These are examples of raku pottery. This technique shows the beginning potters how a change in atmospheric conditions can affect claybody and glazes. Raku is usually fired at or around cone 06 (1800 degrees F) in a gas kiln, the piece is then pulled out of the kiln and placed in a covered container filled with combustible materials (sawdust, paper or straw).
With this technique, Reduction atmosphere inside the container is produced. Reduction in an atmosphere that lacks oxygen. Unglazed surfaces that are exposed to this reduction turns carbon black. The more porous a surface is the more carbon it tends to absorb. Oxygen is pulled out of the clay surface and burned through this reduction process. The by product in the burning of combustibles is carbon, which is then left behind and trapped in the clay body.
When pots are fired, "heated", like anything, it expands. When pieces are fired with glazes both clay and glaze expand. Since glaze is "glass" and it is the first to be exposed to the difference in temperature, the glaze quickly hardens at its expanded form, while the clay body stays hotter longer, it slowly shrinks. since the clay and glaze are fused, the shrinking claybody pulls the already harden glaze resulting in a crackle effect. Cracking the glaze intentionally by blowing air or spraying a light mist of water exaggerates this effect. When the piece is then reduced, the carbon is absorbed through the cracks where clay body is exposed.
I will post images of the actual technique, step by step, sometime in the next few weeks. I have to make pots for it first. I will also try to explain another type of raku involving copper glazes. Stay tuned!!!