Glaze cracking bottom of pots

[Bill Kielb]

@Babs

Hats off to you!

When I met Jennifer McCurdy and she said, Bill I don't know about you but I find it easier on the insides of my vessels to begin ribbing from the top and moving downward.  At that point I had an aha moment so we decided to test.

The end result, we immediately got 10 students previously incapable of throwing a round uniform bowl now able to make one that was leaps and bounds above their previous ability. Guesss what else - no cracks!

PS- when I misjudge how thick to leave the top of my bowl for a given final diameter, I sometimes roll  the edge over to finish the project as planned with slightly thinner walls. Not the best but often saves the project.

So hats off, you found an effective way and as a compression believer I am certain this likely works well for you.

[Babs]

Thanks and we are compressing on every pull up also imo

[neilestrick]

When stretching bowls, I always work from the top down not because of the compression factor, but for stability. Here's what I mean: The more vertical a form is, the more stable it is (less likely to collapse). If you stretch/widen a bowl from the top down, then the area below your rib is always more vertical than where your rib is pushing, and therefore more stable. If you work from the bottom up, the area below your rib is always more horizontal than where you're pushing, and therefore more likely to collapse, especially because as you push outward with the rib there is also some downward pressure. I disagree that this will prevent cracking, though. It may help, but uneven walls, or walls that are too thick, can crack regardless of how you stretch.

[Bill Kielb]

2 hours ago, neilestrick said:

When stretching bowls, I always work from the top down not because of the compression factor, but for stability. Here's what I mean: The more vertical a form is, the more stable it is (less likely to collapse). If you stretch/widen a bowl from the top down, then the area below your rib is always more vertical than where your rib is pushing, and therefore more stable. If you work from the bottom up, the area below your rib is always more horizontal than where you're pushing, and therefore more likely to collapse, especially because as you push outward with the rib there is also some downward pressure. I disagree that this will prevent cracking, though. It may help, but uneven walls, or walls that are too thick, can crack regardless of how you stretch.

No worries, this was originally about S cracks in the bottom. My post was to get some folks thinking about compression and what it really means to them and the benefit of looking at it from a strength and materials viewpoint as well as some aspects in pottery that are counter intuitive.  All of these methods are generally positive or good practice. Some will likely benefit from the thought and exploration.

[Babs]

Thanks and we are compressing on every pull up also imo

Neil the cracking I hope to avoid by opening downward is the cracking which can occur at the rim because of the overstretching at the rim of bowl.

Your explanation above makes fine sense.

I have more control doing it the way I do.

TheOP 's pots were too thick and uneven  as I stated before.