Abby Heingartner Posted July 11, 2019 Report Share Posted July 11, 2019 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! Link to comment Share on other sites More sharing options...
liambesaw Posted July 11, 2019 Report Share Posted July 11, 2019 And a nice slow 108f/hr ramp from 1200-1700f for those dark clay bisques to get rid of the organics in the clay (sulfides mostly). Link to comment Share on other sites More sharing options...
Magnolia Mud Research Posted July 11, 2019 Report Share Posted July 11, 2019 I have had success by wetting the bisqued pot before applying the glaze. The pre-wetting water helps the glaze slurry to fill the pores of the bisque surface. Try wiping the surface with a wet sponge just prior applying the glaze. LT Link to comment Share on other sites More sharing options...
Denice Posted July 11, 2019 Report Share Posted July 11, 2019 I bisque at 04 when I work with red for dark brown clay, wiping the pot with a wet sponge also helps. Denice Link to comment Share on other sites More sharing options...
Babs Posted July 12, 2019 Report Share Posted July 12, 2019 That glaze almost looks underfired. Definitely doesn't look a high gloss. Unfired is there any pitting? Link to comment Share on other sites More sharing options...
neilestrick Posted July 12, 2019 Report Share Posted July 12, 2019 Looks more like a glaze problem than something to do with the clay. Can you post the glaze formulas? Link to comment Share on other sites More sharing options...
Abby Heingartner Posted July 14, 2019 Author Report Share Posted July 14, 2019 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. Link to comment Share on other sites More sharing options...
S. Dean Posted July 15, 2019 Report Share Posted July 15, 2019 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. Link to comment Share on other sites More sharing options...
Abby Heingartner Posted July 15, 2019 Author Report Share Posted July 15, 2019 Ah, I didn't realize that was the only difference between 710 and 266. Thanks for the info! Link to comment Share on other sites More sharing options...
Bill Kielb Posted July 15, 2019 Report Share Posted July 15, 2019 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. Link to comment Share on other sites More sharing options...
Min Posted July 15, 2019 Report Share Posted July 15, 2019 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? Link to comment Share on other sites More sharing options...
Abby Heingartner Posted July 16, 2019 Author Report Share Posted July 16, 2019 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. Link to comment Share on other sites More sharing options...
Bill Kielb Posted July 16, 2019 Report Share Posted July 16, 2019 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.... Link to comment Share on other sites More sharing options...
neilestrick Posted July 17, 2019 Report Share Posted July 17, 2019 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. Link to comment Share on other sites More sharing options...
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