| Abstract: | Method of measurement.—The apparatus used (figure 1) consisted of a hardened tungsten-steel point which was allowed to bear on the glaze surface for 3 min. under a pressure of 50 lbs. The hardness was then computed from the dimensions of the resulting indentation. Results were reproducible to about 5 per cent. (See table III.) The presence of ridges and bubbles in the glaze was the principal cause of discordant readings. Results and conclusions.—The enamels show the lowest and the porcelain glazes the greatest hardness, while the whiteware glazes occupy an intermediate position. The difference in hardness between the enamels and the whiteware glazes is quite marked. There is also a rather well defined difference between the whiteware and the porcelain glazes, but not a sharp separation between the lower and the higher fired porcelain glazes. Enough evidence has been accumulated to show that increased firing of the same glaze will increase the hardness. It appears also that increase in the alumina content brings about greater hardness. |
