Black Coring in Structural Clay Products

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Abstract
The formation and removal of black cores in structural clay products was studied to provide the industry with a broader understanding of this defect. Analytical procedures indicated that carbonaceous matter was responsible for the formation of black cores, but the properties of the core were actually dependent on the state of oxidation of the iron. For the removal of black cores, time, temperature, and kiln atmosphere were varied to find the optimum conditions for core oxidation. Such conditions were found at higher temperatures than were usually recommended and with more excess air than was heretofore thought necessary. Ammonium compounds were incorporated into clay bodies to assist in core oxidation. The action of these reagents was found to be more mechanical than chemical. Ammonium chloride and ammonium hydroxide increased the porosity of the ware in such a way as to allow a better penetration of oxygen into the center. These additions greatly accelerated the oxidation of clay products, but they did not substitute for atmospheric oxygen. The experiments conducted indicated the importance of cooling as an oxidation period.

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