8" Mirror Polishing, Part 1: The Power of Teeth
Previously, @Beasmeeply generously donated me an 8" mirror blank, kick-starting an attempt to finish grinding it and make an 8" telescope. There are four stages of mirror grinding, so to figure out which step I needed to start with, I put the mirror into a Foucault mirror tester, and it gave a smooth-ish image, telling me BeasMeeply had gotten through the first two stages of mirror making, rough and fine grinding, and it was ready to polish.
The next step is polishing, and to do that, I need a pitch lap! A pitch lap is a disk the same size as a mirror with a layer of pitch poured onto it and used as a grinding surface.
Step 1: Make a disk.
Nowadays the cheapest way to do it is with waterproof plaster called dental stone. A local astronomy club had some and was willing to mix up some so I could use it… but they only had a paper bowl to mix it up in, we used too little, and the result was a thin pancake that cracked easily. I needed more stone and a thicker tool.
I looked online for where to buy dental stone. Assuming you don’t want to buy a 25 lb bulk box ( I only need 1lb), it’s very hard to find anywhere but the slightly expensive firsthanddiscovery.com .
And then: by chance, I had a pre-scheduled dentist appointment. During the appointment, I asked nicely, and they sent me home with two ziplock bags full of dental stone!
A trip to Goodwill got me a big plastic jug for mixing lots of dental stone in, and which I was okay ruining with bits of hardened stone. (The flexible plastic walls ended up being convenient for cleanup later - bending them meant large bits of hardened-on plaster cracked off!)
I got help online with the water-to-powder ratio 30 ml water/100 g powder, grabbed one of my failed wooden carvings from the meniscus mirror project to contain any spills, and got mixing. Dental stone hardened so quickly - within two minutes it was noticably hard to mix. I thought I had messed up the water-to-powder ratio because it felt too dry, so I added more water and then it was suddenly very runny. When I dipped a plastic fork in it, it didn’t drip off the fork, however.
Worried that if I waited any longer it would harden, I poured the plaster (using my mirror blank as the mold with a layer of plastic wrap on top). The dental stone was already so viscous it didn’t smooth itself out to a level surface, giving me an uneven back.
Then I had to wait a week for it to dry before I could turn the stone disk into a proper grinding tool!