We have a cello in the shop with four very thin and multiply cracked ribs, the traditional fix is to repair the cracks and then “double” the rib by laminating a second layer of new wood to the inside of the original
Structural and tonal considerations
- How to add enough wood to the rib to reinforce the repair without adding too much mass? The traditional approach is to thin the original rib to 0.5mm or less before laminating, this is destructive and not conservative. Our approach was to use a combination of silk to reinforce the cracks and then laminating only as much new maple as necessary to bring the rib up to good playing dimensions
- A repair we often see for multiply cracked ribs is to add a cross grained section of wood, often spruce, to reinforce the damaged area. There are two problems with this, firstly the cross graining significantly changes the normal bending and flexing patterns of the rib such that it will likely have a negative effect on the instrument’s tone. Secondly cross-graining is generally avoided wherever possible in woodworking because the different longitudinal to radial shrinkage rates of wood eventually leads to more cracking (this is why most cello rib cracks occur next to the corner and end blocks, the ribs shrink but the blocks shrink less causing a crack). We believed that our combination of silk for crack resistance combined with a same-grain maple laminate will give a better outcome.
Procedural and aesthetic considerations
- Traditional doubling uses wooden counter forms incorporating the outline shapes of the adjoining front and back plates. These counter forms are used to support the rib when the doubling laminate is clamped into the original rib. This method has several problems besides the laborious and fiddly (read “expensive”) process of producing the forms: using this type of counter form tends to flatten out the characteristic waves and undulations present in the old ribs, making them look unnaturally flat. Besides asthetic considerations, flattening the ribs can force repaired cracks to open again. We decided to make plaster casts of the original ribs and then use a vacuum bag for the lamination process. The resulting repair preserved the wavy look of the original old ribs
- Laminating an entire cello rib requires applying a lot of animal glue at one time, it would be difficult using traditional hot hide glue because of its quick gel time, instead we used fish glue which has a much longer working time.
Caution
When using a vacuum bag, glue and water on the inside of the rib can get sucked right through the rib by the vacuum and there is a chance, particularly with shellac varnishes, of water damaging the finish. For this reason I recommend:
- Use fish glue which is much more viscus than regular hot hide glue.
- Don’t leave the newly glued laminate in the vacuum bag longer than needed to tack the parts together, let it dry in the mold outside of the vacuum bag.
- Try small test piece on an inconspicuous part of the rib
Note: this idea of laminating maple and silk is not our original idea, it’s been used by Canadian bass maker and innovator James Ham
Taking casts from the cello ribs
The rib is shielded using 1.5 inch wide, smooth, low-tack masking tape, if you suspect that the varnish is water sensitive, add a layer of plastic packing tape. Lightly coat the tape with mineral oil for further water resistance and easy mold release.
Peter Bingen lays up a cello rib mold. Mix a small batch of plaster, wait until it starts to go off and trowel a thin (approx. 10mm) layer of plaster onto the rib. allow this to set for about 15 minutes then mix a second, larger batch of plaster, again wait for it to start going off and lay up 3-4 layers of plaster-impregnated burlap.
Using the remaining plaster trowel the outer surface of the mold smooth
When the plaster begins to heat up it’s time to remove the mold. The resulting cast is very strong, I whacked a test one down onto the workbench and while it cracked in the middle the two halves stayed in place, perfectly aligned. It is tried and true technology, I remember my father laying up molds this way with plaster and burlap, for bronze casting.
Mold preparation and cello rib doubling
While we want to preserve the natural shape of the rib some corrections are need, for example at the corner recurve the rib needs flattening so that it can be joined to the corner block. (in fact the rib never fitted the corner block properly which is part of the reason that it cracked all the way through to the end. In this repair we also have to add more wood to the curved face of the block so that rib and block can finally be united <3 )
To correct the mold high areas can be scraped down, while low areas must be filled with more plaster. Mix a small batch of the same plaster, wet the mold before applying. Scraping the new plaster while it is still green will endure a smooth transition between old and new layers. Adding a different color to each subsequent layer is a good way to make sure only the new plaster gets reshaped.
The corrected mold. Other parts of the rib will be left with its natural undulations
An internal counter form was made by laminating four layers of stiff card (check the grain direction of the card before cutting) glued with Titebond and placed in the vacuum bag with the plaster cast. The resulting counter form is quite stiff, has all the undulations of the rib and is moisture absorbent . It will be used to hold the newly laminated rib for a week or so while it dries. The plaster has been lined with a low-tack adhesive vinyl obtained from a sign printing shop.
Wooden stops were glued to the plaster and alignment marks pencilled on, these will be visible through the vacuum bag to ensure correct positioning of the rib in the mold.
Fish glue is applied to the rib, the new maple laminate and counter form wait at the side…..
… the silk is located, more glue is applied and the excess squeegeed off.
With the doubling located the whole thing goes into the vacuum bag. We left it for two hours, long enough for the layers to be sucked close and for the glue to tack up. Everything was then removed from the vacuum bag and after clamping the cardboard counter-form was clamped into place, it was set aside to dry properly
After the doubled rib is throughly dried the new maple is thinned down to give the rib normal dimensions and flexibility. Because we are relying on the silk and not the new maple to hold the cracks together, there is no need to leave the doubled rib thicker than a new cello rib.
We were working with ribs that had been removed from the cello. I think that using a plaster mold and card counter form an effective doubling could be made with the rib in place on the back of the instrument. Final clamping would be done using many traditional clamps rather than the vacuum bag,