On Thursday, April 30, 2015 at 3:25:46 PM UTC-7, wrote:
Not exactly metalworking. I needed to make a replacement glass piece
...for proper focusing. I used a 1mm glass slide, the type made
for microscopes. The piece needed to be 8 x 9 mm. I mamaged to get one
and then discovered I had scratched it. So I tried again and was
rewarded with a scratch free piece. I was surprised how hard it was to
cut this glass, for it to break where I wanted it to

The usual scheme, is to rotate a brass or copper cylinder tool, painted with
abrasive slurry, and trepan out a disk. A kind of hotmelt glue (jeweler's wax,
or dop adhesive) holds the work. It's slow, because the glass will crack
if you allow the cut to heat it too much.
I've done it with a tinning swab, a few drops of glycerine, and SiC grit.