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Unglazed ceramics - how to fire?


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Hi All! I'm a beginner out there and have been working on a stoneware vessels using PSH stoneware 519S clay body firing at cone 6. I wanted to keep the vessels unglazed as they won't be holding water etc and I work really hard on creating a silky texture. They all survived the bisque firing no cracks etc but on the second glaze (high temperature) they have all cracked - incredibly disappointing! If the intention is to keep them unglazed what is the best method of firing to vitrify the clay as I can't leave it in it's bisque format? Firing once at a high temperature (no bisque firing), firing twice at  a lower temperature? Really need some help on this one so it doesn't happen again?

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Hi and welcome!

So, if these pieces aren’t going to be glazed, why the need to fire them twice? Is there a decorating process in between the two firings that doesn’t involve glaze?

If you only want to once fire them, you would normally just start off the firing as you would for a bisque (slow), and expect it to be longer than a typical glaze fire to top temperature because of that. 

Knowing how the pieces were made, or what the pieces even are would help with diagnosing what when wrong. 

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Posted (edited)

Thank you all for your response - much appreciated! I've attached some photos of the cracks - these were in areas that there weren't even hairline cracks previously from the bisque firing or areas where I wasn't aware of stress/or that I patched in the making process. The color was dirty almost too, supposed to be ivory but ended up being greyish tinged in some parts and there were lots of banding marks too that weren't visible on the first firing.

How would you recommend firing them once (what cone) and no there isn't any added decoration etc between firings? Any thoughts on what happened here?IMG_7919.jpg.27c9f5963a4e5e1bb14764995cc7b480.jpgIMG_7918.jpg.2b6e922d5ddca939816314c504bdb6ac.jpgIMG_7917.jpg.ddfc32aab5b06d4581bab5e55f0db1c5.jpg

 

 

Edited by analibertine
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4 hours ago, analibertine said:

The color was dirty almost too, supposed to be ivory but ended up being greyish tinged in some parts and there were lots of banding marks too that weren't visible on the first firing.

It’s odd that the coloring was different so the clay could be in question. You mentioned that you worked hard at smoothing these which brings to mind a friend that would work very hard at burnishing his greenware even to the point of smoothing it with water and ribs after it was nearly dry. Whenever he did this, almost always he would get these style cracks from the variation in clay density that he created. They would almost always make it through the bisque, but rarely made  it through the glaze.

The cure was to make sure he did all his compressive smoothing and burnishing during construction (throwing for him and basically carefully metal rib work) and only burnish reasonably at leather hard. Not sure if that will help, but overworking the clay can cause issues.

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Thank you Bill, I really appreciate the feedback! I was worried that they were removed from the kiln when they hadn't cooled down enough! How would you fire a piece like this - would you put it through just one firing instead?

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So. Looking up this form of crack in Hamer and Hamer. The cracks in your image most closely resemble the images in illustration A, page 85.

”Crack A in biscuit and glazed ware:

The crack is probably the development of one that was latent at the raw stage. The stress at the raw stage may have ruptured the clay without visible signs at the surface. Such cracks are called latent or dormant cracks and no matter how careful the firing they are forced to develop. Mishandling of raw ware when packing a biscuit kiln may have induced the cracks, but the cause should first be sought as poor design, rapid drying, unequal drying, incorrect method and readsorption.” (Not a typo.)

My personal guess is that whatever happened had to do with water being reapplied at the wrong time somehow. It could be that you built too much weight on top of the piece too rapidly. The lovely sheen that you’ve built up on that surface makes me wonder if you used terra sig in your process. If you added it when the piece was drier than your clay wanted to be and then burnished, that might have caused hidden issues. Also, if you covered your piece the wrong way, adsorption can cause pieces to develop faults or even collapse in extreme cases.  

 

Edited to add: It wasn’t the firing that did the damage. The firing just revealed the damage.

 

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Thank you Callie, so you'd still recommend firing twice - I will change how I'm creating the pieces in the first place but is there a better way to fire as it would be great to fire only once instead considering I'm not glazing or adding decoration in between the first and second firing? Not using terra sig or adding anything to the clay just burnishing with a stone. How do you control the drying, I keep mine in bin bags to reduce the speed of drying?

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1 hour ago, Callie Beller Diesel said:

So. Looking up this form of crack in Hamer and Hamer. The cracks in your image most closely resemble the images in illustration A, page 85.

”Crack A in biscuit and glazed ware:

The crack is probably the development of one that was latent at the raw stage. The stress at the raw stage may have ruptured the clay without visible signs at the surface. Such cracks are called latent or dormant cracks and no matter how careful the firing they are forced to develop. Mishandling of raw ware when packing a biscuit kiln may have induced the cracks, but the cause should first be sought as poor design, rapid drying, unequal drying, incorrect method and readsorption.” (Not a typo.)

My personal guess is that whatever happened had to do with water being reapplied at the wrong time somehow. It could be that you built too much weight on top of the piece too rapidly. The lovely sheen that you’ve built up on that surface makes me wonder if you used terra sig in your process. If you added it when the piece was drier than your clay wanted to be and then burnished, that might have caused hidden issues. Also, if you covered your piece the wrong way, adsorption can cause pieces to develop faults or even collapse in extreme cases.  

 

Edited to add: It wasn’t the firing that did the damage. The firing just revealed the damage.

 

Looks like a copy of the relevant pages is online: http://ceramicsfieldguide.org/pdf/materials-handouts/ClayCracks.pdf

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1 hour ago, analibertine said:

Thank you for your help! My second question is how would you fire pieces that are unglazed, how does a single firing work? Do you fire at a glaze temperature so cone 6 in my case?

A single firing is sort of like combining both firings in one in that I mean a bisque goes slowly : less than about 200 degrees per hour(Prox 10-14 hours) and ends at let’s say cone 04. In this time it has off gassed, and done significant changes in dewatering etc. so in essence if we just continued this speed firing to let’s say cone six  that would be a single fire. Sort of like adding a very slow glaze fire at the end  of a bisque fire. No need to really let it cool to decorate, it’s not getting decorated.

single fire is fine,  saves some energy for sure, it’s just a bit more difficult in a shared kiln environment for everyone to go that slow for a glaze firing. Glaze firings can go much faster, say 400 degrees per hour (prox 7-8 hours) because all the burnout and dewatering has already been done in the bisque. If it’s your own kiln, single fire on a bisque schedule to your finished cone is just fine and saves a fair bit of energy.

Edited by Bill Kielb
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5 hours ago, analibertine said:

Thank you Bill, I really appreciate the feedback! I was worried that they were removed from the kiln when they hadn't cooled down enough! How would you fire a piece like this - would you put it through just one firing instead?

I would first make sure not to overwork it, so adding any significant amount of water and smoothing would be a no go. For the smoothest this can be I would metal rib / burnish as smooth as practical definitely green but also realize that this will change when bisqued. Even if single fired.

I have sanded bisque (With appropriate protection)  to get things very smooth. Actually Jen McCurdy and husband  (Last I knew) spend lots of time sanding to perfect her look. https://jennifermccurdy.com/

I do not think the firing is likely causing these cracks though, especially if other wares in the same firing were fine.

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17 hours ago, analibertine said:

so you'd still recommend firing twice

Not necessarily. If your situation allows it, you have no reason not to. Just go really slow as if you were firing a bisque. The firing didn’t do the damage.

You can still use terra sig too, you just can’t put it on bone dry ware. I know there are some tutorials that tell you to do that, but they involve low fire clays that might have different tolerances for adsorption. PSH 519s seems like a pretty finely grained white stoneware.

I think your problems are arising from the bin bag being wrapped too tight. Most people associate drying slow with no cracks. But it’s drying evenly that’s more important. If you slow the drying down to the point where evaporated water is condensing on the surface of the piece and then being covered in plastic, that’s what’s causing your issues.

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