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Abby Heingartner

Pitting issue with dark brown clay

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Hi there,

I've been having issues with pitting with one particular clay/glaze combination I use. It's a dark brown clay (Standard Ceramic 710), with a white glossy glaze, fired to cone 6 in an electric kiln. I've attached a photo of the result for reference. This is not the first time this has happened--there's always some amount of pitting with this combination, but this is one of the worse examples. I have used the same glaze with my other clays (a speckled tan and a red clay, both also fired to cone 6) without any issues, so I suspect it's the clay body giving me problems. I have had a little bit of pitting with other glazes used on this clay, but it is most apparent with the white. I bisque fire to cone 08. 

I'm wondering what the best way to go about correcting this problem would be. Should I be bisque firing to a higher temperature? Thinner glaze application? (I don't usually apply it very thickly, but I definitely could water it down) Changing the firing cycle? Any advice would be very welcome. 

Thanks! 

IMG_7489.jpg

Edited by Abby Heingartner

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Have never worked with that particular clay - but you might want to start with higher bisque-fire.  The Standard  ^6 clays I have worked with generally recommend ^04 bisque.

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Thanks everyone for your replies! I'm afraid I don't have the glaze formula--it's the studio white at the community studio where I glaze and fire all my work. It definitely isn't underfired--I had a cone pack in the firing and cone 6 was all the way down. I also had other pots in the same firing that were a different clay and glazed with the same white, and the came out totally fine. @Babs, when you ask if there was any pitting when it was unfired, do you mean in the clay itself or the glaze after it's been applied? I did rub over the surface of the glaze with my fingers to try to smooth out any imperfections, and that usually works fine. 

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710 is the grogged version of 266.  There's lots of info here on 266 and  when you search the forum you will find it that is a beautiful clay that can be temperamental.  266 prefers a higher bisque  than 08 (try 04).  Although Standard calls it a cone 4-6 clay they recommend firing it at cone 5.  Frequently bloats at Cone 6. 

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18 hours ago, Abby Heingartner said:

Thanks everyone for your replies! I'm afraid I don't have the glaze formula--it's the studio white at the community studio where I glaze and fire all my work. It definitely isn't underfired--I had a cone pack in the firing and cone 6 was all the way down. I also had other pots in the same firing that were a different clay and glazed with the same white, and the came out totally fine. @Babs, when you ask if there was any pitting when it was unfired, do you mean in the clay itself or the glaze after it's been applied? I did rub over the surface of the glaze with my fingers to try to smooth out any imperfections, and that usually works fine. 

If that is supposed to be a gloss glaze it definitely looks underfired. Does it ever have a gloss finish? See below.

81EA9AC8-DDE2-4C05-AFAA-AB8640EA3496.jpeg

Edited by Bill Kielb

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22 hours ago, Abby Heingartner said:

I had a cone pack in the firing and cone 6 was all the way down. I also had other pots in the same firing that were a different clay and glazed with the same white, and the came out totally fine.

Cone pack on the same shelf as this pot? Some single zone or manual kilns can fire very unevenly. Could you post a picture of what the glaze is supposed to look like? I agree with Babs and Bill that it doesn't look like a gloss in your picture above.Could be a combination of things, too low a bisque and/or a bisque that wasn't vented well enough plus application plus firing a bit cool. Do you have the option to change the bisque cycle to one that works better for dark claybodies?

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It's a digital kiln--I don't remember which shelf the cone pack was on, but as far as I know this kiln usually fires pretty evenly. This picture is of the same glaze on the speckled tan clay--maybe not "high" gloss, but definitely towards the glossy end of the spectrum.  

I do have the option of doing a higher bisque, although it won't be convenient. I think that's where I'm going to start. 

IMG_6847 copy.jpg

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It’s a semi gloss! Just kidding, looks much better on that body. Since you have no control over the glaze formula, higher temperature bisque seems to be the  logical choice.

If it’s a packed bed porosity issue then using the wet sponge prior to glazing may help. I would suggest trying one of each on two pieces of your new 04 bisque just to be sure the best way to glaze it for future reference. Bisque to 04 will close this body off more a bit when glazing. 

As to slightly under-fired, it does not seem like you will have precise control over that in the future anyway. Kilns can fire unevenly depending upon loading, time, control etc....

Edited by Bill Kielb

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We've used a fair amount of the 266 in my studio, and it does all sorts of odd things to glazes. Some of our glazes are unrecognizable, some look underfired, some change for the better, and a couple of the gloss glazes look about the same. The satin and matte glazes seem to change the most.  The 266 doesn't like to be pushed past cone 5, but the 710 is supposed to handle cone 6 a little better because the grog makes it slightly more refractory.

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