Tool marks left by the old masters are always fascinating to me and I keep looking for them trying to interpret the type of tool which created them, the way of handling it (direction and depth of the cuts for example) which may at the end give me a clue about the overall working method.
On this Stradivari violin from the makers golden period some interesting scratches can be seen, mainly on the pegbox walls. My assumption is that they are the result of a very strong and agressive abrasive, possibly sharkskin.
For a while now I have been useing dried skarkskin on my own work, experimenting how deep the marks need to be in the white wood to be still apparent after the whole varnishing process. Most of the scratches disappear during wetting and scarping the wood- the first step which is done to the raw wood in order to let it swell up, raising the grain a few times before putting the ground and varnish on to have better control over the final surface texture.
If you´re interested in how sharkskin can be prepared from fish to finish please visit this blog by Charline Dequincey, a colleague in Canada who presents a detailed photo essay on the subject. Shark skin