CLAY SEWER PIPE MANUFACTURE. II.?The Effect of Variable Alumina, Silica and Iron Oxide in Clays on Some Properties of Salt Glazes

Abstract

ABSTRACT Introduction. —A study was made of the relations between the alumina, silica and iron oxide contents in clays and their ability to take a salt glaze. Color. —Under the conditions of these tests it was found that low silica clays containing 0 to 1.00% iron oxide and high silica clays containing 0–2.19% iron oxide in terms of calcined weight produced white to tan glazes; low silica clays containing 1.00 to 3.50% and high silica clays containing 2.19 to 3.5% produced light brown glazes; those having 3.5 to 4.75% produced brown glazes; clays containing 4.75 to 8.2% produced mahogany glazes; and those having more than 8.2% possessed dark brown to black glazes. Brightness. —With clays similar to those tested in this work and which are fired and glazed to the same temperature, the brightness of a salt glaze which will be produced on it, may be predicted by calculating G in the following expression: x1, x2 and x3 represent the percentages of alumina, silica and iron oxide in the clay in terms of calcined weight. If G is greater than zero the salt glaze will be bright; when between —0.1337 and 0, it will be semi matt; and when less than —0.1337 it will be matt. Thickness of Glaze. —-The thickness of the salt glaze may be increased 600 per cent by increasing the silica content in a pure clay.