SOME CERAMIC MANUFACTURING DEVELOPMENTS 275 



In developing an enamel to be used for resistances, it was desirable 

 that the melting temperature be as low as possible consistent with good 

 durability in order to maintain at a minimum the thermal shocks re- 

 ceived by the porcelain cores and any changes in the resistance of the 

 wire during firing. A very high viscosity during fusion was also de- 

 sirable in order to avoid running of the enamel during firing, an unde- 

 sirable feature which would result in exposed wires and unsightly lumps 

 unless the enamel was applied in numerous thin coats. Conversely to 

 these requirements, it was necessary that the enamel coating be glassy 

 in appearance, smooth, free from blisters and pin holes, and capable of 

 being fired in a relatively short time. 



These factors indicated the desirability of investigating lead-boron- 

 silica mixtures and the elimination of any raw clay or similar refractory 

 substance in the enamel slip. The enamel finally developed was as 

 follows: 



Red Lead 48.0 



Boric Acid 24.0 



Flint 10.0 



Soda Ash 3.7 



Cryolite 6.0 



Tin Oxide 1.6 



Manganese Dioxide 0.5 



Cobalt Oxide 0.6 



Iron Oxide 4.0 



Zinc Oxide 1.6 



100.0 



White Lead 10.0 



Light Calcined Magnesia 1.3 



All of the materials other than the white lead and light calcined 

 magnesia of the above composition were fritted or melted to a glass 

 and then quenched in water. The fritting of these materials was done 

 to insure complete formation of stable compounds and to permit more 

 rapid firing of the enamel coating on resistances to a smooth homo- 

 genous glass. The proportions of sodium, lead, boron and silica were 

 selected to obtain a stable coating with the desired viscosity character- 

 istics at as low temperature as possible. Sufficient opacity of the 

 coating was obtained through the use of cryolite and tin oxide. The 

 cryolite also functioned as a flux. A pleasing dark color was obtained 

 economically with the iron, cobalt and manganese contents. The zinc 

 oxide functioned as an additional flux and also aided considerably in 

 the formation of a smooth coating. Slight variations in the sodium 

 content of this enamel affected its viscosity markedly and also affected 

 its expansion characteristics. 



