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Fire polishing observation and question

Posted: Tue Nov 04, 2014 11:23 am
by Geo
I am making pendants (ranging in size from about 1.5 to 2 inches) from Bullseye frits and powders, no sheet glass at all. Some I fired following a basic full fusing schedule, and some I made in small plaster/silica molds and fired following a schedule I use for pate de verre. After grinding the edges and the backs of some pendants, I now am trying to fire polish them.

I did a test firing with interesting results. My schedule for fire polishing was 300 degrees to 1250, then 600 degrees to 1325 for 15 minutes, AFAP to 900, then slowly cooled.

1. On one of the pate de verre pieces I essentially did no coldworking. There was no change at all in the finish after fire polishing.

2. One of the pate de verre pieces I grounded the back using a flat lap. After fire polishing, the back was very smooth and shiny, with which I am pleased.

3. The edges of the fused pieces rounded slightly, but some rough spots remained.

So I want to adjust my schedule for the fused pieces and am wondering if I should increase heat or time at my processing temperature. Any thoughts?


Re: Fire polishing observation and question

Posted: Wed Nov 05, 2014 3:31 am
by Morganica
I think what you just discovered is that different glasses firepolish differently, and the combination of glass and coldworking make a difference. So it's hard to say what you should do with your schedule--it depends on what you're firing and how you've coldworked.

Harder glasses (whites, for example) take more heatwork to soften, so they will tend to be the last to firepolish. (IOW, if you want to firepolish them, you may need to boost the schedule). But soft glasses such as black will firepolish quickly, so by the time the whites have firepolished, the black will be overfired.

That's why firepolishes can be tricky if you've got a wide variety of hard and soft glasses in the kiln.

The answer is to either fire similar glasses in the same firing, or make up for the differences with coldwork. If you coldwork the harder pieces to a uniform 200- or even 400-grit, you give them a headstart on the firepolishing and they're better able to keep up with the softer glasses.

Re: Fire polishing observation and question

Posted: Wed Nov 05, 2014 8:56 am
by Geo
Thanks for the reply! I really find all of this quite fascinating.

In the future I'll be firing the different pieces separately. As I adjust the schedules accordingly, would you suggest increasing the time I am holding the glass at the highest temperature, or increasing the temperature?

Re: Fire polishing observation and question

Posted: Wed Nov 05, 2014 3:23 pm
by Valerie Adams
It may be that one of those changes is what you need, or both. I'd advise taking good notes, and visually observe pieces while in the kiln. Set up a few experiments and peek at the time when you think they should be done; if they're not glossed up to your desired result, add a little extra time. Repeat as necessary.

On another batch, try ramping slower and peek. On another batch, try firing hotter, and peek. It may take awhile to find the sweet spot, but well worth your time.