Creases formed around the rim during fire polishing

[Charlotte Miller]

I had an interesting thing happen to a piece I just fired. The piece is 15" round, BE black irid on bottom, BE black with mica fired on top. I had blasted a design on the top surface. The design included 6 holes(i.e., I blasted all the way thru the glass) evenly spaced around the rim (approx 1/2" from the edge). Each was the shape of a shallow crescent, but each end was straight and slightly flared (like bell bottom pants). The crescents face inward toward the center (boy, I hope this makes sense). When I fire polished the piece after blasting, I got creases around the rim of the 15" circle (i.e., not the rim of the crescent shaped hole). It looks like the crescent shapes pulled in slightly during the firing, causing the rim of the piece to crease. I fired it at 1410 for 8 minutes. I'm wondering if I could prevent the creases if I fire polished at a lower temp. What is the lowest temp and least amount of time needed to fire polish a blasted area (I'm using 100 grit aluminum oxide). I'm also thinking that the fact that the back of the piece was iridized glass may have played a role? I'm interested in your thoughts on this.

Thanks for all your wisdom,
Charlotte

[Mimi Rivera Vellek]

Hi, Charlotte! 1410 for a fire polish is pretty high (in my kiln, a Paragon 24", 1420 for 20 minutes is fusing temp). I fire polish at around 1360, depending on the piece. The creasing might also have something to do with the irid; as a surface coating it likes to stay on top and will try to migrate to the surface if it's not completely capped.

Hope this helps,

Mimi