PROPERTIES AND USES OF SEVERAL CLAYS IN PORCELAIN ENAMELS

The comparative effect of several clays on the physical characteristics of typical ground-coat and cover enamels was investigated. In the ground-coat enamels, the effect of the clays on the firing characteristics, the development of the bubble structure, reboiling, and the set characteristics were studied. Suitable amounts of each of the clays were used as mill additions to the following cover enamels: (1) superopaque antimony frit, (2) zirconium frit, (3) a clear frit as used for colors, and (4) fluoride frit. The effect on reflectance, gloss, and the resistance to gouging was noted on these samples. Spectrophotometer curves were run on selected cover-enamel surfaces. The clays showed a widely varying effect on the set characteristics and the bubble structure of the ground-coat enamels. The different clays with the antimony frit caused a wide variation in the gouging characteristics while, with the zirconium frit, the clays had less effect, the gouging values being uniformly high. Summary The general effect of the various clays in sheet-iron ground-coat and cover enamels may be summarized in the following manner: (1) In the clays tested, the chemical analysis and the physical data, such as particle size, hydrogen-ion concentration, or pyrometric cone equivalent, do not give a satisfactory basis for predicting the general usability of clays in porcelain enamels. (2) When the various clays were milled in the same ground-coat frit and then fired, photomicrographs showed a widely varying bubble structure in the fired enamel. (3) Some of the clays brought about an earlier development of adherence or bond between the metal and the ground-coat enamel. (4) The clays which gave a large bubble structure in the ground-coat layer, when impacted with the falling-weight test, gave a large fracture area, while those giving a small bubble structure caused a small fracture area. (5) The freshly milled ground-coat enamel containing the various clays was adjusted to give (a) a pickup of 40 gm. per sq. ft., thus yielding a variable solids-to-liquid ratio and (b) a specific gravity of 1.65, yielding a constant solids-to-liquid ratio in the slips. The enamel slips were tested for the weight picked up after specific aging periods; the various clays were found to cause an increase in the weight picked up in some cases, in others a decrease, while others gave practically no change in the weight-pickup value after various aging periods. (6) At an application weight of 60 gm. per sq. ft. the different clays caused a maximum variation of about 6% reflectance in the antimony, 12% in the zirconium, and 25% in the clear frit. (7) The spectrophotometric curves of the various clay and frit combinations showed that the clays gave uniform spectral values in the antimony and clear frits; in the zirconium frit, a greater variation in reflectance was found in the red portion of the spectrum than in the blue or green portions. (8) The various clays in the antimony frit showed a marked difference in the gouging characteristics of the surfaces, while in the zirconium frit the clays caused only a slight variation in gouging as all of the values were uniformly high.