To make a mirror, the sheet glass needs to be in your inventory, not in the bench. The temperature of the bench must be higher than 5000, or the silver powder will blow away and be lost. There doesn't seem to be any upper limit to the heat, as it works even with 50000+ temperature.
It takes about 30 seconds to make each mirror.
- 6 used in a Mutagenics Lab.
- 3 used to upgrade an Acoustics Laboratory with a reflectory.
- 4 used in a Funerary Temple.
- Various amounts used in a Metallic Obelisk.
- 10 needed for Crystal Acoustics research.
- 16 needed for Obelisk Construction 2 research.
- 50 needed for Mutagenics research.
Historically, these test buildings also required Mirrors: