Hill's Space

Meniscus mirror: twyman time!

 |  2 min  |  228 words
My mirror with tiles laid on top and a row of duct tape around the edges. I mix dental stone powder with water and pour it on top, where it hardens into a tool with the same shape as the mirror's curve.
The dental stone, poured onto the mirror, hardens over time like quick setting plaster.
Top left: my mirror, after removing the new concave tool. Bottom: the freshly hardened concave tile tool made of dental stone and tiles. Top right: my old, too thin concave tool - notice the notches around the edge where crumbs kept breaking off during grinding attempts.

When grinding thin mirrors, there is something called the “twyman effect”: grinding one side adds stress that can slightly curl the mirror’s other side. It can lead to slight astigmatism in thin mirrors that disappears once you grind the back side to relieve that stress. (See quinsightspectre.com/16-25-f-3 !)

To avoid any problems, I decided to follow the wisdom and grind my meniscus mirror’s convex back through #220 grit. Plus, it would get rid of any saddle shape, which I could see existed when beginning to grind the front. I needed a concave tool to match the convex side, and while I had one already to rest the mirror on, it didn’t work well as a grinding tool because it had no tiles. The grit seems to have destroyed the soft plaster more than the hard glass, leaving a slurry of sticky yellow mud that grabbed the mirror. Tiles are useful!

I mixed and poured a new tool with ceramic tiles using 1.2L of water and 2.49 kg of dental stone; my ratio had bit too much water, I think. Since dental stone hardens at 8 minutes I way overcompensated and poured at 3 minutes, giving a nice level top. The end result: a 1in thick (probably thicker than I needed) sturdy tool I could use for grinding the back and afterwards as a stand for grinding the front.

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