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ALUNITE AS SOURCE OF SCUMMING ON STONEWARE BODY *
Author(s) -
Knizek Jan O.,
Fetter H.
Publication year - 1945
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1945.tb14541.x
Subject(s) - alunite , anorthite , mineralogy , gypsum , potassium , mineral , chemistry , sulfate , nucleation , salt (chemistry) , potassium sulfate , chemical engineering , materials science , metallurgy , hydrothermal circulation , organic chemistry , engineering
A bstract A heavy, disagreeable scumming developed on a surface of stoneware articles. This scum, having the appearance of a wrinkled skin, prevented the body from taking the salt glaze on account of its refractory character. It consisted almost entirely of the mineral anorthite, and its formation is attributed to the aluaite content of the clay. Summary The scum coating which appeared on a stoneware body on firing proved to be composed essentially of the mineral anorthite as determined by chemical analysis and microscopic examination. The substance to be blamed for the scum formation seems to be the alunite, which has been found in the clay in exceedingly large quantities. Although the alunite content of the clay remained at 15%, no troubles have been observed from its presence. The explanation for the scum‐formation mechanism is that the potassium sulfate, which remains as the only soluble and readily fusible compound after the decomposition of the alunite, dissolves the calcium sulfate, alumina, and silica and migrates to the surface at temperatures closely approaching that of the clay vitrifying point. The anorthite may easily crystallize out of the melt of the potassium sulfate, which is volatilized and disappears completely.