When a top crack passes over the sound post area it is the usual procedure to inlay a patch on the inside so that the pressure from the soundpost will not reopen the crack. In some circumstances we just glue the crack and hope it holds, as would be the procedure on an inexpensive instrument. This instrument is a 1927 violin by Kurt Gutter, one of the better Markneukirchen makers.
First, the crack is cleaned out and glued (not illustrated). With this violin the more challenging job was to glue the other crack, about a centimeter to the left of the soundpost crack. Some “professional” glued it with epoxy. It was lots of fun cleaning that one out, but back to the sounpost crack.
Click on photos to enlarge.
In order to have a surface upon which to clamp, a perfect counterpart must be made. This is easier than you might think. First, a piece of 3 cm. thick pine or basswood is roughly cut to the shape of the violin between the c-bouts. It is hollowed out to very roughly match the arching, and put aside. The violin is then covered with aluminum foil–you’ll see why next.
A generous amount of Bondo autobody filler is mixed and deposited on the violin (the reason for the foil) completely covering the area between the c’s. Then the piece of wood is placed over the Bondo so that it oozes out, covering the area to be clamped. The Bondo cures very quickly, and you have to watch the temperature as it is an exothermic reaction. Usually the mold is hard enough to remove before it gets really hot. Within an hour, it has cooled and can be used. The foil is removed from the violin top and the sticky stuff that held it on is washed off. The Bondo counterpart is corrected, if necessary, and lined with thin plastic wrap. The violin top is clamped in place, with the camp handles acting as support legs, allowing access below the counterpart.
A template is used to lay out the area to be excavated. A gouge and scrapers are used to create a uniform trough to within .5 mm of the exterior of the top. That’s why you need a solid counterpart. The photo at left shows how thin the hollowed-out area is by how much light it will pass.
Now the new piece of spruce is prepared. It is split from a seasoned block that matches the grain characteristics of the top. The outside diameter is cut using the same template that was used to mark the trough, but it is cut about 1 mm larger. Rough fitting is done with knives and planes; then chalk is used to attain a more precise fit. Final fitting is achieved by gluing studs around the perimeter of the trough and pressing the patch directly downward to pick up the chalk.
Once fit is achieved, the chalk is cleaned out of the patch bed and from the patch. The top 2/3 of the patch are split off, but the top piece is left in place to help spread the clamping pressure. The studs are left in place and the patch is glued with hot hide glue. After a couple of hours, the clamps are removed and the top removed from the counterpart in order to clean up any glue that has oozed out. Then the whole thing is reassembled and the clamps replaced. It is allowed to dry overnight.