I've got some porcelain insulators that I picked up near a hydroelectric dam in south Georgia and it knaps very well, much better than toilet porcelain. The toilet porcelain I've worked tended to be tougher and grainier, making clean flake removal difficult. Also, it seems easier to break. The insulator porcelain I worked was black on the outside, but was snow-white on the inside and had a much finer texture. I really like it! I've made several points out of it as well as one knife blade. It can be a somewhat brittle, so prep your edges well and grind them heavily, otherwise the edge can collapse. Enjoy that porcelain!