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once fire vs bisque plus final fire for unglazed pieces


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#1 flowerdry

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Posted 24 February 2013 - 11:14 AM

Hello CAD forum folks!

I don't think I've seen this topic before and had no results on a search. I am making a japanese lantern textured to look like stone. It consists of 5 separate unglazed parts made of standard 112 which will just be set on top of each other for the final piece. I am in a small class at a community art center and there will be only one bisque(cone 04) and one final firing (cone 6). Due to logistics, it looks like I might have to fire some of it in bisque and some of it only once in the final fire. So here are my questions:

1.Might the parts that have been fired differently look different?
2. since there will be no glaze applied, I think the once firing on a glaze schedule will be ok with respect to the impurities burning out, etc. Am I correct?

3. I want a matte finish. The piece will be outside. Should I leave it bare, or find some type of sealant to protect it that won't impart a sheen?

Oops, sorry. I think I should have posted this in the other section.

Doris Hackworth

"Promoting the joy of handmade pottery"


#2 bciskepottery

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Posted 24 February 2013 - 01:42 PM

I would not recommend firing some pieces in bisque and then some pieces in single fire glaze. Either bisque all pieces, or single fire all pieces. But don't mix and match. If the pieces are going to sit on top of each other, you want consistent shrinkage . . . best they shrink together, and at one time. Plus, you have different temperture ramps between a bisque and glaze firing that could affect your once fired pieces (shrinkage, warping due to lack of preheat often used for bisque, etc).

If the lantern sits on the ground, you might have a problem with absorption, especially if the temperatures get to freezing. Freezing/thawing will stress the clay body.



#3 weeble

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Posted 25 February 2013 - 05:06 AM

They will shrink differently, the color will probably be slightly different (I don't know your clay, but most every clay I've worked with looks different between 05 and 5) and the low fire pieces will absorb water and be much more susceptible to freeze damage as well as much more likely to grow moss/mold/crud. The growing thing can be a cool effect, but it'll definitely be different between the two firings.

I know when I fire, a bisque fire (cone 05) takes 12 hours to fire to a lower temperature, to burn crud out. Glaze fire (cone 5) goes for 8 hours to a higher temp, much steeper cycle. You might have a problem with things blowing up, or you might not.
Maryjane Carlson

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#4 neilestrick

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Posted 25 February 2013 - 01:43 PM

Will the bisque pieces also then go into the high fire, or will some only go to bisque and some to high fire? Assuming they all go to cone 6 eventually, they should look the same whether they were bisque fired first or not. If they need to stack, it's best to fire them stacked together so they still fit together after firing. Wax the joints with some was that has alumina hydrate in it to keep them from sticking. Depending on the kiln, the cone 6 firing may be faster at the beginning, and could cause them to blow up if they weren't bisque fired first. At cone 6 that clay will be well vitrified, so you shouldn't need to seal it any further.
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#5 flowerdry

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Posted 27 February 2013 - 08:01 PM

Will the bisque pieces also then go into the high fire, or will some only go to bisque and some to high fire? Assuming they all go to cone 6 eventually, they should look the same whether they were bisque fired first or not. If they need to stack, it's best to fire them stacked together so they still fit together after firing. Wax the joints with some was that has alumina hydrate in it to keep them from sticking. Depending on the kiln, the cone 6 firing may be faster at the beginning, and could cause them to blow up if they weren't bisque fired first. At cone 6 that clay will be well vitrified, so you shouldn't need to seal it any further.


Sorry I did not make that clear. All pieces were meant to be bisque fired first, then high fired. I think I will try harder to get the 2 pieces I have at home to the studio so they can get in with the bisque fire too. (It's a long drive). I would hate for them to blow up as they were a lot of work...not to mention ruining other students' work. Or I could try harder and work faster to get my own kiln up and running!

Thank you all for your thoughts and taking the time to write.

Doris Hackworth

"Promoting the joy of handmade pottery"


#6 perkolator

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Posted 28 February 2013 - 07:39 PM

you should try to get it all in one kiln if possible. as long as your sections aren't really tight against one another, there shouldn't be too much issue firing in separate pieces - but together is usually ideal. i don't really see any problems with getting half in one kiln and the other half in another firing so long as the piece was built all at once (hope this makes sense) - there may be slight color variation, but since it's outdoors unglazed it's going to change color with time anyways.

the main problem i see with your original post is the fact that your studio bisques and you're planning to put greenware inside a glaze firing. you need to make sure whomever's firing that kiln KNOWS and adjusts the schedule accordingly - or you risk blowing up your work.




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