This last week I desided to cast a light weight 10" blank, but not a cellular one! This blank would weigh in at 2.5kgs and be @20mm thick when finished. Now as this wasn't a high priority project and so didn't need a Pyrex substrate I used some scrap glass that I thought was soda-lime glass, boy was I wrong! As it turned out once I had it upto the working temperature for soda-lime glass (SLG) I checked up on it, what I found was that it had not moved AT ALL, so I had an inkling that this might be a borosilicate glass (AKA Pyrex) so I raised the temperature to the lower end of the working range for Pyrex type glass and yes it did move, albeit quite slowly.

Now the bad news about this little discovery is that the glass spent nearly an hour at 840'C which is right in the middle of the devitrification range of Pyrex. Not a good thing, as devitrification is a one way process, once the damage is done that's it you either scrap the peice of live with the effects. In this case I got lucky the is only a relatively heavy case of surface devitrification, its unsightly, but as this a mirror blank it will be ground off.

This is what the surface looks like.....



Click on the image if you want a close up, it is a big file so it might take a while to download.

Now I did give the blank an extended annealing, so in theory if should have come out with next to no strain/tress in the glass, but as you can see from the picture below there is still quite a bit of strain present, I beleive that this is due to the surface having devitrified, basically that is a crystaline layer around the vitrious glass core, this can't be good for the glass!



So after I grind off the devitrified layer I will re-test and re-anneal the blank if needed, I expect it will be needed!

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