Big bubbles on flat fuse
Posted: Mon Jul 21, 2014 5:06 pm
I'm a little reluctant to ask this in case someone tells me to go do research - I've been doing that and I still don't know what's going wrong. This is a gorgeous piece of deep royal purple with rainbow irid, about 10.5" x 4.5". I was planning to slump it into a tray mold (am hoping to make an incense burner by drilling a hole for the incense stick).
So ... there must be air trapped between the glass and the shelf.
The thing is, I could fire blanks just fine straight on my kiln shelf until just recently.
Then my Skutt Hotstart's lid was dropping kiln brick dust onto the glass while it was still fluid enough for the dust to get incorporated in the glass. The pins had been falling out, the element (which is in the lid) was loose, apparently it was rubbing against the brick and dropping the dust. So I got new pins and wasn't able to push them in to move and secure the coil; the coil wire had gotten too stiff over time. So I ordered a new kiln lid and my husband drilled holes, etc., and installed it.
That's when the real trouble began. After various failed attempts at firing on flat molds (with the plan of making tiles), I decided to try firing on the shelf again. That was actually working well (and I found a solution for internal bubbles) with 5/25" square blanks until today with the 10.5" x 4.5" blank. With this result.
I've been using kiln wash instead of fiber paper because Id rather have as little exposure to particulates as possible.
I'd dried the kiln-washed kiln shelf at about 500 degrees for 20 minutes.
This is the schedule (from Bullseye TechNotes on reducing internal bubbles):
300/hour to 1200, hold for 20 min
25/hour to 1250, hold for 20 min
300/hour to 1490, hold for 10 min
9999/hour to 900, hold for 30 min
150/hour to 700
9999/hour
I've read that big bubbles seem to form in the 1350 - 1450 range (so if I'd tack fused, there wouldn't be bubbles, since the top temp programmed for that is 1325 - likely irrelevant).
The kiln is maybe 1mm - 2mm off level (if I raised the back legs that much). Is that enough to be a problem?
This is actually from a second firing. The first result was lumpy. I'd used powdered frit between layers of clear and purple irid, per Bullseye TechNotes, and it wasn't really an even layer (though it was thin). After the firing, I held the piece up to the light and looked from the back. There didn't seem to be many bubbles at all, and they were really small. (Yay!) The back was flat.
I fired again to see if the piece would flatten out (after kiln washing shelf and 500 degree drying cycle again).
Can someone give me some advice? Please?
It seems likely that there were biggish internal bubbles that I just didn't see, but I looked really well and it seems as if I'd have seen them if they were there.
I'm thinking that firing a blank of 1/4" thick clear would give me good information about whether bubbles are forming between the shelf and the glass.
Thanks much for any suggestions ...
So ... there must be air trapped between the glass and the shelf.
The thing is, I could fire blanks just fine straight on my kiln shelf until just recently.
Then my Skutt Hotstart's lid was dropping kiln brick dust onto the glass while it was still fluid enough for the dust to get incorporated in the glass. The pins had been falling out, the element (which is in the lid) was loose, apparently it was rubbing against the brick and dropping the dust. So I got new pins and wasn't able to push them in to move and secure the coil; the coil wire had gotten too stiff over time. So I ordered a new kiln lid and my husband drilled holes, etc., and installed it.
That's when the real trouble began. After various failed attempts at firing on flat molds (with the plan of making tiles), I decided to try firing on the shelf again. That was actually working well (and I found a solution for internal bubbles) with 5/25" square blanks until today with the 10.5" x 4.5" blank. With this result.
I've been using kiln wash instead of fiber paper because Id rather have as little exposure to particulates as possible.
I'd dried the kiln-washed kiln shelf at about 500 degrees for 20 minutes.
This is the schedule (from Bullseye TechNotes on reducing internal bubbles):
300/hour to 1200, hold for 20 min
25/hour to 1250, hold for 20 min
300/hour to 1490, hold for 10 min
9999/hour to 900, hold for 30 min
150/hour to 700
9999/hour
I've read that big bubbles seem to form in the 1350 - 1450 range (so if I'd tack fused, there wouldn't be bubbles, since the top temp programmed for that is 1325 - likely irrelevant).
The kiln is maybe 1mm - 2mm off level (if I raised the back legs that much). Is that enough to be a problem?
This is actually from a second firing. The first result was lumpy. I'd used powdered frit between layers of clear and purple irid, per Bullseye TechNotes, and it wasn't really an even layer (though it was thin). After the firing, I held the piece up to the light and looked from the back. There didn't seem to be many bubbles at all, and they were really small. (Yay!) The back was flat.
I fired again to see if the piece would flatten out (after kiln washing shelf and 500 degree drying cycle again).
Can someone give me some advice? Please?
It seems likely that there were biggish internal bubbles that I just didn't see, but I looked really well and it seems as if I'd have seen them if they were there.
I'm thinking that firing a blank of 1/4" thick clear would give me good information about whether bubbles are forming between the shelf and the glass.
Thanks much for any suggestions ...