Re: [ATM] question about tiles on a beveled glass tool

[Jan Bentz <jbentz@inreach.com>]

Mark Holm wrote:

>I don't know if an Edmund tool is good enough glass that it can be used
>to make a mirror.  The important thing is the amount of strain in the
>glass.  If low enough, and if it isn't already convex from rough
>grinding, you can make a dental stone - tile tool for a modest cost,
>rough with something else and get two mirrors, one Pyrex, full thickness
>and one plate lightweight thickness.
>
>Probably the tool is good enough for a mirror, but to be sure, you would
>have to check it with crossed polarizing filters.  If it is evenly dark
>all over when viewed with crossed polarizers, one on either side of the
>glass, then it is good.  If there is a cross shaped light and dark
>pattern, or any other pronounced light and dark pattern such as
>blotches, then there is too much strain for it to be used as a mirror.
>
>
>Mark -- I've used those Edmund plate glass tools for several perforated Cass mirrors because of the weight factor.  I can tell you that the strain was not enough to affect the figure before perforating, that is as a "Newtonian" but after coring there was a slight alteration in the figure around the central hole larger than what you might expect from   the Twyman effect.  There was some strain visible in polaroids but not enough for concern but I guess enough to cause a problem in relatively thin glass.
>
       Jan Bentz

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