firing schedule for small pull knobs?

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sammysrocks
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Joined: Tue Jun 04, 2013 11:43 am

firing schedule for small pull knobs?

Post by sammysrocks »

Hello all

Newbie here with some questions on firing schedule for a 4 piece mold for small pull knobs, like a kitchen drawer. The problem is the glass at the very bottom, where the screw hole would be, is not melting together. It is just "fusing", leaving lots of air spaces. I am using an Auber ramp/soak with SSR, started by setting ramp up at max speed with a safety start at 1365 for 20 minutes, max speed to 1400 for 5. Then I tried 1400 at 25, 1450 for 15, knobs turned out about the same. Should I just try increasing soak temp and/or time again, or will slowing down ramp up speed help? How important is ramp up speed? Will a slower ramp up help get the air spaces out of the bottom of the mold? Why are 2 soak temps suggested by mold manufacturer instead of just one?
We are making our own frit out of stained glass pieces if it matters.

Thanks

Eric
Morganica
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Re: firing schedule for small pull knobs?

Post by Morganica »

Uhm....what is an Auber ramp/soak with SSR? Or a max speed with safety start? ;-)

Small castings can actually take quite a bit of heatwork to fully fuse into a solid piece. I cast small pendants with a lot of detail (faces and whatnot), and if I want the face to have a fully formed nose and lips the mold must stay at 1485F for up to four hours, depending on the design.

You may want to invest in a book or two about kiln casting with glass. Try "Pate de verre and Kilncasting of Glass," by Kervin and Fenton. It has lots of very good information.
Cynthia Morgan
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Jenna
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Re: firing schedule for small pull knobs?

Post by Jenna »

Hi Eric,
If you're only firing to a top temp of 1450, you haven't really reached a full fuse temp yet. Also increase your hold time. 5-15 min is not long enough. Try 1485 with a hold of at least 35 min. As Cynthia said, it can take up to 4 hrs for some molds. The glass in the bottom of your mold is the "furthest" from any heat/elements. It helps to think of heat as radiation (like the sun). The heat needs to either pass through the glass above (to reach the bottom of your mold) or up from the bottom elements, through a kiln shelf and the bottom of your mold which act as insulators. So this part of your mold "sees" the heat last.
BTW, what are the two soak temps suggested by the mold manufacturer?
sammysrocks
Posts: 3
Joined: Tue Jun 04, 2013 11:43 am

Re: firing schedule for small pull knobs?

Post by sammysrocks »

Thanks for the replies. We built our own kiln control system, Auber is the brand of PID controller we used. The SSR is an electronic contactor without any moving parts, so it will last longer and is very quiet when running, so there isn't any clicking when the elements switch off and on. Safety start (Auber's term) means if you set a time for a step, the time wont start until the first temp is reached, instead of starting just because the time ran out. The Auber programs by time to next step, instead of degrees per hour.

Manufacturer recommends the first settings we used, 1365 for 20, then 1400 for 5, then stress relieve. We tried increasing to 1400 for 25, then 1450 for 15, then stress relieve. The top part turned out both times, shiny curved surface, the bottom barley fused both times.
Why two temps? Preheat the glass below devitrification temp?

We tried both medium and fine BE compatible prepared frit in the bottom, neither fused well.

Thanks

Eric
Morganica
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Re: firing schedule for small pull knobs?

Post by Morganica »

Ahhh. SSR, i.e., "solid-state relay?" And so "safety start" is what the more popular glasskiln controllers do then, i.e. DO-UNTIL...only I didn't know the fancy name for it. ;-)

I'm not sure why you'd hold for 20 minutes at 1365F, then 5 minutes at 1400, and I'm assuming your stress-relieve is the anneal soak at 900/960? You might do that for larger pieces where you needed to equalize temperature through the mold/surface/core, but if you're working with frit it's really not going to do much more than add heatwork. And 1365F is smack-dab in what people usually call the devitrification zone.

You typically do a "bubble squeeze," i.e., hold the frit right at the temp where the glass is starting to soften. That slows down the settling of the frit, giving time to push the air out. But those are typically done somewhere between 1200-1250F, not 1365F.

As Jenna said, you've got a lot of insulation at the bottom of the mold and with smaller pieces there's not a lot of heat retention/gravity helping to finish the forming. I had that problem too, when I switched from casting large pieces to doing small pendants. I was a bit astonished to find that the detail really didn't form until at least 3 hours into the schedule. If I'm dripping glass in from outside the mold, i.e., with billet, it doesn't take as long, but frit in the mold takes forever.

You might try paging through the Bullseye tech notes (http://www.bullseyeglass.com/methods-id ... icles.html). In particular, look at Heat and Glass.
Cynthia Morgan
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http://www.cynthiamorgan.com

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sammysrocks
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Joined: Tue Jun 04, 2013 11:43 am

Re: firing schedule for small pull knobs?

Post by sammysrocks »

We increased to 1575 for 75 minutes, with a fairly fast ramp up of 120 minutes. Also used the paper punched fiber paper discs on the wire without the wrap of paper around them. Used 2 molds, one spent extra time tapping mold to settle glass and get out all the air we could, other did minimal settling. Both turned out great, sounds like wrapping the discs with more paper was the issue, at least for us. Possibly using wrong paper, but just the discs are working.
Next we will try gradual reductions of time/temp to save time/money in firing.

Thanks all

Eric
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