Casting Devit

[revjerry]

Hi folks.
Sometime back I cast an original sculpture of a dolphin, 10" high that devitrified pretty badly. I used Spectrum series 96 glass in a pot melt to get some transparency. I went up slow from Room Temp to process which was 1700, held for 2 hours and then cooled through annealing.

I suspect I went too high and held too long; any responses? I'm getting ready to do the dolphin again and would appreciate your thoughts about temperature and hold times.

Thanks in advance,
Jerry

[Brock]

Hi folks.
Sometime back I cast an original sculpture of a dolphin, 10" high that devitrified pretty badly. I used Spectrum series 96 glass in a pot melt to get some transparency. I went up slow from Room Temp to process which was 1700, held for 2 hours and then cooled through annealing.

I suspect I went too high and held too long; any responses? I'm getting ready to do the dolphin again and would appreciate your thoughts about temperature and hold times.

Thanks in advance,
Jerry

I don't have any experience casting Spectrum, but I know the process temperatures are very close.

I would have done a long hold at 1450. Brock

[Lauri Levanto]

Brian keeps reminding
that devit occurs only within a narrow temp zone.
Hot enough that the atoms can arrange to crystal lattice,
coll enough that the movement of glass prevents
formation of solid crystals.

My understanding is that a pot casting is very safe,
because the glass is in movement all the time.
If yoy got devit on the way up, it must appear as stria
within the bulk of glass. If you got devit on the way down,
it will be on the surface.

My experience is that glass deeper that about 2"
looks matt, not because of devit but
1. because it takes the not glossy mould surface "too exactly"
2. because some paticles of mold material are
attached to the surface.

-lauri

[revjerry]

Lauri,
I've heard all that before and would agree with part two except for the way the devit showed up. It's really strange. It is on the lower half of the piece (upper as you look at the finished product) while the rest is clean! That tells me that the glass in the bottom of the mold, which was in there the longest, got too hot for too long.

I'm still wondering if I should knock off some temperature and not hold as long. Maybe I can set this thing up so I can see when the drip has filled the mold and give it another 30 minutes.

Any recommendations sincerely appreciated.
Jerry

[charlie holden]

Lauri,
I've heard all that before and would agree with part two except for the way the devit showed up. It's really strange. It is on the lower half of the piece (upper as you look at the finished product) while the rest is clean! That tells me that the glass in the bottom of the mold, which was in there the longest, got too hot for too long.

I'm still wondering if I should knock off some temperature and not hold as long. Maybe I can set this thing up so I can see when the drip has filled the mold and give it another 30 minutes.

Any recommendations sincerely appreciated.
Jerry

You can go up as fast as your mold can stand with a pot melt. The glass will be fine since it is in relatively small pieces. You might want to soak around 2000 F to squeeze out bubbles before the glass starts to fuse. No reason to go slow higher than that.

You could knock off 150 F at pouring temperature but you would need to soak there longer.

But really it is not that the glass got too hot but that it is cooling too slowly. It seems like what's happening is that the glass in the bottom of the mold is cooling much more slowly than the rest of the glass. It is in the range between 1400 and 1300 F that devit develops. Were the mold walls much thicker at the bottom or was your mold half buried in sand?

Were you using 96 scraps or 96 casting billets? A glass designed for casting will give you better results.

ch