I agree with Chris my throwing teachers told us to think compression, compression, compression while we were throwing and spent our first year throwing and cutting up cylinders and bowls. We only got to glaze three pieces at the end of the class. I had another professor that taught us to throw with just the water that was on our hands, no water on the piece and clay was fairly soft. He would throw two foot platters in a few minutes this way, it was a thing of beauty to watch him throw. Don't give up keep practicing and you'll get it. Denice (Wichita, KS)
Yep, after looking at the crack, which is quite familiar. I looked it up in Hamer's Potters Dictionary. Page 80 shows the crack occurring in greenware, and bisqueware, and glazeware. In your case since you had patched the work before or after the bisquefire-Hamer says in so many words that it is because of - floor being thinner than the walls when thrown, overly wet, or drying too quickly. If the crack occurs after bisque-Hamer says it was probably unnoticed in the greenware. If the crack occurs after the glaze, it is because of glaze fit, or too thin of a base where compression of glaze and clay comes into play.
If this is the case, I highly recommend that you refrain from using a sponge in the later stages of throwing. The sponge just puts too much water into the clay. At the same time slow your drying time of the pieces before you trim them. You have said that you use a rib to compress, but are you compressing or are you scraping the wall shape. Two different things. A rib laid at an angle on the clay will push the particles together, where as a rib held perpendicular will not.
Thank you so much everyone for the information!!!...I am guilty of several "wrongs" you listed. You have all been so helpful. Deb