The term "soundpost crack" is not a reference of the soundpost itself cracking, but is a reference to a crack that has formed in the belly or the back, around the soundpost area, in a cello or violin.
This tends to happen when an instrument is dropped, or when the instrument case is too small and, in closing the case, the instrument gets squeezed. It can also happen if the case is closed while the shoulder rest is still attached to the violin, or when someone inexperienced moves the soundpost.
After a crack develops, it will tend to get worse, and likely eventuate in the instrument becoming unplayable.
As regards repairing ordinary instrument cracks, the process requires that the instrument be opened, the crack glued, that special tabs/studs be made and fitted to the inner surface of the cracked area.
However, as regards repairing a soundpost crack, given the huge pressure of the bridge and soundpost, the cracked area needs to be strengthened as opposed to simply gluing the crack and fitting tabs. The strengthening process requires that the cracked area be "shaved" from the inside and the shaved area replaced with a small wooden patch (which needs to be perfectly crafted and fitted). Whilst this work is undertaken, the plate needs to be supported in a manner that will prevent it becoming warped so as to keep the original arching shape of the instrument. To address this, after re-gluing the crack, a plaster cast of the outside of the plate needs to be made. The cast on the outside surface means that, as the wood is being shaved from the inside leaving a thin and delicate layer of the original wood, the outside is protected from becoming deformed or damaged. At this stage, the plate is ready for the soundpost patch to be fitted.
Here are the pictures of the process of making a soundpost patch in the belly of a violin.
This repair was done by Sylvie Raynaud at Le Violon Rouge NZ
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