Close Inspection Reveals Crawling is a Crack

I've fired nine foot-soaking bowls, and I thought I had a minor problem with some crawling of the glaze around the joints. I figured it was some dust left behind after sanding some rough spots off the green pots before glazing. So this last firing I really hit the pots with some pressurized air before I glazed.

After I unloaded the kiln today, there was more "crawling" than ever, so I thought I'd take pictures and blog about it, maybe get some advice from other potters. I took some macro shots and when I blew them up to 100 percent with Photoshop, I realized that the crawling was actually due to my join coming apart. It's not a crack that goes all the through the wall, just a fissure of sorts.

So, now I've got about 20-some bowls that I've finished and are waiting to be fired, and I'm thinking I might try to bisque fire a load before glazing to see if they'll do better. That's a lot of work, so I want to figure out if I can salvage them. I've been raw glazing with a spray gun.

I've still got to make another 130 bowls, so I have to start thinking about what part of the process to change to keep this from happening again. I've been drying them slowly, and when the bottoms don't show any more color change from wetness, I set them upside down outside in the sun to let them fully dry.

My firings are slow, between 14 and 15 hours to cone six with an hour soak at 250 degrees F. and 1,000 degrees F.

I throw the bottom of the bowl (a platter shape) and the top of the bowl (a bottomless bowl) and join them at the leather hard stage, scoring and slipping each section, tapping the sections together lightly with a tool and putting a coil on the inside and the outside.

There's so many possible causes and solutions. Which one will I find? Which one will help me?

Mary says we're having a technical difficulty. We need to investigate the fault, and find a solution.

What I'm thinking right now is I'm possibly using too much slip when joining. I'm scoring each section and applying two coats of relatively thick slip on each section. Once joined, I brush the joint with slip, not to apply more but to smooth it over before I tap it with a tool. Maybe I need to do more tapping.