Edges needle when slumping

[Wini Kraft]

This had driven me crazy for too long.... when slumping into a bowl - shallow or deep the edges of my piece often get very sharp needles. This then requires that I do quite a bit of finish work, removing the needles with the grinder and then fire polishing the piece. Is this typical or is there a better way to get my slump?

My typical Slump program is:
300* to 1270* hold 15 m
ASAP to 960* hold 1 hour
100* to 700* no hold

This is a Jenn Kenn Kiln EZ Pro 15-6

Thanks, Wini

[jim simmons]

What kind and how much separator are you using?
jim

[Valerie Adams]

Try 250° per hour to 1150°, hold 30-45 minutes (visually confirm slump), then proceed with annealing.

[jim simmons]

What's happening, is the glass is catching on the mold as it slumps.
Valerie has a good suggestion. SLOW and easy does it.
Jim

What kind and how much separator are you using?
jim

[Morganica]

Glass is more likely to needle when gets too hot too fast for the process, so I'd agree with Val and Jim; reduce the heatwork and slow down a bit.

Also consider what that edge looks like BEFORE you slump it. If you're using a fresh-from-the-kiln blank and you've been firing hot, the bottom edge of the glass may already be on the sharpish side and more likely to form needles.

Try knocking off the bottom edge of your blank (the side that will be touching the mold) with a tiny, roughly 45-degree bevel. Simplest way to do that is with a block and some 400-grit wet-dry sandpaper, but diamond pads or machinery will do the same thing. Then slump. The bevel allows the glass to slide more easily.

If your top edge is really rounded over, you can also do this simply by flipping the glass over and slumping it upside down, but the bevel makes a neater edge (I think).

[Wini Kraft]

Thank you for the helpful suggestions! I do often bevel my edges and that does help, but now I will attempt this new slump schedule.