Re: [ATM] The results of my 1st 16" casting attempt.

["Thomas Janstrom" <thomas@moiler.com>]

Hi Oscar,

Thank you!

The black stuff is scale from the stainless steel ring that I had around the
outside of the mold as a safety feature, it (the scale, not the ring) was
laying loosely on the surface, I vacuumed it up before I loaded the extra
glass. 

The glass is fully fused, what you can see is trapped alumina dust from the
kiln wash I used to coat the mold to stop the glass sticking to the fibre
material (I now know that this wasn't required, one lesson learned!) that is
trapped in the spaces that were the contact points of the chunks used (so in
a way you are right), as I re-melt the glass this will float to the surface
fully (at present it is trapped in the top 4mm of the glass), btw I'm not
the first to re-melt a cast mirror of this type; the first 8.4m mirror for
the ESO observatory in Chile had a leak during casting and they "lost" three
tonnes of glass into the kiln. After the first casting run was finished they
measured the face plate thickness and found it to be just passable at the
outer edge, but to be only 9mm thick near the central perforation, which was
only 1/4 what it was supposed to be! So they loaded a further two tonnes of
glass and recast the beast, only took three months!

And yes, Pyrex is rather pricy over here in Australia, like AU$50 per kg for
E6 borosilicate glass, Schott Duran/Borofloat33 is only $45/kg! There is no
good reason for this other than this being Australia as far as I can see.

I did think about using a boron salt to aid with reducing devitrification,
but I don't know how this will affect the bulk of the casting, so I'll leave
that alone til I have some time to do some small scale experiments, again.

Cheers, Thomas.

-----Original Message-----
From: Oscar González Regueira [mailto:ogry@oculares.com] 
Sent: Tuesday, 27 November 2007 7:41 PM
To: thomas@moiler.com
Subject: RE: [ATM] The results of my 1st 16" casting attempt.

Hi, Thomas

Congratulations, very promising results, even if not fully satisfactory.

The very first impression is that some black particles (dust) had polluted
the surface. Are into glass or simply over the glass?

The pieces from what you have started are imposed in the final piece. One
can recognize each single piece of glass! It seems that is needed a lot more
of time/temperature to get a full fusion. May be time to experiment with
chemical aids (fondants, as boron salts)? In a old book I found that you can
use a wet wood piece to remove the melting without problems. The wood fires
and vanishes clean, but the core persists until the work has been deed.

Did you say to use steel for the mould? In my knowledge (from lectures of
diverse books, but not actual expertise), is better to get ride of Fe in the
Kiln. It can vaporize some material and ruins the process.

Did you mention to reuse the same glass adding 200gr? Did you think that
this is a good idea? May be the borosilicate is expensive at your location,
at mine is 0,5€/kg.





Saludos,      OSCAR.

> -----Mensaje original-----
> De: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] En nombre de
> Thomas Janstrom
> Enviado el: martes, 27 de noviembre de 2007 8:03
> Para: atm@atmlist.net; atm_free@yahoogroups.com;
> glassblowing_topics@yahoogroups.com; Mirror-O-Matic@yahoogroups.com;
> gingery_machines@yahoogroups.com
> Asunto: [ATM] The results of my 1st 16" casting attempt.
> 
> Hi All,
> 
> 
> 
> OK the kiln is cold and I have taken a look at the blank, as cast I'm not
> over the moon about the results, but it wasn't a failure by any means
> either!
> 
> 
> 
> I have uploaded the results here:
> http://tjanstrom.com/16inchmirrorproject.htm they are towards the bottom
> of
> the page, and I decided to re-melt the blank and try to correct some of
> the
> defects I found. One thing I'll note, which I'm REALLY happy about, is
> that
> there are NO bubbles in the blank! So I'm back to waiting, it will be 13
> days till I'll know if the corrective action I have taken has worked or
> has
> cost me a kiln!
> 
> 
> 
> Cheers, Thomas.
> 
> 
> 
> PS. As per usual comments suggestions etc are always welcome!
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/

_______________________________________________
ATM mailing list http://www.atmlist.net/