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This is an example of one of John Prosser's "house mugs". They have been fracturing. Partially broken ones are spring-loaded like this. All broken pieces have black coring. Of course when thick-walled, high carbon, high iron bodies are fired without a previous bisque in heavy reduction one can expect true black coring (where Fe2O3 and CO2 react to form a body matrix hostile to even slight thermal shock). But none of these factors are present. Of course, testing could be done to bisque these higher, soak longer in the bisque, start reduction later, and oxidize longer at the end. But these measures will not likely be enough. The outer surface could be put on as an engobe over a vitreous body (but lots of work using the EBCT test would be needed because of the difference in firing shrinkage).
Stepping back consider: These black cored sections are unglazed. When iron reduces it turns black so the color black alone does not mean official black coring. When there is enough feldspar to form a good measure of vitrification (as is the case with this body) one can expect it to be suitable for light duty functional ware. Magnesia mattes like this have low thermal expansion because they contain a lot of MgO, a super low expansion flux. That puts them under compression on the body, a lot of unglazed external surface like this compounds the problem. The solution is to raise glaze expansion, something fairly easy to do in high fire. Just increase the KNaO at the expense of CaO.
These are iron reduction test bodies. The L4168G (left) is far stronger yet it has a higher percentage of Fe2O3 and much more black coring. How is that possible? Because it has 10% added feldspar. The black FeO iron is staining the feldspar black (bleeding out from each pyrite particle), helping it do its job of producing a glassy black color is not coming from the kiln atmosphere? Because the buff-burning bodies in the same kiln did not have any of this. On the right, the iron is restricted by its ability to vitrify the body by limited glass development, ending up destabilizing it instead (by increasing body thermal expansion).
Troubles |
Black Coring
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black. |
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