The is an email I send out when people’s pots are too thick.
Subject: Trimming Issues
I literally pick up every pot that is made at Fireborn three times, because I put your pots under plastic when they are leather hard, and I load the bisque and glaze kilns. I noticed some of the pots you trimmed were too thick to fire. Your instructor must have overlooked them. So this does not happen again, I am contacting you directly, right away. Please ask your instructor for help with this issue. (Please note this is a generic email I send whenever I find over weight and thick pots).
The wall thickness of pots should be 1/4 of an inch or less. Usually, the bottom of a thrown pot is slightly thicker than the top. The maximum thickness allowed at Fireborn is 1/2 inch. You have three opportunities to achieve this thickness.
1. Throw the pot so it is light and has thin walls. This is difficult and beyond the ability of most beginners, so don’t feel badly if you can’t do this.
2. After throwing the pot, and before you cut it off the bat, take a wooden tool or a metal trim tool and cut away excess clay from the sides of the pot near the bottom. You can do this.
3. When the pot is leather hard, trim it to the proper thickness. You can do this too.
You can show your instructor your trimmed pots and ask for help.
The reason we don’t fire thick pots is that they might explode in the bisque kiln with enough force to damage other people’s work. Furthermore, flying shards of pottery can land on the kiln controller and cause the kiln to over fire and ruin all the pots in the kiln, and even possibly damage the kiln. These are risks we simply won’t take.
The cause of explosions is that, even when a pot is bone dry, there is still some moisture in the clay. As the pots heat up, that moisture needs time to escape. If the internal temperature of the pot reaches 212 degrees before the moisture has escaped, the moisture turns to steam, expands rapidly, and BANG, your pot explodes with amazing force. The thicker the clay is, the more time it takes for the moisture to escape. We fire on a 12-hour cycle and 1/2 inch wall thickness is the maximum allowable. You might note that bricks are several inches thick and made of clay. So how are they fired? The answer is “very slowly”, on a 60-hour cycle, which gives the moisture plenty of time to escape.
In conclusion, keep working. This is a common problem and easy to remedy. Soon you will gain more control and be able to make pots that are pleasing to the eye, functional and structurally sound.
Thick pots are sometimes be relocated to the “graveyard” on the white table. There you can see other pots with common problems. You should discard yours after inspection.
Sincerely,
Dan Vito
