Re: [ATM] RE:anealing

["David Harbour" <scarab2@cox.net>]

Ken, all:

Some of my friends and I made some very creditable mirrors out of industrial
sight fluid flow glasses; about three quarters of an inch thick and nearly
seven inches in diameter. We got them for nothing. They were FULL of strain;
I broke one after getting it perfectly spherical. Put the largest piece back
on the stand and tested it; figure was not even approximately spherical. (I
put a "glass catcher" on my mirror cradle on my bench after that episode).

The others showed the planets and Lunar detail exquisitely at over 300X; but
the temperature was not changing. We did not parabolize them; left them
around f/11 or a little longer. The one I broke broke into three main
pieces, in descending order of size. I have made a survey of guys who have
had their mirrors slip off the stand when they have turned their backs on
them, or gone in for a phone call, or just dropped them like I did; the
majority of them reported my experience: three major pieces, in descending
order, and then the chips. One of my friends reported going in to take a
call, and his 16" blank was on the garage floor when he got back, in three
pieces, in descending order of size, and assorted chips. He had it polished
and close to spherical.

I just laughed at myself for being such a careless fool. Hey- it's just for
"killing time" anyway, according to one of the authors of the Albert Ingalls
ATM books. Got another one and started over (long time ago when I was
newbie).

A. A. Common made a successful 36" plate glass mirror that held its figure
unless temperature was changing- it was so full of strain that when it was
cored for a Cassegrain, very slowly and very carefully, it shattered.
(Article did not say how many pieces, or sizes, of the sherds).

Dave


----- Original Message -----
From: "Ken Hunter" <atm_ken_hunter@yahoo.com>
To: "Ken Lowther" <hermit@outofoptions.org>; <atm@atmlist.net>
Sent: Sunday, May 22, 2005 11:21 PM
Subject: Re: [ATM] RE:anealing


> True... But the original PYREX doesn't grow on trees
> either. It has to be made from original component
> elements, melted, held in the melt stage long enough
> for the bubbles to rise and is poured into large
> sheets before entering the annealing phase. The
> remaining glass, once cooled, is broken out of the
> casting pots and sold to the manufacturers of our
> glass blanks, jars, cooking utensils etc..
>
> The old Pyrex cast blanks from Edmunds etc skipped the
> pulverizing stage and just used broken pieces or
> ingots of Pyrex glass as described above that were
> remelted, stirred, poured into molds and cooled (not
> necessarily annealed). They exhibited rather large
> amounts of striations, bubbles, seeds and other unpaid
> for goodies.
>
> Ken Hunter
>
>
> --- Ken Lowther <hermit@outofoptions.org> wrote:
> >
> > Hmm... And I was told that would lead to bubbles.
> >
> > Ken Not Hunter
> >
> > Quoting Ken Hunter <atm_ken_hunter@yahoo.com>:
> >
> > >> From what I heard and seen at Kitt Peak years ago
> > (and
> > > probably severely outdated by now), the best way
> > to
> > > remelt PYREX is to pulverize it and mix before
> > > remelting to get consistant glass. Mixing in the
> > melt
> > > stage is very difficult due to the viscosity (and
> > of
> > > course the temperature) of the molten mass.
> > >
> > > Ken Hunter
> > >
> > >
> > > --- Mel Bartels <mbartels@bbastrodesigns.com>
> > wrote:
> > >> > Hi Mel, can you please explain the cause of the
> > >> remaining strains
> > >> > to fill a gap in my knowledge of glass  ;-)
> > >> >
> > >> > For a guess i would think inhomogeneity from
> > poor
> > >> mixing,
> > >> > anything else ?
> > >>
> > >> That's what comes to my mind, also.  At the Univ
> > of
> > >> Ariz Steward Obs Mirror
> > >> Lab, they check each piece of 'pyrex' from OHara
> > for
> > >> index, and if out of
> > >> bounds, that piece is rejected from that melt and
> > >> spin.  I don't know if
> > >> pyrex can be thoroughly mixed.  From my
> > >> understanding, it's more an issue of
> > >> finding consistent blocks of pyrex from the
> > initial
> > >> casting.
> > >>
> > >> Mel Bartels
> > >>
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