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Will crazing happen earthenware if bisque firing is one cone off


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#1 icyone

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Posted 04 July 2012 - 06:13 PM

My question is I did a load of earthenware bisque at 04, set up cones and fired and it looks like my cones barely moved.

In the past we always fired to 03 in an older kiln to achieve our 04 bisque.

This time I used one of our newer kilns with the evironvent thinking it would give me a nice 04 but as mentioned it only barely bent the 04 cone.

So my question is will crazing be a problem or is the problem just going to be outgassing if not fired hot enough? I know I'll be able to tell if outgassing will be a problem by doing one of the pieces however I don't want crazing to happen like I've seen happen sometimes months later.

thanks, Sloan

#2 Kabe

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Posted 04 July 2012 - 10:16 PM

My question is I did a load of earthenware bisque at 04, set up cones and fired and it looks like my cones barely moved.

In the past we always fired to 03 in an older kiln to achieve our 04 bisque.

This time I used one of our newer kilns with the evironvent thinking it would give me a nice 04 but as mentioned it only barely bent the 04 cone.

So my question is will crazing be a problem or is the problem just going to be outgassing if not fired hot enough? I know I'll be able to tell if outgassing will be a problem by doing one of the pieces however I don't want crazing to happen like I've seen happen sometimes months later.

thanks, Sloan



I may be wrong here and if so someone will correct me and I would be thankful for that. But my understanding of crazing is on how well the glaze fits the clay body. If you glaze your bisqueware and then fire it to it's maturing temp, your glaze and you clay body will all be fired to the correct temperature and it should not craze, That is, if the glaze fits the clay body. Crazing is created by the different rate of shrinkage between the glaze and the body it is covering. The glaze is stretched so tight that is sort of shatters on the surface. I bisque to 08 and fire to cone 6 or if it is in a gas kiln it fires to cone 10. I never thought it made to much of a difference to what temp I bisqued to, as long as I heated the clay to the point that I have driven off all the water that was chemically contained by the clay. It starts at about 350 C and should be done at 500C . ( Clay and Glazes for the Potter, Daniel Rhodes) My fear is when it over fires and then my bisqueware will not absorb the glaze. I think underfiring a bisque load by one cone would not create a crazing problem. I am not sure what you mean by outgassing unless you mean the gasses that are released as it is fired, to me this could create a blistering and pinholing problem problem but if you fire everything to what the glaze calls for and let it cool at the right speed it should be ok. Happy firing Kabe

#3 icyone

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Posted 05 July 2012 - 12:08 AM

Thanks for your input,

I have out-gassing when I fire bisque too low. For instance once I fired a test piece of bisque in an 06 glaze load.

Painted, glazed and fired it

However out-gassing happened ) as I got big holes and craters from the gasses still being released as the bisque was not fired hot enough. I googled a bunch and found out about out-gassing.

However that was 2 cones difference, this time it's just one so I am willing to do another test. I just don't want it to craze later on down the line as I've sometimes have had happen before. I thought maybe the crazing happened later cause the fit was almost right but not quit (like one cone off was one guess I had)

Not sure but it sounds like you might be working with stoneware by looking at your cones so I believe earthenware is a bit more finicky than stoneware because of the low temps but I'm just guessing about that part as I've never done stoneware.

I don't believe it will craze either however I have had crazing happen months after a piece was done and was hoping that would not happen with this load.

03 came out to hot so I might try and refire the whole load at 04 with a 10 to 15 min hold this time but I'd much rather have the glaze stick than go too hot again.

#4 neilestrick

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Posted 10 July 2012 - 08:18 AM

Most clay bodies behave just fine being bisqued anywhere between cone 08 and 03. I find it a bit odd that you're getting gassing from a cone 06 bisque. What clay body are you using? You may want to try slowing down your bisque firing if it's that touchy.
Neil Estrick
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Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

neil@neilestrickgallery.com




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