From: "Stan Kohut" 
To: "Andreas Derwahl" 
Cc: "aaatm" 
Reply-To: "Stan Kohut" 


Hi Andreas and Everyone !
I have so-far made 3 attempts at melting glass into something that could be
used as a blank using a Plaster-sand mold and a graphite mold. I have made
2 pages on the first couple of attempts:
http://www.geocities.com/natsp2000/casting-glass.html
http://www.geocities.com/natsp2000/casting-2.html
The biggest problem I have run into are the bubbles that get trapped in the
molten glass.
My first 2 castings used large chunks of broken 1/2" plate glass and
had a moderate amount of bubbles, the third attempt used 1/8" plate
glass that was broken into small pieces and resulted in considerably more
bubbles, as far as I can tell, the larger the chunks the better. I believe
the graphite mold burns off at these high temperatures and adds to the
bubble problem as the mold has been reduced in size about 1/8" in two
firings, on the last attempt I had two large bubbles that were streaked
with a brown color that I believe is from the mold. Devritification occurs
between 1100*f -1400*f so you need to melt at about 2100*f
(for borsilicate) for about 4-6 hours and then crash cool back down to about 1000*f
Here is a webpage on annealing with a chart:
http://www.glassnotes.com/Simple%20Annealing.html
 I would worry about trying to get a good melt and use a simple annealing
scheme, you can always go back and fine anneal it later if you come out
with something decent enough to anneal.
Good luck !
Stan



----- Original Message -----
From: "Andreas Derwahl" 
To: "Atm-US (E-mail)" 
Sent: Monday, December 09, 2002 9:02 PM Subject: ATM casting a 10"
pyrex blank - rfc


|
| 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
|
|
|
|
|

--- BBBS/NT v4.00 MP