Hello all! I am a potter of several years but know that I still have a lifetime of learning to do. I have a very particular aesthetic I try to accomplish with my mugs-- essentially they are liner glazed interior with a thick coating of multiple glazes on the exterior. Many potters do this, amiright? I have been creating these intricate glazes myself for 3 years now, without issue. It seems like all of a sudden (last 4 months) I have had customers come to me saying that their piece has been fine during multiple uses, when all of a sudden they pour their hot liquid in, get a ping, and a horizontal crack appears on the vessel moving along in a ring. To me, this sounds like thermal shock dunting. Right now, I am feeling lost among the vast amount of variables and don't know where to begin trying to solve the problem. I will list all of the variables/info I have collected, and hope someone out there can tell me where to focus my attention. I appreciate this community, immensely-- Thank you in advance!!
-I fire with Laguna Bmix 5 and Standard 225. Both mature at cone 6. I fire to cone 6. Majority of issues have been with the 225, with one or two bmix pieces. I worry it is a clay body issue.
-My firing cycle is quite standard and the same each time. I fire at a medium rate with no slow cool (I can provide the schedule if needed).
-I throw my mugs thin.
-My liner glaze is always the same. Black Licorice, Cone 6 (Mastering Cone 6 glazes). Recipe below. I do add Epsom Salt to this to prevent settling.
-The exterior glaze always has a bit of the glossy liner glaze above and a matte as the undercoat. Then-- there are SO MANY different glazes I use from there in my laying techniques-- application and amounts differ from piece to piece.
-Each time the issue has happened with a mug, the glaze has been thick and the issue arose when the customer added freshly boiled water. My glaze is alwayyyys thick on the exterior though...
What does everyone recommend? If I had my biggest wish come true, there would be an adjustment to the Liner Glaze (better COE) that would allow a more balanced thickness of glaze overall in the piece. Can you see an adjustment I can make in the recipe below? Is there a better black recipe for this?
Or, Should I throw thicker to ease the tension of exterior vs interior glazing?
Orrrr... Am I missing something entirely, hah!
Ferro Frit 3134
Total base recipe
Red Iron Oxide
Cobalt Oxide 2