From: 
To: "Andreas Derwahl" 
CC: atm{at}shore.net
Reply-To: 


Lower your soak temp to about the 900'C-1100'C mark, you want the glass to
flow thats all, remember the plaster will out gas a lot. See my site for
the effects of this out gassing. http://www.tjanstrom.com/glass1.html

Consequently I'd suggest a graphite mold.

Crush the glass as fine as you can with out making dust out of it, as this
will blend the different types as much as possible with out the need to
stir the melt.

As for your annealling schedual I'm no expert there are more knowlegable
persons on the list, Richard please weigh in here;)

Well that's all I can contribute at this point.

Thomas.
www.tjanstrom.com
>
> From: "Andreas Derwahl" 
> Subject: ATM casting a 10" pyrex blank - rfc
> Date: 10/12/2002 13:02:39
> To: "Atm-US \(E-mail\)" 
>
>
> Hello everyone,
>
> I'm a newbie to atm-ing and read this list for a couple of months until the
> email load got too much to handle. Now I signed up again because I am
> thinking of a way to produce a cheap blank. I have access to a lot of broken
> labware glass (pyrex, duran etc. - most if not all will be borosilicate
> glass) I would like to try and cast a 10" blank. I plan on
building a 10" f5
> dobson as a portable scope and have a very limited budget, hence the thought
> of using the availiable resources (including a glassblowers oven).
>
> I've searched the archives and found some recommendations and annealing
> schemes so I came up with this plan and questions:
>
> - mould from  1:1 plaster of paris and silica
>
> - this I want to fill with the crushed glass. I'm not sure, though, if I
> should leave the glass pieces as big as possible or crush them into smaller
> pieces. I assume I will get less bubbles and a more homogenous blank if I
> use larger pieces, but since the glass is mostly curved (beakers, flasks and
> erlenmeyers), it might be difficult to fill the mould high enough.
>
> - heating to 1400 to 1500 deg C and holding for 6 hours. Am I to expect
> devitrification over this period?
>
> - lowering the temperature to annealing point 565 deg C and holding for ca.
> 15 h (overnight). I understand the cooling rate at this stage is not crucial
> (except possible devitrification)?
>
> - lowering the temperature to 370 deg C at 0.04 deg C/min (i.e. over 3.5
> days). I changed this from the recommended 0.01 deg C/min, which was said to
> be a very conservative rate. Can I do this or is 0.04 deg C/min too fast (or
> could I even increase the rate)?
>
> - lowering the temperature to 320 deg C at 0.055 deg C/min (i.e. over 15 h,
> overnight). Again I changed the cooling rate from 0.02 to 0.055 deg C/min.
>
> - and finally lowering the temperature to 60 deg C at 0.14 deg C/min.
>
> That would mean I need about a week for the whole process.
>
> As I said, I'm a newbie and would very much like your comments on this plan.
>
> Cheers,
> Andreas Derwahl
>
>
>
>
>
>

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