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


Hi Stan/All

Is the following technique worth considering ..... ?

Instead of melting a number of glass chunks, stand a glass rod vertically
in the centre of the mold. As it melts, it will (hopefully) flow radially
outwards in the mold and fill it.

The intention is that it won't trap air bubbles as it spreads out.

Problems / comments:

* You need to start with a rod that has enough volume of
glass to fill the mold - It may not fit in the kiln.

* You could feed it in through the peep-hole !!!

* It may start to melt a long way above the mold and
drop into it like honey off a spoon.

* Bubbles should not be a problem if they are far away
from the surface. Perhaps you could use a combination of your technique and
that described above to obtain a blank that has a bubble free top layer.

As I mentioned in a post a long time ago, myself and a few friends
investigated making our own blanks. We had access to a kiln and all the
right equipment but when we did our sums, it was much less expensive to buy
a blank.

However, I am still VERY interested in your endeavours.....

please keep us posted

Cheers

Frank Q

----- Original Message -----
From: "Stan Kohut" 
To: "Andreas Derwahl" 
Cc: "aaatm" 
Sent: Wednesday, December 11, 2002 11:25 PM Subject: Re: ATM casting a
10" pyrex blank - rfc


>
> 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