Dave K Posted September 19, 2016 Report Share Posted September 19, 2016 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? Link to comment Share on other sites More sharing options...
Babs Posted September 19, 2016 Report Share Posted September 19, 2016 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? Link to comment Share on other sites More sharing options...
neilestrick Posted September 19, 2016 Report Share Posted September 19, 2016 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. Link to comment Share on other sites More sharing options...
Mark C. Posted September 20, 2016 Report Share Posted September 20, 2016 Whats kind of clay body is it? stoneware? Link to comment Share on other sites More sharing options...
Dave K Posted September 20, 2016 Author Report Share Posted September 20, 2016 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. Link to comment Share on other sites More sharing options...
neilestrick Posted September 20, 2016 Report Share Posted September 20, 2016 240 is a tricky clay body. It llkes to crack when uneven. Link to comment Share on other sites More sharing options...
JohnnyK Posted September 20, 2016 Report Share Posted September 20, 2016 You might try cutting a few of your pots to check the cross section and the thickness in the critical places... JohnnyK Link to comment Share on other sites More sharing options...
GEP Posted September 20, 2016 Report Share Posted September 20, 2016 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. Link to comment Share on other sites More sharing options...
Min Posted September 20, 2016 Report Share Posted September 20, 2016 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) Link to comment Share on other sites More sharing options...
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