Glass Slumping Bowls

by Janet Rogers
(Durham)

When I slump a 12" bowl at 740c hold for 4 minutes, the result is fine, the outside has a nice shine. However when I slump a 5" small bowl using the same kiln firing schedule, it does not slump totally into the bowl. If I move the temp up to 760c they slump fully, but I am left with a frosted effect on the outside. What am I doing wrong?

Milly's suggestion: Good question Janet. Slumping glass is a case of lots of trial and error, as you've probably found out! The additional weight of the 12" glass will make it slump more effectively than the 5" bowl - that's why you're having to increase the top temperature.
You don't say what type of glass, or what glass slumping mould you're using, so I'll guess that the 'frosted' look is either devitrification, or the glass is picking up the surface of the mould because of the higher top temperature.
I'd have a go at not increasing the temperature - keep that at 740C - but increase the soak to 10-15mins, depending on your kiln. That will give the glass time to sink down into the mould. Alternatively, you could keep the top temp at 740C and the soak at 4mins, but do the heat work on the way up by slowing down the ascent. As I don't know your kiln firing schedule, it's a bit difficult to say any more than that! I hope that helps.


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