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.
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
Mold preparation and cello rib doubling
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,