Hi! I have been making pots for about 7 years and in the past 4 have struggled with small s-cracks appearing on the bottom of about 75% of my pots. The s-cracks are usually around a half inch long, thin, and only on the underside of the pot - never go all the way through and therefore the pots are all fully functional, just annoyingly cracked. In the photo attached, you'll see 3 cracked pots, and 2 planters that did not crack. My theory is that something is amiss in the center of my pots that causes stress & cracking during drying - because typically, when I make planters that have the centers removed, cracks do not appear.
The cracks don't show up to my knowledge til after the bisque fire (though because my studio fires my pottery, there are usually a few days that I'm not seeing the piece before it's loaded in the kiln.)
I have tried just about every remedy I can get my hands on and still can't manage to kick these cracks to the curb. My current regimen is:
1. Wedge each piece of clay (ram's head) 40x each.
2. Align clay on wheel so the spiral-y part is horizontal per this video: https://www.youtube.com/watch?v=f7oIZiXmXFU
3. Tap clay to center and use a finger to create a seal between clay and bat.
4. Add water, press down on the clay and get into flat hockey puck-ish shape.
5. Cone up and down three times. I make sure my cone is very cylindrical, with the bottom being almost as narrow as the top, which I read was important for preventing cracking. I make the top of the cone teeny-tiny and focus on getting air bubbles out of that portion.
6. During coning down process, I bring the piece back into a hockey puck shape and run my finger across the top towards the center, occasionally finding air bubbles there.
7. Bring out the walls and compress bottom religiously with my thumb, a kidney rib and my sponge. About 10 times, compressing from the outside of the base to the center. Occasionally, I'll feel little air bubbles during this step and will continue compressing until I've popped them.
8. Continue to compress and flatten bottom as I shape the piece. I don't let water sit in the piece.
9. Let the piece dry slowly and evenly on absorbent surface like canvas or foam (covered after a few hours). Dry with bottoms up once sturdy enough to stand.
Something I've noticed: Often when I use the wire to take pieces off the bat, there is a small air pocket right in the center of the clay that appears both on the remaining clay on the bat (picture attached), and on the piece itself. The indentation in the bottom center of the piece is never more than 1/16-1/8 of an inch deep I'd say. It's something I trim off later in the process, but seems to correlate with the pots that end up cracking.
A hypothesis: Could I be creating these air pockets during the coning up process? I.e. as I press the clay in & up the bottom air pocket forms? Maybe making my cone so cylindrical is part of the problem?
Another potential hypothesis: My wedging is bad and introduces too many air bubbles. Would I be better off using clay straight from the bag? (Or, sigh, trying to get better at wedging?)
Last note: I prefer a flat bottom to a footed bottom. It's possible I get less cracks when I trim feet, but I haven't really investigated.
Any advice you have on ending this years-long struggle would be so, so appreciated!
Bat with Air Bubble