| Abstract: | The network of fine cracks often seen in the surface of glazed products not only mars the surface but shortens the life and lowers the price of the ware. Since the relative expansivity of the glaze and body is a very important factor in controlling this condition reliable measurements of both are of the utmost importance. Until recently the difficulty has been that the expansivity of the glaze has been measured on specimens of the material melted to glass and drawn into rods or cast in molds of sufficient length. Owing to the difference between this treatment and that received by the material when spread on the ware, it was realized that these methods might lead to unavoidable errors. By the interference method, specimens 0.2 mm. in length can be measured with sufficient accuracy. Therefore in measuring glazes the samples may be taken directly from the finished ware. In this paper the thermal expansion of glaze materials prepared in the form of drawn glass or cast rods is compared with that of the same material taken from the finished ware. From the measurements, the expected divergence is found to be as much as 20% in the case of drawn glass specimens, and 10% for cast specimens. |
