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Glaze Problem


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#1 BrittanyDelany

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Posted 02 November 2012 - 12:17 PM

I have just got a new kiln after graduating from uni. Its is a second hand Cromartie.

I create porcelain slipcast ware. I did a test of a biscuit firing to 1000degrees and everything seemed fine. I have fired alot of my work to this successfully.
When testing the glaze firing I used the same firing cycle as uni:

100degrees/hour to 600degrees
FULL to 1260degrees
SOAK for 30 mins
END

When I opened the kiln up the pieces are dry looking and are rough with a few craters on the surface.

The glaze I am using is as follows:
Nephaline Syenite- 35
Dolomitie-20
Whiting-5
China Clay- 20
Flint-20

It is a matt smooth white glaze and is the same as I always used at uni.

I thought maybe the kiln had overfired so re-tested it again and just sent it to 1200degree with a soak of 30 mins. The same thing seems to happen although the pieces are a little smoother, could this be because they havent fired high enough?

I was wondering if the kiln has overfired or under fired at the 1260degrees? I have had various opinions on this.
What would you suggest to do?

Please help! I have craft fairs next weekend and need to get some work fired desperetely!!

#2 Pres

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Posted 02 November 2012 - 02:59 PM

I have just got a new kiln after graduating from uni. Its is a second hand Cromartie.

I create porcelain slipcast ware. I did a test of a biscuit firing to 1000degrees and everything seemed fine. I have fired alot of my work to this successfully.
When testing the glaze firing I used the same firing cycle as uni:

100degrees/hour to 600degrees
FULL to 1260degrees
SOAK for 30 mins
END

When I opened the kiln up the pieces are dry looking and are rough with a few craters on the surface.

The glaze I am using is as follows:
Nephaline Syenite- 35
Dolomitie-20
Whiting-5
China Clay- 20
Flint-20

It is a matt smooth white glaze and is the same as I always used at uni.

I thought maybe the kiln had overfired so re-tested it again and just sent it to 1200degree with a soak of 30 mins. The same thing seems to happen although the pieces are a little smoother, could this be because they havent fired high enough?

I was wondering if the kiln has overfired or under fired at the 1260degrees? I have had various opinions on this.
What would you suggest to do?

Please help! I have craft fairs next weekend and need to get some work fired desperetely!!


Without seeing the pieces, and just going by the description you gave, I would say that it is underfired. I would have thought overfiring may have had a glossier than usual surface or crawling on the surface. You don't mention cones or how you are measuring temp or even if it is a full load or partial load. This is just a personal opinion.

Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . .                                                                                 http://picworkspottery.blogspot.com/


#3 neilestrick

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Posted 03 November 2012 - 10:39 AM

What cone is your glaze formulated to mature at? Are you using cones, or just the pyrometer?
Neil Estrick
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#4 Ben

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Posted 04 November 2012 - 08:59 AM

Try another firing and use cone packs (04, 02, 1, 2, 4, 6 etc)

put them in the bottom, middle, and top. Check them throughout the firing and chart their progress against the reading on the pyrometer.
That will tell you if your pyrometer is accurate. I suspect it isn't.
Most computerized kilns have a way to set the calibration of the pyrometer. you'll need the manual or instructions from the manufacturer.




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