Preventing Cracks and Explosions
Cracking in water based clay can be caused by any one of a number of issues, but usually involves one or more of these conditions:
* If there is a rigid armature inside that doesn’t collapse while the piece is drying, then the clay will crack around the armature. Solution is to not have an armature inside, or use crumpled newspaper that will “give”
* If the piece dries too fast or is left to dry in the open air it will crack. The outer layers dry faster than the inner layers and as clay shrinks as it dries, the outer layers will crack. Solution is to dry the piece slowly, covered in plastic.
* If the clay is unsupported in certain areas, the weight will cause the piece to crack under the stress. Solution is to prop any portion up that tends to sag with pieces of wood or other solid structure until that portion has dried or is very firm and will support itself.
* If the piece is not totally bone dry when it is fired, the escaping steam will crack the clay. Solution is to make sure the clay is dry. Put it in a roasting turkey bag in the oven and turn it on about 180 degrees overnight, then turn off the oven. If you see steam on the inside of the bag, the piece is not dry enough to fire. A piece that merely cracks or breaks can be easily repaired. A piece that explodes into dust cannot. Poke a dozen holes in the bagand then bake in your oven to 180 degrees all day long. Turn it off at night time and leave it sit. Keep doing this until the morning you check it, it doesn’t have any water vapor on the inside of the bag. Then is when it’s completely dry.
* If you put too dry clay on top of leather dry clay while modeling, the new clay will not adhere to the existing clay and will tend to separate and crack. Solution is to always keep the piece you are working on nice and moist and to only add really wet clay while modeling so it will “glue” itself to the existing clay and form a bond.
* If you build a piece thick and when quickly building up the initial layers get air trapped inside the clay, the piece will burst when firing. Solution is to form your initial mass, then while the clay is still quite soft, paddle the clay to smash out all the air bubbles, then build more layers with wet clay. Also, if firing thick, the clay needs to be a clay that contains grog and the firing must commence slowly over a period of at least a day. If the temp is brought up too fast, the clay can burst. Otherwise, the piece must be cut and hollowed out to about 1/2 inch even thickness and holes pierced on the inside of the clay as is instructed in a typical ceramic clay class. When putting the halves back together, the clay must be super wet and the exposed edges scored, or the clay will crack along the seam due to improper adhesion issues mentioned in the previous bullet.
* EM217 is a modeling clay only and really should never be left to dry past leather hard since it should not be fired. It is a very fine clay with no grog and will almost certainly crack if it is left to dry out. It can be molded while still wet.