A mirror is made by fusing 200 silver powder to 1 sheet glass in a glazier's bench. A Sheet Glass Fabrication skill of 7 is required to make a mirror, and it takes about 30 seconds to fuse.
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. There doesn't seem to be any upper limit to the heat, as it works even with 50000+ temperature.