Re: [ATM] convex secondary

["Steven L. Dodds" <steve@pcmw.net>]

----- Original Message ----- 
From: "Joe" <nss@mwt.net>
To: "Ken Hunter" <atm_ken_hunter@yahoo.com>; <atm@atmlist.net>
Sent: Saturday, March 12, 2005 9:05 PM
Subject: Re: [ATM] convex secondary


>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


The old Corning blanks with the shiny sides and the ridge on the bottom were 
molded in saucepan molds. Pyrex Sauce pans were made by dropping a glob of 
molten Pyrex in the mold then pressing it with the top mold.  Telescope 
blanks were made by dropping a double glob of Pyrex in the bottom mold and 
letting it sit until cool.  The molten Pyrex without pressing would fold 
over on itself creating creases and voids, hence the poor quality.  It is 
also the reason for the standard blank sizes.  4.25" 6" 8" 10" 12.5".. It 
was the size of saucepans that Corning sold.

There is your piece of trivia for the day.

Steve



Steven L. Dodds
Nova Optical Systems
http://www.nova-optical.com
>
> 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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