CoveArt

Hello Everyone, I'm 100% new to this forum but not new to teaching and firing ceramics. I just moved to this school however and I'm running into problems I've never run into before when firing some of my students work. My understanding of the materials I've inherited is that the teacher before me didn't really use them in their few years here but the teacher before them was here for years and used them all the time. I have 2 ceramics classes this semester (grades 9-12) and so we've been doing the basics. We've been using a low fire clay body in both red and white and these issues oddly enough have only been appearing on the white. I will do my bisque firing at cone 04 and then the students are able to use either 05 or 06 glazes and then I'll do 2 glaze firings. Both are brands I've not had a lot of interaction with before but it's what we've got and as we all know, public schools gotta use what they've got. What I'm noticing is some of our cone 05 glazes are flaking off. Some a lot, some a little, and (for some students who use the same color in the same firing) not at all. It doesn't happen to everyone in every firing but it's happening A LOT to some pieces and I feel for the kiddos as they were so looking forward to seeing a completed piece only to have it flake off. I'm noticing it more with certain colors so I'll be asking those kiddos which ones exactly they used (we've got 3 types of purple in the same brand so I'm not sure which is which on their pottery). What finally sent me over the edge is when a students piece practically completely flaked off and then a few days later when I picked it up to move it, it simply broke in half (see attached picture). I will freely admit, I'm no ceramics expert. Before this school I was K-12 and was teaching everything from Kinder-basics up to 12th graders on the wheel. I've been a jack of all trades but master of none at my small town school so I'm out of my depth at this point. I want to be able to give the kiddos an answer when they ask me what's wrong other than the 4 or 5 guesses I've got. Clay Brand and Type: Continental Clay, Low Fire Clay (upon googling looks like this says bisque 03 gives best results so this may be my issue right there) Glaze Brand and Type: Spectrum Glazes Inc., Low Fire Glaze (says fire to cone 05/04 but we have other 05's so I do them at the same time) Kiln Brand: Skutt Automatic Kiln Firing Cycle (bisque): 2 hour preheat, medium speed, cone 04 Firing Cycle (glaze): medium speed, cone 05