262 BELL SYSTEM TECHNICAL JOURNAL 



the development and plant expenditures associated with the installa- 

 tion of manufacturing facilities would be warranted. A survey of 

 commercial practices indicated that mechanization of the forming 

 operation and simplification of the finning, firing, and material prepara- 

 tion procedures were possible. 



The two general methods of processing first considered were: (1) 

 extrusion of a plastic column having the end cross section of the block, 

 cutting this column to block lengths and forming complete in the 

 plastic state; and (2) automatic pressing of damp granules. The first 

 method offered some advantages in forming because of the thin walls 

 of the protector blocks, but the greater shrinkage from raw to fired 

 states which would result from the use of plastic material would 

 involve greater dimensional variations. Because of this factor it was 

 decided to confine the development effort to a study of the possibilities 

 of automatic pressing of damp granules. 



The uses of the porcelain protector blocks required a body as highly 

 vitrified as was consistent with dimensional requirements, to minimize 

 moisture absorption in service and to prevent the adherence of car- 

 borundum particles during lapping operations in assembly. High 

 vitrification was also required to insure sufficient mechanical strength 

 to withstand handling during assembly and service. Accurate dimen- 

 sions were essential for satisfactory functioning in service. 



Two general types of bodies were considered, talc-clay combinations 

 and feldspar-clay-silica combinations. An investigation of talc-clay 

 mixtures indicated that the eutectic proportion of the two minerals 

 was approximately sixty-five per cent talc and thirty-five per cent 

 clay with small variations dependent upon various clay compositions. 

 The fusion temperature of this eutectic was approximately cone 12 or 

 2390° F. This combination, however, was not satisfactory since it 

 softened over an extremely small temperature range and formed a 

 very fluid glass in the melted state. A longer temperature range for 

 softening and greater melted viscosity was obtained by the addition 

 of feldspar. A eutectic composition of twelve and one-half per cent 

 talc and eighty-seven and one-half per cent spar was found which 

 fused at cone 6 or 2174° F. Using ten per cent to twenty per cent of 

 this flux, a well vitrified body was obtained at cone 8 or 2237° F 

 The firing range of this body was still much narrower than desired 

 and any excess firing resulted in blistering. Although it was evident 

 that commercial use of this body would require extremely close regu- 

 lation of temperature, it was decided to investigate its pressing char- 

 acteristics in view of the small amount of abrasive material it contained 

 and the importance of abrasion on dies and equipment with automatic 

 molding. 



