>Once again I opened the kiln today . . . projects broken!
>Does anyone have any suggestions
WHY DOES CLAY BREAK IN FIRING?
Nearly all firing breakage is caused by heating it too fast at the critical boiling temperature of water. This is worst if the clay is thick or not quite bone dry. Some clay is more open and can tolerate faster firing because the steam can get out easier. One quantity of water converts to about 600 parts of steam. This explode the clay soon after it reaches 212 F at sea level (sooner in Denver). Sometimes the steam gathers in an air pocket, forcing the clay apart, making it appear as if the air pocket caused the breakage.
Seven WAYS TO AVOID FIRING DAMAGE
1. Fire slower until kiln is well past the boiling temp. Some kilns have limited slow controls, so you have to leave the lid partially open at first, but most kilns can be fired very slowly at first and then turned up after all steam is removed from clay. Thicker pieces take a while to get heated all the way through, so be patient before turning up the switches. Once a kiln shows red heat, it can go full speed ahead.
2. Pre-dry the clay longer or use a fan, heat register, or even place in thermostatically controlled kitchen oven at 180 degrees F.
3. Students may learn as much or more with several smaller sized (thinner) projects instead of one larger and thicker project. What if the assignment is to make three of something and then the students selects the best one of the three to be fired? Unfired clay is used again.
4. Ventilate thicker clay sculpture pieces by poking lots of holes in them from the bottom with a piece of coat hanger wire. Poke a hole nearly through to the top. Put a hole every half inch or closer to let the steam out as it heats. Do this before the clay is dry.
5. Have students add sand (or grog) to the clay. It opens the clay and allows the moisture and steam out easier. White sand from builders supply stores (sold for plaster) is generally high quality silica and works fine. Add as much as you want to as long as the clay is still workable. Some river sand, beach sand, or common pit sand may not work because it may include limestone or sea shell pieces (causes pop-outs).
6. Teach students to make thinner pieces. Encourage them to work the clay together well when they add parts.
7. Have student construct thicker sculpture by wrapping clay slabs or pinch pots around wads of paper when a larger mass is needed. The newspaper can stay in the clay, but poke a hole in the bottom to let the pressure out. These cores need to be soft enough to shrink as the clay dries. Paper wads formed and taped are good cores for animals and head sculptures. Wood paddles help form the clay.