
frame on the bandsaw. Using this jig, any kind of clamp will get the
pressure right down to the plate, though it should only be used in
conjunction with a cast (photo 8). The plate might also need to be clamped
into the cast outside the stud area to make sure it does not lift up. |
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Hints and tips
- If a crack wants to slide out of register, a slightly angled clamp
can help pull one side up (see left)
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- A similar effect can happen accidentally if you are using clamp B,
since one side can clamp lower than the other
- Position the studs so that the endgrain surfaces of corresponding
studs face each other. This will minimise the effect of a swelling
stud on the adjustment while glueing.
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To prevent the clamp from digging into the endgrain of the stud, you can
heavily gluesize the parts concerned.
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If you use spruce for studs you will be able to split off the studs above
the glueing surface and reuse them
- For an average crack - if such a thing
exists! - positioning a pair of studs every 30-40mm will work well. This
will create enough space between for fingers or clamps to help levelling.
For tougher cracks you can increase the frequency or for easy ones, lower
it.
- Do
not exert more clamping pressure than is absolutely necessary to close the
crack.
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It is very important that you leave the integrity of the inside surfaces
of the plate intact. A lot of restorers maintain that, to make it easier
to soak the stud off cleanly, it is a good idea to glue a piece of paper
between stud and plate. In reality, though, most of them glue their studs
directly on the plate. To remove the studs they split them off above the
glueing surface and soak and wash the remains off with water. If this is
done carefully the plate surface will not suffer.
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COUNTERPARTS
Fitted
counterparts, or clamping blocks, on the outside of the plate allow you to
apply levelling pressure without using a cast. They can be made out of
hardwood • at roughly the same size as the counterpart or pair of studs
on the inside of the plate and should be lined with thin plastic. Fitting
the counterparts to the arching is most easily done using carbon paper.
This method has the advantage that you can check the crack from the
outside between the counterparts.
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SHARING THEIR IDEAS
A number of London-based
restorers and repairers contributed their expertise to this article, John
Dilworth and Michael Byrd both worked at J.&A. Beare before setting up
their own making and restoration workshops. Florian Leonhard came through
the restoration workshops of the Hills, Machold and Withers before
establishing his own business. Another former Withers employee, Paul
Gosling, now works at Phelps. Annette Farjado was g
employed by Michael Becker of Chicago and World of Basses in
Germany before joining the shop of Peter Biddulph. The current staff of
J.&A. Beare also provided suggestions
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