8" Mirror Polishing, Part 2: Curveball Pitch
I’m making a tool called a pitch lap to grind an 8" mirror. Previously, I discovered the best way to find dental stone is a dentist, and made a yellow plaster disk.
Pitch lap step 2: Pour the pitch!
Pitch is a weird material. It’s a liquid so viscous it looks like a solid. At high temperature it’ll pour like honey, at low temperature it’ll act solid but very very slowly flow. When grinding a mirror, we use pitch so it’ll keep flowing by a few nanometers and keep touching and grinding the mirror even as the mirror’s shape changes.
Hot pitch sticks to anything. Cold pitch cracks and sprays tiny dust crumbs everywhere. It’s annoying to work with. I bought a sacrificial pan from goodwill for $5 to hold the pitch, and I’m glad I did because that pan is never getting the pitch out.
The individual steps themselves were pretty simple: first heat up the pitch (this took hours to heat it evenly up to 150F), then pour it on top of your tool, then press the mirror onto your pitch so the top surface becomes shaped like your mirror.
Before that I had to cut away the edges of my dental stone tool with a razor blade, which I’d never done before and my slowness made the local astronomy club impatient.
The very very final step is to cut lines into the plaster so it can flow easier. Normally this is done right after pouring… but I had to leave for something time sensitive, and that meant the 99% complete pitch lap sat in a box for a month.
A month later, I cut some channels using a razor blade (the others made it look so easy!), warmed up the pitch by putting it in warm water, and then pressed the mirror face onto it. A bag of onions helped create tiny channels called “microfacets” so the pitch can flow easier.
Pitch lap complete! All I need is some cerium oxide polishing agent and a way to test my mirror, and I’m ready to start polishing.