Westward Group Abrasive Tools sale in Paris: Amateur Telescope making
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:
Most of the disatvantages can be eliminated by taking precautions, while the advantages are tremendous!
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.