Causes For Cracks

[Dave K]

I have recently been making 8-9 pound pots. I am having a problem with the bottoms cracking. I think I am compressing the bottom correctly and getting any excess water out. The last two to crack seemed totally dry before bisque firing and I do a 3 hour preheat. I don't think the bottoms are overly thick or thin. One was starting to show a small crack before bisque firing and the other one nothing. Coming out of the bisque firing the were both cracked enough to see light through them. I am using stoneware. Does anyone have thoughts to what I am doing wrong?

 

[Babs]

Are the bottom of your pots the same thickness as the walls of your pots?

Are you flipping them as soon as the rims are dry enough to support pot.

Uniformity of drying?

[neilestrick]

It's a thickness issue usually. If the bottom of the pot isn't close to the same thickness as the walls you'll get cracking. Also make sure the bottom is uniform thickness throughout. If the bottom on the inside curves upward, the bottom on the outside must also curve upward. Also make sure the wall near the bottom isn't too thick. All those variations can cause uneven drying and cracking.

[Mark C.]

Whats kind of clay body is it? stoneware?

[Dave K]

I use Standard 240. Neil, you are probably correct about the same thickness throughout. I am not use to throwing that much clay and am struggling in most aspects of it. I will have to keep trying and be more aware of the consistence of overall thickness. Thanks to everyone for the help.

[neilestrick]

240 is a tricky clay body. It llkes to crack when uneven.

[JohnnyK]

You might try cutting a few of your pots to check the cross section and the thickness in the critical places...

JohnnyK

[GEP]

Before trimming, take a needle tool and measure the thickness of your bowl before trimming it. Measure it right in the center of the bottom, and in the area where you plan to establish a foot ring. Just knowing this in advance gives you a much better chance of getting even thickness when trimming. Don't worry, the act of trimming will seal up the needle holes. I have been throwing for 20+ years, and am now a full time professional, and I still do this when trimming large bowls.

[Min]

Another way slightly different to get even bottom thickness is to make one of these depth measuring pin tools. Tap a sewing needle or pin into the bottom of a hardwood dowel, chopstick, paintbrush handle, whatever, then snip it off with side cutters to the depth you want the base. For wide items you use it in a few spots across the base. (this one is for a no trim base on wide pots, the needle is fine so it's kinda hard to see)