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19mm

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Posted by Robert May on August 7, 2002 21:08:33 UTC

Considering that a 10:1 blank is a fine starting point, I'd consider that a 150:19mm blank to be a fine starting point. The general "standard" now is a 15:1 for working without having to risk grinding in astigmitism to a blank when using a moderate amount of care so that means that you can actually get a piece about 300mm in diameter and be able to work it. Some mirrors have been done with 20:1 and above but there you have to start looking at the depth of the curve that you are putting on the glass and slumping the glass is usually recommended in that kind of ratio of diameter to thickness.
Have fun and enjoy making a nice mirror. Don't accept anything less than the best shape when you finally figure the mirror and you will have something that you will treasure for years to come until you make a much larger mirror.

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