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Craters In Glaze


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#1 twinmom

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Posted 09 May 2011 - 08:41 AM

Hi, Everyone.

In our hurry to get everything done at our local middle school before the end of the year, I've been rushing things a bit. We're using Cone 04 clay and Cone 06 glazes. The glaze instructions say to brush the glaze onto bisqued (Cone 04) pots and then fire to Cone 06.

So I loaded a bisque firing but threw in some glaze-test pots. The net result was that I brushed the glazes onto green clay (unbisqued) and once-fired those pots to Cone 04 (slightly higher than the manufacturers wanted). There were several craters from blisters that hadn't healed. I suspect that either the kiln fires and cools very quickly, or the craters were caused by once-firing.

This electric kiln is old and has only a kiln sitter and timer, and it's hard to intercept it at the end of the firing because the school day is only 7 hours long. The firings are taking 8 hours. If I'm correct in my suspicions, how can I correct this defect with this kiln?

Thanks for any ideas.

#2 Lucille Oka

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Posted 09 May 2011 - 11:27 PM

Hi, Everyone.

In our hurry to get everything done at our local middle school before the end of the year, I've been rushing things a bit. We're using Cone 04 clay and Cone 06 glazes. The glaze instructions say to brush the glaze onto bisqued (Cone 04) pots and then fire to Cone 06.

So I loaded a bisque firing but threw in some glaze-test pots. The net result was that I brushed the glazes onto green clay (unbisqued) and once-fired those pots to Cone 04 (slightly higher than the manufacturers wanted). There were several craters from blisters that hadn't healed. I suspect that either the kiln fires and cools very quickly, or the craters were caused by once-firing.

This electric kiln is old and has only a kiln sitter and timer, and it's hard to intercept it at the end of the firing because the school day is only 7 hours long. The firings are taking 8 hours. If I'm correct in my suspicions, how can I correct this defect with this kiln?

Thanks for any ideas.




You are trying to rush the firings. Because you haven’t much time I understand this. You need a firing schedule that will allow for more kiln work. It could help if you pre-fired the ware. For example, if you load the kiln on Monday afternoon, you can start to pre-fire when you come in on Tuesday morning. Set the kiln to low with the door propped open for about four hours, then close the door leave settings on low and fire to late afternoon or until you leave the school. Set an alarm clock if you need help to remember to shut off the kiln before you leave. It will cool down over night. This pre-fire will be just to remove the physical water and some of the gases. You can then come in on Wednesday morning with the Kiln still already loaded; place your 04 cone in the setter for the bisque firing. You can then start to fire on the medium setting. Prop the door for an hour or so to get rid of lingering gases and then close the door to start the maturation firing. Use your regular method to determine when to step up to the high setting. This schedule requires less time to reach the cone 04 bisque temperature.

Also, if glazes are fired higher or lower than the recommended temperature defects can occur. Firing the cone 06 glazes to cone 04 you fired them too hot and they began to boil without time for ‘soaking’. And ‘once fired ware’ needs an extra long ‘candling’ time to drive off gases and water in the greenware and the glaze.

Some glazes however do have a firing range as do some clays check with the manufacturer to be certain.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

#3 pent19

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Posted 13 May 2011 - 07:42 PM

***Please use caution when running the kiln with the lid propped open while at school!! I set off my fire alarm (fire trucks and all) when the heat from the kiln set off our heat sensor alarm in my kiln room(from propping). The technique will work but be cautious with the amount of heat that escapes even with the kiln at low! I was the banished from running my kiln for a month while the school made sure there wasn't a bigger problem (just an impatient art teacher taking care of business!)



You are trying to rush the firings. Because you haven’t much time I understand this. You need a firing schedule that will allow for more kiln work. It could help if you pre-fired the ware. For example, if you load the kiln on Monday afternoon, you can start to pre-fire when you come in on Tuesday morning. Set the kiln to low with the door propped open for about four hours, then close the door leave settings on low and fire to late afternoon or until you leave the school. Set an alarm clock if you need help to remember to shut off the kiln before you leave. It will cool down over night. This pre-fire will be just to remove the physical water and some of the gases. You can then come in on Wednesday morning with the Kiln still already loaded; place your 04 cone in the setter for the bisque firing. You can then start to fire on the medium setting. Prop the door for an hour or so to get rid of lingering gases and then close the door to start the maturation firing. Use your regular method to determine when to step up to the high setting. This schedule requires less time to reach the cone 04 bisque temperature.



#4 Lucille Oka

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Posted 11 June 2011 - 06:19 PM

Thank you for that. It never occurred to me that a school would have such an issue. I guess I should not take it for granted that a school will properly install a kiln. The ventilation is inadequate. Was the kiln installed near the heat sensing device? Those are two of the kiln installation no nos. They also should not be installed near fire sprinklers. There are other no nos but folks need to read a kiln instruction manual before installation.
John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".




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