For grinding my 6" mirror, I used a Tile on Plaster Tool . instead of the second 6" glass that I got with my grinding kit.
Using that kind of tool offers several advantages, despite the little effort in making the tool. Since I never ground a mirror on a glass tool myself, I will give a short summary of what people say, who tried both:
• You can save your second blank to make another mirror.
• The channels in the tool minimize the danger of seizing.
• You can grind faster and more efficiently by using less abrasives.
• For big mirrors, the weight of the tool can be reduced.
• Even distribution of the abrasive across the tool.
• No trapped air-bubbles.
• You have to make the tool yourself, which might be bit of an effort the first time.
• Cleaning the tool is more difficult.
• Unbeveled tile edges are a potential danger for scratching the mirror.
• Plaster or cement particles coming off the tool can cause scratches.
Most of the disatvantages can be eliminated by taking precautions, while the advantages are tremendous!
• Dental plaster; or plaster of paris; or any kind of cement preferably without sand and as fine as possible. Dental plaster cures very fast, which makes it the best choice.
• Slow (2-4 hours applying time) curing epoxy glue. I made good experiences with white Porcelain epoxy. Of course it should be waterproof.
• Porcelain tiles cut in small squares (~2,5 cm) . Porcelain is best, because it is very hard. If you can only find big tiles, then cut them yourself with a tile cutter.
• Masking tape and aluminium foil.
• Carbo grinding stone. The kind that you use to sharpen carpentry tools. (Buy a cheap one!)
First you cover the mirror face with thin plastic kitchen foil and create a mould by wrapping aluminium foil and masking tape around the blank. The side which later on will become the mirror face looks up. Take care that there are no air bubbles between the plastic foil and the mirror.
To avoid that the plaster leaks out while pouring, you should put masking tape around the plastic foil-covered blank before attaching the aluminum-foil. On the upper side of the glass, let the tape overlap 1 cm and fold it down, so you get a sticking outer rim. On this rim you can easily attach the aluminum foil strip. To get a firm foil strip of the required mould broadness (min. 1 1/2 blank thickness), you can fold a length of foil until it matches the size.