lin_c Posted September 14, 2021 Report Share Posted September 14, 2021 Hello everyone, I have a problem with one new glaze I want to use and I could use some advice on how to fix this problem. I bisquefired at 800, than wiped the plate, than poured the glaze in and emptied the plate right away. Looked perfectly even. I've let it dry, fired it at 1160 with a soak of 30 minutes. The maximum temperature of the glaze is 1170. When I got the plates out of the kiln, all of them had round empty circles with something in the middle that seem to be a grain of overfired glaze. Why is this happening? What am I missing? I do not think it is crawling because I've diluted the glaze and the same thing happened. I've also used another type of clay and the result was the same. The glaze was sieved when I've mixed it water. It is a commercial glaze, I do not understand glaze chemistry although I wish I do. Please let me know if someting similar happened to you and how did you solve the problem. Thanks! Link to comment Share on other sites More sharing options...
Babs Posted September 14, 2021 Report Share Posted September 14, 2021 800C quite a low bisque, prob too low More usual to bisque cone 04 1060degC or cone 06 1000degC. Not sure if this will affect the result. Are all the circles in the inside of pots? Wipe your pots with a damp clot befire glazing. A 30 min soak will bring the heatwork done to way over 1170 in my op. Very frustrating for you Link to comment Share on other sites More sharing options...
Hulk Posted September 14, 2021 Report Share Posted September 14, 2021 Are the circle marks open to the clay? Looks like gas from the clay. Are there more marks where the clay is thicker/thickest? Per Babs suggestions, try higher bisque (with adequate oxygen), fire to target without overfiring. Link to comment Share on other sites More sharing options...
lin_c Posted September 14, 2021 Author Report Share Posted September 14, 2021 2 hours ago, Babs said: 800C quite a low bisque, prob too low More usual to bisque cone 04 1060degC or cone 06 1000degC. Not sure if this will affect the result. Are all the circles in the inside of pots? Wipe your pots with a damp clot befire glazing. A 30 min soak will bring the heatwork done to way over 1170 in my op. Very frustrating for you Hi Babs, I'll bisque at higher temp next time. I was in a rush now and since the plate is only glazed on the inside I risked. The circles are not from gas because they are glazed with glossy glaze. Now that you mentioned it, I think the end temperature+ 30 min. soaking might be a problem. The point in the middle of the circle looks like burnt blue glaze pigment crawled together but something transparent from the glaze covers the circle. I have a last glazed but unfired plate that I will fire lower, at 1130 deg C with a soak of 30 to see what happens. Will let you know. Thank you so much for your time! Link to comment Share on other sites More sharing options...
lin_c Posted September 14, 2021 Author Report Share Posted September 14, 2021 34 minutes ago, Hulk said: Are the circle marks open to the clay? Looks like gas from the clay. Are there more marks where the clay is thicker/thickest? Per Babs suggestions, try higher bisque (with adequate oxygen), fire to target without overfiring. Thanks Hulk! Overfiring might be the problem and the blue in the glaze is more affected by high temperature. The circles are not gas since they are still covered in glaze left transparent. Gas circles make deep holes in the glaze, at least in my experience. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted September 14, 2021 Report Share Posted September 14, 2021 Is there a silica based grog or chamotte that’s common in the clays in your area? Crushed granite or feldspar or something? The glaze looks like a boron based floating blue of some kind, and if there’s enough active flux in the glaze, it could theoretically react with any silica or feldspar bits to leave that glassy spot. It could also be a little over fired. If the top end for the glaze is 1170, and you fired to 1160 plus 20 minutes, it’s possible the heat work pushed it too high. Link to comment Share on other sites More sharing options...
lin_c Posted September 15, 2021 Author Report Share Posted September 15, 2021 12 hours ago, Callie Beller Diesel said: Is there a silica based grog or chamotte that’s common in the clays in your area? Crushed granite or feldspar or something? The glaze looks like a boron based floating blue of some kind, and if there’s enough active flux in the glaze, it could theoretically react with any silica or feldspar bits to leave that glassy spot. It could also be a little over fired. If the top end for the glaze is 1170, and you fired to 1160 plus 20 minutes, it’s possible the heat work pushed it too high. 73% SiO2 in the clay, 40% chamotte. I will test the overfiring but let me know if you think there is a chance these 2 will never work because of the silica. Thanks! Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted September 15, 2021 Report Share Posted September 15, 2021 @lin_c I think the spots are coming from an impurity of some kind, because there’s a black speck right in the centre of each. The usual place to look for something like that here would be the clay body. If the impurity is silica or feldspar of some kind, it could explain why the spot is glassy instead of bare. One way of testing that theory would be to try the glaze on a different clay. Link to comment Share on other sites More sharing options...
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