Re: [ATM] convex secondary

[Joe <nss@mwt.net>]

I loved those old corning blanks,,  I always wondered though  what they 
used as a mold?

Could handle the temps required and not melt, and also have a polished 
surface.

Joe

Ken Hunter wrote:

>In the old days, Pyrex blanks were moulded in forms
>and were very full of in-homogenities and striations
>(SP)? These could be easily seen on most any blank
>from several y. Lately, I've not seen visible (by eye)
>defects in the Pyrex disks that are currently
>available. I would think that testing through the back
>would be possible with today's glass.
>
>Ken Hunter
>
>
>--- Greg Jones <gregj888@comcast.net> wrote:
>  
>
>> "Pyrex is not homogenous and has a lot of fine
>>grain boundaries.."
>>
>>I've read this in all the old texts too but am
>>wondering if it's true (or still true).  Has anyone
>>checked a Pyrex blank in the last 10 yrs?  Seems
>>like to be low expansion it would need to be
>>homogeneous, and how do you make it so it is not? 
>>Has to be annealed too.  I'm not saying it's any
>>better now than 40 yrs ago, just suspicious.
>>
>>It's on my "to do list" to check, but still a ways
>>from the top.
>>
>>
>>Greg Jones
>>http://home.comcast.net/~dac20/dac_001.htm
>>
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>
>
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