Pushing Glass.

The pitch tool is used with a very fine abrasive, cerium oxide, that embeds itself in the pitch and both wears away and pushes around glass molecules to smooth the surface. I often tell people that making your own telescope mirror is easy: You just put two round pieces of glass together with grit in between them and push the top one back and forth over the bottom one about a million times, and voila, you're done. 2: After the pitch has spread over the tool base, coat the mirror with cerium oxide and press it into the pitch in order to form a matching surface to the mirror's sagitta. Ring tools and tile tools have replaced glass tools, and the Ronchi test has gained ground over the Foucault test, but the general method of making mirrors is pretty much the same as ever. [Extracted from the article]