rough edges & needles after slumping - what am I doing wrong? -

rough edges & needles after slumping - what am I doing wrong?

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rough edges & needles after slumping - what am I doing wrong?

Postby helenM » Tue Nov 15, 2016 10:01 am

Hi, I've just completed my second ever slump (yay!). The first one was in a small shallow square mould and came out fine. For this second one I've used a shallower mould ( ... Path=96_73) with blue transparent glass, and the finished shape is wonderful .

The problem is the finish on the bottom of the glass isn't that good. I've used Primo Primer (following the provided instructions and making sure I stirred well before every application). Basically, on the underside of the dish the center part of the dish is lovely and glossy, as is the front side. But around the edges of the underside (about 1 centimeter all round up to the edge of the dish) it's not glossy but looks rougher almost as if I had used fine sandpaper on it. Additionally I have one little needle on each of the four corners of the dish.

I can't work out if this means that I need to increase my slump top hold temperature, or lower it. I thought lower it because of the needles ? And I'm not sure what's causing the roughness. Could it be my application of primer wasn't even or something? I cleaned the glass before use, but I wasn't sure if this was devit or not. Could anyone advise what might be the causes please? And if I remove the needles with a diamond pad can I fire polish the dish? I've tried to find some answers online and in books I have but am not finding anything that can help.

Slump temps used (recommended by warm glass):
Segment Rate Temp Hold
1 300F/167C 1225F/663C 20 min
2 AFAP 900F/482C 60
3 100F/56C 700F/371C End

Kevin Midgley
Posts: 659
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: rough edges & needles after slumping - what am I doing wrong?

Postby Kevin Midgley » Tue Nov 15, 2016 1:41 pm

more kiln wash.
lower top temperature.

Valerie Adams
Posts: 587
Joined: Mon Apr 19, 2004 2:49 pm
Location: Santa Rosa, California

Re: rough edges & needles after slumping - what am I doing wrong?

Postby Valerie Adams » Fri Nov 25, 2016 12:47 pm

Fire polishing temperatures are higher than slumping temps, which means you'll lose the shape of your piece if you just put it in the kiln and try polishing.

Most slumping problems occur because of firing too hot/too fast. For 95% of what I slump, I use the same schedule: 200° per hour until 1150°, hold 30-45 minutes (I visually confirm the slump), then anneal.

I've got a few coats of kiln wash on my molds. I applied it when I started fusing in 2001. I've never reapplied kiln wash to my molds because at that low temp (1150°) the kiln wash doesn't degrade. I'd only replace it if I scratched it.

I always explain slumping like this: if you gently coax your glass into the mold, it won't grab the edges, or get that frosty stretched look. Treat it kindly and it'll respond. :D

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