More help with frit casting mold, please :)

[Shelley Lewis]

I used my first sunflower frit casting mold 4 times, had 4 beautiful flowers, then the mold cracked. I replaced it with the same mold, and the first flower I cast came out beautifully, but a small piece of mold chipped off when I removed the glass. I smoothed off the rough areas of the small depression and I could barely tell where the chip had been. The next casting I tried turned out like the pic below, with a huge depressed area. I thought it was a fluke, so I tried again and got the same result. Can anyone explain what is going on? So frustrated! I don't think this could be happening because of the mold chip...the chip was at the tip of a petal and the craters formed near the center of the flower. Any answers are appreciated!

[Stephen Richard]

The raised area in the brown looks like a bubble.
What kind of firing are you using? let us know the ramp speeds, soaks etc. to see if we can help diagnose the issue.

[Shelley Lewis]

The raised area in the brown looks like a bubble.
What kind of firing are you using? let us know the ramp speeds, soaks etc. to see if we can help diagnose the issue.

Yes, it is a crater on the top of the cast glass, and a hump on the back. I thought it was a bubble too, but I cast 5 flowers using the same firing schedule with perfect results, so I'm at a loss . Here's the schedule in F:

300 1100 20
300 1410 20
Full 960 60
60 700 off

These were cast with fine frit if that makes a difference. Many thanks for your help!

[Bert Weiss]

Whenever air is trapped either under glass or inside glass, and the glass gets hot and viscous enough to expand, the air will expand from the heat, and blow a bubble with the glass. Sometimes the bubbles break. Many pate de verre artists don't heat their glass so hot that it is viscous enough for this to happen. Their result is like a grainy casting, more so than a transparent glassy one.

[Shelley Lewis]

Whenever air is trapped either under glass or inside glass, and the glass gets hot and viscous enough to expand, the air will expand from the heat, and blow a bubble with the glass. Sometimes the bubbles break. Many pate de verre artists don't heat their glass so hot that it is viscous enough for this to happen. Their result is like a grainy casting, more so than a transparent glassy one.

Would you say 1410 is too hot? Also, any idea why I would have success the first 5 times, and then bubbles on the next 2?

[rosanna gusler]

Check the position of your thermo couple . You might be over firing due to a malfunction. R.

[Shelley Lewis]

Check the position of your thermo couple . You might be over firing due to a malfunction. R.

Will do, thanks for the suggestion.

I had a thought... If I used too much glass tac in the flower's center, which is where the problems occurred, could that be the cause of bubbles?

[Morganica]

1) Yep, stop using so much glasstac. The less gunk you use to stick the glass particles together, the less outgassing you're going to get, the fewer bubbles get blown.
2) Not sure what your second segment is doing--it's not really hot enough for a bubble squeeze, and you shouldn't need to equalize temps with fine frit at that point. If you're doing a squeeze, add another 100 degrees or so to the soak temp, to around 1225F-1240F.
3) Pate de verre artists don't do "grainy castings," but we do deliberately trap air bubbles between glass particles to change the way light travels through the piece. Done properly, the piece should look translucent and waxy, like jade. If you reduce the level of heatwork, so that the frit retains its shape, it's not really pate de verre. (I think Charissa Brock coined the term "patisserie de verre," because it looks a bit like sugary pastry)
4) Where was your first crack? And in the second mold, where was the chip in the relation to the big bubble/crater you're getting now? (for example, is the crater on the same side of the flower with the chipped petal?) Are they always in the same place? Are you placing the mold in the same part of the kiln?
5) When you smoothed out the edges, how did you smooth it? Did you angle it out gradually to the floor of the mold, create an undercut that might trap air...etc?