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I've recently had problems with dark rings or patches on my clay body, just underneath the glaze application, in a cone 6 glaze fire. It looks as though my clay body actually burned. 

My witness cones look great - just as they should. I'm firing to 2232. It's happening with my mid fire white stoneware as well. I have a fairly new kiln, and this hasn't happened before.

Could I be over firing even though my witness cones look good? My kiln is 2 years old and fired, on average,  once a month.  Do I need new elements already? 

Thanks for any advise! 

rsz_20200416_141406.jpg

rsz_20200416_141352.jpg

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Welcome to the Forum, Meghann...Does it make any difference where in the kiln these pieces sit? Are they right next to the elements or away from the elements? Are they all glazed the same or are you using different glazes? Are you mixing your own glazes or are you using commercial glazes? I have found that each of these situations could have an effect on the outcome of a particular firing. This may or not be related, but Glazenerd just posted some interesting pix of an experiment that he is doing on clay bodies here . I'm sure he and others will be weighing in on this to give you more info on your situation...

JohnnyK

 

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Glazes high in alkali metals can fume a clay like this. If your cones are reading a correct firing and this is happening on both white and brown claybodies I would be looking at the glaze recipe. Is this happening with all your glazes or just this one? If it's one you have the recipe for it might help if you post it.

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1 hour ago, JohnnyK said:

Welcome to the Forum, Meghann...Does it make any difference where in the kiln these pieces sit? Are they right next to the elements or away from the elements? Are they all glazed the same or are you using different glazes? Are you mixing your own glazes or are you using commercial glazes? I have found that each of these situations could have an effect on the outcome of a particular firing. This may or not be related, but Glazenerd just posted some interesting pix of an experiment that he is doing on clay bodies here . I'm sure he and others will be weighing in on this to give you more info on your situation...

JohnnyK

 

 Thanks for your reply! Right now it seems as if it happens anywhere in the kiln. One piece was close to the elements and the other piece was in the center of the kiln. I'm using commercial glazes - this is new albany brown (Amaco). Clay is mid-range oxidation from Continental. The burning effect happens on some and not (as much) on others. Can happen when other glazes are applied as well, which is why I wondered if it was an issue firing the raw clay body rather than a glaze issue.

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Meghann:

Min and Neil pointed out fuming from glaze and clay material- to which I agree. You clay body obtains its color (lt. brown) from iron disulfide. The iron disulfide and the fluxes in your glaze produces a gas as they melt- that gas is the source of "fuming" as Amin pointed out. You are not over firing by any evidence I can see. 

I do have two questions: you mentioned your white stoneware does this as well. Does this occur when you fire only white stoneware, or is it mixed in with this body? Second: you said this has not occurred before- is either the clay or glaze a new batch?  Can I assume that you have fired this clay with this glaze with no prior issue?  Is this the first time you have left some clay exposed when glazing? Do you have a kiln vent? Was it running?

Nerd

 

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18 hours ago, glazenerd said:

Meghann:

Min and Neil pointed out fuming from glaze and clay material- to which I agree. You clay body obtains its color (lt. brown) from iron disulfide. The iron disulfide and the fluxes in your glaze produces a gas as they melt- that gas is the source of "fuming" as Amin pointed out. You are not over firing by any evidence I can see. 

I do have two questions: you mentioned your white stoneware does this as well. Does this occur when you fire only white stoneware, or is it mixed in with this body? Second: you said this has not occurred before- is either the clay or glaze a new batch?  Can I assume that you have fired this clay with this glaze with no prior issue?  Is this the first time you have left some clay exposed when glazing? Do you have a kiln vent? Was it running?

Nerd

 

Thanks for your response! I guess I typically fire both white and brown clay bodies together - I'll test with just a load of white clay to see what happens. And no, I've been firing this combination for the last two years and although I shouldn't say it never happens - it has rarely happened to the extent that it is now. Over the last two years, there has been a very small ring where the glaze meets the clay, and I'm ok with that. I'm just not ok with the large patches that are happening so frequently now.  It is a new batch of clay, and over the last two years I've used two different batches of glaze as well (I'm not a full-time potter, clearly).  No kiln vent, the kiln is in a shed. 

Meghann

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Meghann:

Mined clays can go through subtle changes as they dig the deposits; but unlikely to cause radical differences. Stoneware bodies have been changing over from potassium to sodium fluxes: which off gasses a bit more. The more likely culprit is kiln pack density. The denser and more tightly loaded (kiln pack) your kiln is: the greater the likelihood fuming will occur. A glaze with high levels of cobalt blue for instance will stain (fume) adjoining pieces. Have you changed  how you load your kiln? Denser, closer together?

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On 4/17/2020 at 7:45 AM, Meghann said:

I've recently had problems with dark rings or patches on my clay body, just underneath the glaze application, in a cone 6 glaze fire. It looks as though my clay body actually burned. 

My witness cones look great - just as they should. I'm firing to 2232. It's happening with my mid fire white stoneware as well. I have a fairly new kiln, and this hasn't happened before.

Could I be over firing even though my witness cones look good? My kiln is 2 years old and fired, on average,  once a month.  Do I need new elements already? 

Thanks for any advise! 

rsz_20200416_141406.jpg

rsz_20200416_141352.jpg

These pictures seem to have some correlation with the movement of colorant  in the glaze above.

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17 minutes ago, Bill Kielb said:

These pictures seem to have some correlation with the movement of colorant  in the glaze above

I agree with Bill's observation.

Question to  Meghann:  Is the  it the darkness of the "unglazed areas" that is annoying?  or would you prefer a uniform darkness?  I ask, because I have been  spraying unglazed areas with  dilute baking soda water to create contrasts  between light and not-so-light areas.    The sprayed areas are a tad darker than the non-sprayed areas.   

As a trouble shooting exercise place some unglazed mugs nearby the glazed ones and carefully compare the unglazed mug with the glazed ones.    Also try washing a mug or two with clean water prior to glazing and see what the results are.  

LT 

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On 4/17/2020 at 8:32 AM, oldlady said:

looks perfectly OK to me.  what do you want to change?

There is another thread here that shows the fuming/flashing on the claybody, it's easier to see what's happening here than on the brown pots from the op but I believe it's the same issue.

@Meghann, you mentioned you have used two different batches of glaze but is it two batches of the same named glaze or a second totally different one? Also, what are the claybodies that it's happening with? 

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If you're using this glaze a lot could it be that the fuming materials have impregnated the kiln materials and so make this effect more likely.

Place a glazed pot next to an unglazed surface of another pot.

Do the same in a subsequent firing with a different glaze and see what the outcomes are. 

What cone are you bisquing to?

Less porous bisqye ware may stop the toasted edge...

But fuming is my bet.

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