Q: What are Scotch stones, and what are they good for?
A: Scotch stones are a natural abrasive stone, a type of slate, I think, from an area in Scotland (hence the name). They’re also called “Water of Ayr” stones, and the main use of the material is for making blocks used for sharpening knives and tools. Small shaped pieces of Scotch stone are also held in the hand, as with the types of various pencil stones and slips commonly used by tool and die makers, and are used to smooth metal areas and surfaces by rubbing. They have a number of advantages over powered abrasives, in that, when used with water, they wear down somewhat rapidly, so the point in use conforms to the surface of the metal being treated, thus being much less likely to leave swirls and drag lines such as one might get if trying to get into details with a rotary tool. As an abrasive, these things, while rigid and somewhat brittle, are fairly soft and gentle to the metal, wearing away, as I noted, fairly quickly in use.
That doesn’t mean their action is particularly slow, however. You can smooth tool marks and irregularities (such as solder scars, file marks, etc) on a surface remarkably fast with them, and with practice, get a surface that’s almost ready for a bit of rouge, as the stones have a very fine grain, and leave a surface finish that I’d compare to about what a 400 to 600 grit emery paper will leave. They are especially useful for getting into things like square corners, or corners and seams, recesses, or other blind or hard to reach surfaces in general, where the linear motion they facilitate conforms to the surfaces better than a rotary point might do.
They generally are available in a “stick” shape, usually four to six inches long, in shapes ranging from 1/8 inch square on up. The largest ones I’ve got are 1/2 inch square, but I’m pretty sure they can be had larger than that. I’ve used them to good effect in finishing virtually all of the jewelry metals, including silver, gold, and platinum. I don’t think I ever tried them on titanium, but I’d assume they’d have some use there, too, though perhaps they’d work slower.