For the first time in this project, I went to a class with no progress from the previous session. Thanks to a busy study schedule, I’d managed to do absolutely no work whatsoever on my mirror. A side effect of the week’s inactivity was that I found the work more strenuous than usual – I haven’t had such back pain after an afternoon’s grinding since my very first day. I either need to spend more time grinding glass, or get some proper exercise.
By the end of the day, about two hours grinding, I’d brought the mirror down from f/8 to f/7.5, which is… well I’d hoped for faster work, but haven’t really got anything to complain about. Two hours out of an entire week is not very much work at all. Still, the good news is that a few more hours at home and I’ll be getting very close to my target focal ratio. Once I’m near my target, I can switch to the next finest level of grit – #240 – and I’ve brought a supply home in preparation.
But there’s one thing I must do before any more grinding: Sand down a new bevel. The old one is almost completely worn away, and is maybe a fifth of a millimeter thick. Not good enough. As before, there’s a risk that a sharp edge could splinter or chip during grinding, which would leave tiny shards of glass between the mirror and the tool, which in turn would leave huge scratches. So one hour of careful work with fine waterproof sandpaper on a block against the edges of both pieces of glass will prevent that problem.