The 12 micron may be contaminated with larger grits but I'd first check whether you are grinding too long with that grit. With 12 micron and finer grits it is easy to keep grinding too long and thus end up with the grit and glass fusing together and making a larger clump in the almost dry interface between the tool and glass and thus you get a scratch.
Also check to see that the tool and glass have a good chamfer (doesn't need to be much but there needs to be some) and there's no chipping at the edges.
The scratches from this tend to have a fat end with several scratches very close to each other rather than a single line.
Also, a dirty workspace will also cause scratches at this step along with a tile tool that has pockets for grit to stay in. A quick wipe of spray paint will hold grit in after you have cleaned the holes out if you need to do so.
A dry tool and mirror will tend to make the scratches come out so make sure that the grit is never dry. You should get a good wipe of the grit/glass on the overlapped tool.
Also, if you are using dental stone, make sure that it's all wet inside before starting grinding or it will suck the water out of the grit.
Lastly, make sure that you have enough grit to do the job on the surface. I use watered grit when grinding and use a spot of about 2" in diameter for a 8" mirror. When the grit has settled out of the water in the bottle, the thickness of the water is about 4 times the thickness of the grit.

• fine gringing - wilgaze - July 11, 2002 - 14:30 UTC