I have slumped a 13inch blank into a shallow dinner plate type mold and the finished piece has a huge bubbble which makes it look more like a hat than a plate! a second attempt produced exactly the same result. The mold looks a bit 'enclosed' ie two air holes in the centre but the edge of the plate has a rim of about one inch which sits on the kiln shelf and I wondered if perhaps the air couldn't escape. The manufacturer however says the mold is OK and they have sold many without any complaints and that I should 'bring it down slower' I'm not too sure what they mean and some help would be much appreciated. I fired to 1380 but didn't flash vent.
Linda, a hole drilled through the vertical support rim of your mold or even a groove across the bottom of the rim where it sits on your kiln shelf will allow any air trapped under the mold to escape. Your complete ramp up schedule is needed if anyone is to offer suggestions about firing.
I have slumped a 13inch blank into a shallow dinner plate type mold and the finished piece has a huge bubbble which makes it look more like a hat than a plate! a second attempt produced exactly the same result. The mold looks a bit 'enclosed' ie two air holes in the centre but the edge of the plate has a rim of about one inch which sits on the kiln shelf and I wondered if perhaps the air couldn't escape. The manufacturer however says the mold is OK and they have sold many without any complaints and that I should 'bring it down slower' I'm not too sure what they mean and some help would be much appreciated. I fired to 1380 but didn't flash vent.
Best regards
Linda
Try setting the mold up on some kiln posts (on edge) or sliced up shelves to allow air to circulate under it. Also, 1380 is way, way too hot to slump most fusing glass. What sort of glass are you using and what was your schedule?
I The mold looks a bit 'enclosed' ie two air holes in the centre but the edge of the plate has a rim of about one inch which sits on the kiln shelf and I wondered if perhaps the air couldn't escape.
Best regards
Linda
I also would reccomend that you lift the mold up with stilts to get it off of the shelf.
ALSO, check to be sure that the vent holes are not plugged with kiln wash.
Jim
Linda-Easiest thing to try is a lower target temperature. Don't know what the mfr. meant by bringing it down slower, but my guess is that you can dramatically reduce your target and get a good slump with no bubble.
Susan-Unfortunately you can't fill the bubble with frit and re-fire. The fusing temp is so much higher than the slump temp that you would have all sorts of new bubble problems, as well as the glass wanting to puddle in the lowest part of the form. If the original slump was not extreme, you could try to flatten it by bringing it up to slump temp without a form, then filling the hole, fusing, and re-slumping. Problem with this is that the "patch" will always be obvious,both texturally and visually.
The mold looks a bit 'enclosed' ie two air holes in the centre but the edge of the plate has a rim of about one inch which sits on the kiln shelf and I wondered if perhaps the air couldn't escape. The manufacturer however says the mold is OK and they have sold many without any complaints and that I should 'bring it down slower' I'm not too sure what they mean and some help would be much appreciated. I fired to 1380 but didn't flash vent.