There is a cheaper alternative to Cerrosafe for casting chambers, and that is sulfur. It will melt at boiling water temperatures, so can be melted in a home made double boiler consisting of a tin can in a larger can or in a regular cooking pan. The addition of a little bit of lamp black helps make it easier to read measurements from it. I haven't looked for sulfur recently, but used to get it from the local drug store. I first used it to identify a Remington rolling block rifle that was supposed to be in .45-70, but turned out to be in an obsolete Danish caliber.
