Draping / slumping Robax glass

[adrian]

HI Folks
I've been asked to look at 'bending' some Robax heat-resistant glass over a (probably stainless-steel') former.

This is the stuff
http://www.us.schott.com/hometech/engli ... cts/robax/

Anybody know anything about this material ?
- it's claimed to be "an extremely heat-resistant, transparent ceramic fireplace glass"
- I'm going to fire some in the kiln and see what happens, but I'm wondering if anybody else has been here before ?

Thanks
Adrian

[Alexis Dinno]

Have you written the company to see if they will provide information about firing?

[adrian]

HI Alexis

Thanks for your reply.
Yes - I did contact them - but they've not replied yet.

I've been running some test - and managed to get the beginnings of a slump
(perhaps a few millimetres across an 8-inch length)
firing at 880c for 15 minutes - don;t really want to go much hotter than that
with my new kiln - so shall try another firing taking it slower, and maybe with 60 minutes at the process temperature...

Thanks
Adrian

[Tony Smith]

A possible outcome is that the glass wll no longer be heat-resistant. If you are depending on its heat-resistant properties, reheating and annealing without a proper (and perhaps proprietary) schedule may make the glass behave like "normal" glass.

Tony

[Bert Weiss]

A possible outcome is that the glass wll no longer be heat-resistant. If you are depending on its heat-resistant properties, reheating and annealing without a proper (and perhaps proprietary) schedule may make the glass behave like "normal" glass.

Tony

Tony, what makes you say that? Isn't this glass simply low expansion borosilicate glass? If so, it is what it is. Low expansion glasses are easier to anneal. That said, I have no clue what temperature one would anneal at. I suspect it is quite high, as anneal soak is about 100ºF below the temperature when it will first begin to slump.

In the olden days, pyrex was low expansion borosilicate glass. These days they use soda lime glass and heat strengthen it (tempering). With this glass, you would surely lose the strengthened qualities once it is heated above the strain point.

[Tony Smith]

Sorry Bert. I've never heard of Robax before. There are lots of glasses that are made "heat resistant" by tempering or other modified annealing schedules as you know. There are also glass-ceramic matrices (Schott Zerodur comes to mind) that depend on specific annealing schedules to maintain their low thermal expansion which render them thermally resistant. If Robax is just a brand name of boro, then in agree that it should have the same heat-resistant properties after slumping as it did before.

Tony

[Tony Smith]

Indeed, Robax is a glass ceramic matrix, and not a borosilicate.
http://www.schottrobax.com/?WTS=1&WTC=H ... -Warm-Safe

Proceed with caution.

Tony

[Kevin Midgley]

Broken stove top so called glass that I tried didn't do anything with 'normal' fusing and firing temperatures and schedules.

[Bert Weiss]

Indeed, Robax is a glass ceramic matrix, and not a borosilicate.
http://www.schottrobax.com/?WTS=1&WTC=H ... -Warm-Safe

Proceed with caution.

Tony

I replaced a piece of glass in my pellet stove after it broke during shipment. Spendy little sheet of glass. I never gave any thought to bending this stuff. It is a whole different ballgame I guess...

[adrian]

HI Folks

Thanks for all the comments...

I've failed to get the stuff to bend, despite having taken it up to 880c (in my homebuilt kiln, waterglass slumps at 680c and float slumps at 780).
I didn't want to go any higher, for fear of upsetting my new kiln...

Schott (the manufacturers of the Robax glass) replied to my email as follows

"Unfortunately I have to tell you that the glass ceramic material which you can buy on the market, you can't bent any more. Only we as a producer of glass ceramic can do this processing step before turning this specific material into glass ceramic. "

So it sounds as if their manufacturing process is 'form to shape' then 'process' -
so, without knowing what the process is, you run the risk of ending up with a formed piece of glass that's no longer heat-resistant!

This is probably why Schott charge a significant premium for supplying formed glass for use in 'bow-fronted' stoves..

Thanks all
Adrian