Glaze Firing Problem

[DMCosta]

I am a ceramics teacher as well as artist myself and today when I emptied the kiln I noticed a glaze defect that I have never seen. (or at least to this capacity) My students made tumblers, and every red earthenware clay tumbler had large bald spots where the raw clay was exposed. (some the size of a lime or larger) This was a glaze firing so I was upset to see this had happened to many students in the final stage. This defect only happened to the red clay pieces, not the gray, which were fired in the same kiln. I used Amaco LG glazes, and It also occurred in different glaze colors. I was on my last kiln firing for this project, I had fired many other tumblers this month both red and gray clay with success. The kiln had been having problems where the fuses had to be replaced but I then re-fired the pieces and used an orton cone to ensure it reached temperature. Anyone have any idea why this had happened and what can be done to prevent this from happening again in the future?

[Lucille Oka]

For this to have happened when all other firings were good means something happened to the ware either in the firing or after the bisque ware was waiting to be glazed. It sounds like there could have been an oily residue on the bisque. Was there an inordinate amount of handling of these vessels after they were bisque fired and were they possibly handled with oily hands or oily wiping sponges? Could oil from any source have gotten on these vessels? To what temperature were they bisque fired was it too high or were they placed too close to the elements causing hard spots on the bisque? Those are some questions you can ask yourself about the ware.

 

Also one tip when students come to glaze have everyone wash their hands before handling the bisque ware and restrict the use of lip glosses, chapsticks, lotions, notions, potions and balms before handling ware to be glazed.

 

You can try reglazing the bald spots.

[bciskepottery]

"The kiln had been having problems where the fuses had to be replaced but I then re-fired the pieces and used an orton cone to ensure it reached temperature."

My first thought was dust or oil on the bisque surface prevented the glaze from fusing to the tumbler, as suggested by Lucille. But I kept coming back to the sentence above which suggests that the tumblers were being fired when the kiln had problems leading you to replace the fuses and then re-firing, and in the re-firing you had the glaze fusion problem. If that is correct, I am wondering if something happened during the re-fire that affected the red clay, which is different in composition than the white clay. Did the kiln cool completely between the initial firing and the re-firing? Was the re-firing faster than the previous firings -- perhaps because the kiln was already heated? Did you visually check the tumblers in the kiln before starting the re-fire? Was there any evidence of glaze on the kiln shelves -- like it splattered off the tumbler onto the shelf? Lots of questions but there are lots of variables to consider.

[DMCosta]

Thank you both for replying to me post. Initially I did think that it could have been some dust or oil that caused these pieces to not take the glaze application but they weren't really handled excessively or had any oils around the room. I do things the same way the whole year and have never had this issue. When I re-fired the pieces, the kiln cooled completely and I did a visual check before re-firing a second time. I also fired to the same temperature both times. This defect will continue to stump me a bit, but thank you very much for the replies, it's something to think about.

[bciskepottery]

Just as a test . . . take a glazed red earthenware piece and re-fire it in your next load and see if it develops the same problem.