RE: ATM casting a 10" pyrex blank - rfc

["Andreas Derwahl" <A.Derwahl@massey.ac.nz>]

Hi Stan and everyone,

I've read your webpage, sounds like a good idea to melt, rough anneal, have
a look and then fine anneal if suitable blanks have been produced. That
could save a lot of time (unless the rough anneal leads to shattering the
glass:-)).

I'm getting a much clearer picture now, thanks to everyone for on and
offlist comments.
If there's interest I'll keep the list posted on the outcome.

Cheers,
Andreas



> 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