The production of Porcelain for electrical insulation — VII

Plant control check tests are of importance primarily to the manufacturers of a product. These tests are, however, of considerable importance to the consumer. The lower a manufacturer's losses are, the cheaper his production costs will be. As high losses are caused by defects in the product it is evident that the fewer pieces there are with noticeable defects in them, the fewer pieces there will be that have minor defects in them that will escape detection even with the most careful inspection. Tests that will pick out the pieces that have these minor defects are of great importance to the consumer. Porcelain should be non-porous and tests which will make it possible to cull porous pieces are of paramount importance. If the fuchsine dye penetration method is to be used for this porosity test the selection of the test specimens must be made in a definite and dependable manner. If the firing of the porcelain is not done in a uniform manner so that the location of the pieces most likely to be porous are definitely known the test of any one piece would be of no value and it would obviously be impossible to test every piece. Where a continuous car tunnel kiln is used and the cross sectional area is relatively small the selection of the proper test specimen is not only possible but has been used successfully for several years. A recent development for testing disk insulators comprises a mechanism for subjecting each of the insulators under test to a pull test of 5000 pounds for two minutes, and simultaneously subjecting the pieces to the high-frequency oscillator test. This eliminates any doubt as to whether the porcelain would resist the two tests when applied at once. A petrographic study of thin sections of various porcelains is of great interest, in fact it is essential if the manufacturer or consumer desires to know something of the structure of the product, the extent to which the pyrochemical reactions have progressed and what the variations are from time to time. Several photomicrographs of various types of porcelain are illustrated. These are selected from a wide variety of wares in order to show the various steps in the development of different qualities of porcelain. The unlike thermal expansion and contraction of various porcelain ingredients is undoubtedly the cause of some of the deterioration of aged porcelain. It is evident that if some of the particles in the porcelain are under stress due to their tendency to contract more than the surrounding glassy matrix during cooling, after firing, there will be a tendency for these particles to rupture and break down in order to relieve this local strain. One of the illustrations shows this very well. Overfiring causes the development of gases in the body at a stage when the glassy matrix is in a molten condition. Continued firing causes expansion of the gases and results in the development of a vesicular structure. The degree of overfiring governs the size of the vesicles and the extent to which they have become interconnected. Too glassy a structure also develops brittleness and is to be avoided. Overfiring is well illustrated in Fig. 50.