Saturday, April 09, 2011
Only When I Have a Deadline
I use a finicky but lovely clay (Standard 240). Its a stoneware but it is almost a half porcelain. Last year I noticed a lot of cracks on the bottoms of mugs and bowls. Simple remedy - dry them upside down in their humidity shelf. Problem solved. This February I was invited to be a show and asked to provide larger pieces. No problem, right? Big problems. For the last 2 months. I kept noticing cracks along the sides just directly above the foot. This was only happening in the pieces with wide bases. I assumed it was one of 2 issues: drying time/method or compression during wedging/throwing.
My hunch was compression so I looked up some tips for throwing large porcelain pieces on the internet. I discovered one remedy was to center the clay, cut it off the wheel, flip it over and re-center thus evening compression throughout the clay. 2 of the 3 pieces like this exploded, of course therefore I only had 1 survivor. It was fine when I took it out of the kiln. A day later I had come back to find it had cracked.
I decided to explore 3 new options all to be fired in the same bisque.
1st. experiment: Use the 182 clay. Slightly grogged and not as pure.
2nd. experiment: 240 with a small percentage of fine grog wedged in.
3rd. experiment: 240 with a large percentage of fine grog wedged in.
My initial concern with using grog was that I like the smooth surface and it suits my glaze palette and aesthetic. In test #3 I could definitely feel it when I threw so I wasn't sure how I would like it once fired. After the bisque firings I learned that it was indeed the clay and not how I was compressing it during the throwing process. The 182 had the highest success rate next to the heavily grogged 240 clay. Some of the light and heavy grogged pieces survived but the 182 did not have an issue with a single piece. After 2 months of struggling it is clear to keep both types of clay hand to use depending on the size and diameter of the piece.
On the bright side after throwing so many large platters and bowls I am now REALLY REALLY good at it!