elisethomas Posted November 20, 2016 Report Share Posted November 20, 2016 I use these 2 commercial glazes together on a regular basis. Just recently, I've started having a problem with the 2 glazes developing this ugly black line where they meet. My guess is that is has something to do with kiln atmosphere? Note sure, though and I haven't been able to do anything to remedy it with the last few batches. I've tried vacuuming the kiln and removing an addional plug with no luck so far. These are fired at cone 06 in an electric kiln. I also use the kiln for fused glass and firing decals, so I'm a bit worried about contamination as well. Any ideas? Link to comment Share on other sites More sharing options...
Pres Posted November 20, 2016 Report Share Posted November 20, 2016 Pictures in something like this would help with the assessment of the problem, Elise. best, Pres Link to comment Share on other sites More sharing options...
elisethomas Posted November 20, 2016 Author Report Share Posted November 20, 2016 Oh! I thought it had attached, let me try again. Thanks Link to comment Share on other sites More sharing options...
Bob Coyle Posted November 20, 2016 Report Share Posted November 20, 2016 My guess is that there is a reaction taking place between the two glazes. Looks like the bottom may be a copper based glaze and the reaction may be metallic copper being formed because of some reduction condition taking place... of copper oxide being formed because the top glaze is more basic than the bottom. This is just a guess. Could be caused by just a contaminant in the glaze. If you are firing electric, then your atmosphere is probably oxidizing. Run a test tile with both of the glazes mixed together and see if the whole thing turns black. PS... I hope those bowels are not being used for food. Link to comment Share on other sites More sharing options...
Min Posted November 20, 2016 Report Share Posted November 20, 2016 PS... I hope those bowels are not being used for food. Ummm...... Link to comment Share on other sites More sharing options...
Babs Posted November 20, 2016 Report Share Posted November 20, 2016 Haggis? Link to comment Share on other sites More sharing options...
elisethomas Posted November 20, 2016 Author Report Share Posted November 20, 2016 It does look like a reaction, but I use these 2 glazes in combo all the time, been doing so for years. This problem just developed within the past month.... No, definitely not using these for food, they are decorative Should I be concerned about fusing or 22K gold decals in the kiln causing this? Thanks for the thoughts! Link to comment Share on other sites More sharing options...
glazenerd Posted November 20, 2016 Report Share Posted November 20, 2016 What kind of clay? Link to comment Share on other sites More sharing options...
Pres Posted November 20, 2016 Report Share Posted November 20, 2016 Is it my imagination or is there also some dark coloration in the rt upper corner in the white semi matte glaze also? best, Pres Link to comment Share on other sites More sharing options...
glazenerd Posted November 20, 2016 Report Share Posted November 20, 2016 No Pres: I saw it, as well as a splotch below the copper band. If the glaze maker used dolomite in their clear/zirco white matte: was thinking it is magnesium pulling. Nerd Link to comment Share on other sites More sharing options...
Min Posted November 20, 2016 Report Share Posted November 20, 2016 Could you post how you bisque and glaze fire re length of time / ramping / if and how the kiln is vented / when you put the peephole plugs in if you don't use a vent plus how densely you pack the bisque. Also, are you using lustres or just gold decals? Link to comment Share on other sites More sharing options...
Guest JBaymore Posted November 21, 2016 Report Share Posted November 21, 2016 Are you firing raw clay to bisque in the same 04 firing that the glaze is being fired in? Is all the work in the kiln YOUR pieces... or do others have stuff in there also? Are you or anyone else using paperclay in these firings? best, .............john Link to comment Share on other sites More sharing options...
elisethomas Posted November 21, 2016 Author Report Share Posted November 21, 2016 These are slip cast, low fired to cone 04, I am unsure of the slip formulation. I don't feel like the clay body is the culprit as I've not had this problem with the same clay until very recently. Guess that's why I'm thinking kiln atmosphere. There is also a dark patch in the upper right, basically this flaw is subject to occur any place where the two glazes meet. (The white may appear matte due to a lighter application in that spot as it is brushed on, not dipped.) I'm only using gold decals, no lusters. My glaze fire is to cone 06, Med speed, not too densely packed, usually only fill the kiln about 1/3 full on 1 1/2 full shelves. One peephole out. No vent. It is digitally controlled and I haven't verified temps with cones recently. ( I did when first starting out with this kiln.) Link to comment Share on other sites More sharing options...
elisethomas Posted November 21, 2016 Author Report Share Posted November 21, 2016 Are you firing raw clay to bisque in the same 04 firing that the glaze is being fired in? No Is all the work in the kiln YOUR pieces... or do others have stuff in there also? All mine Are you or anyone else using paperclay in these firings? nope Link to comment Share on other sites More sharing options...
Mark C. Posted November 21, 2016 Report Share Posted November 21, 2016 Gold decals? Are they on paper backings that burn away?Are there a lot in same kiln load or just a few? That glaze is crazing a lot as well. Looks like a ring holder or candle holder?Just a guess. Link to comment Share on other sites More sharing options...
elisethomas Posted November 21, 2016 Author Report Share Posted November 21, 2016 Yes, Decals are on paper backing. But they are tiny, I just use them to add initials and dates to some pieces. Only one or two at a time. The glaze is a commercial crackle, meant to do that. Yep, ring holders Link to comment Share on other sites More sharing options...
Min Posted November 21, 2016 Report Share Posted November 21, 2016 How are you packing and firing the bisque or are you single firing? How is the kiln vented during both bisque and gold decal firing? Any chance it's just a brush contamination issue? (long shot I know) Link to comment Share on other sites More sharing options...
Guest JBaymore Posted November 22, 2016 Report Share Posted November 22, 2016 Looks to me in the photo like maybe inadvertent carbon trap. Trying to find the source of the reduction atmosphere. (Mark caught my drift there. ) Clay body change with higher organics could be it. Lots of decal paper. Lots of wax or latex. No active vent is a potential contributor. best, ...............john Link to comment Share on other sites More sharing options...
elisethomas Posted December 1, 2016 Author Report Share Posted December 1, 2016 I think you're right, John. I've done a bunch more test pieces - new glaze, new bisque, new brushes, increased drying time. Same issue. I'm going to look into adding a vent next. If it's reduction, is there any way of removing it one the kiln atmosphere is remedied? Link to comment Share on other sites More sharing options...
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