|Monthly Tech-Tip |
This is a closeup of a shard from the wall of a thrown vessel. The clay is an iron stoneware, Plainsman Fire-Red with added feldspar, fired in reduction at cone 10. The reduction was not heavy, the kiln was fired with enough air to burn almost all the gas leaving only a slight yellow (but mostly blue) flame at the damper. Is this black color carbon? Consider the following. Carbon is refractory, this is glassy. During bisquit firing the carbon was burned out of this. These black zones have a hole at their centers. This is black iron, a strong flux. The iron is coming from large particles (20-40 mesh) of iron pyrite. In the reduction atmosphere, the natural Fe2O3 is being robbed of oxygen (from both the decomposition of neighboring particles and the atmosphere of the kiln) and converting to FeO. That is melting and interacting with the feldspar to soak into the surrounding matrix. Contrary to what most people think this does not weaken the clay, it strengthens ware (provided the feldspar is present). Once the black has permeated the entire matrix of a piece it becomes very strong (even with a hammer it was unexpectedly difficult to break ware to get these shards). Note the right side is not glaze-covered, if it had been the entire matrix would be black. But still strong.
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.
|Oxides||FeO - Ferrous Oxide|