SOME CERAMIC MANUFACTURING DEVELOPMENTS 259 



the required light dispersion in certain colors of lenses without the 

 use of sandblasted surfaces. Several methods of dispersing the light 

 were tried including the application of a translucent layer of glass 

 on the back of a clear lens, but as it was difficult to control economically 

 the amount of light dispersion by these methods, it was decided to 

 use opalescent glasses. Calcium phosphate and cryolite were found 

 suitable as opacifiers and satisfactory compositions were developed by 

 means of further progressive changes to suit the particular working 

 conditions in the shop. 



Several serious objections were found to the open pot method of 

 manufacture, the most important of which were the long heating 

 period required for new pots and their relatively short life. The 

 manufacture of opalescent glasses increased these difficulties because 

 of the more corrosive nature of these glasses as a result of which the 

 maximum life of the pots was approximately twelve days. In view 

 of this, a small 500-pound capacity gas fired melting furnace known 

 as a day tank was designed and constructed. This tank consisted of 

 a rectangular box shaped furnace lined with refractory blocks about 

 twelve inches thick. With this equipment, a complete batch was 

 melted each night and the resultant glass formed into rods during the 

 next day. Under continuous operation, furnace life of about three 

 months was obtained which was considered very satisfactory in view 

 of the corrosive nature of these glasses. 



Satisfactory compositions and methods of manufacture were finally 

 developed for the production of glasses in the required colors. This 

 development resulted in the elimination of an unsatisfactory supply 

 situation, reduced the cost of lenses appreciably, and greatly improved 

 the quality of lenses. 



Spirally Grooved Resistance Cores 



At the same time that development work on glasses was being 

 carried on, a preliminary survey was made of the advantages of manu- 

 facturing instead of purchasing the ceramic cores used in filament 

 resistances. As the preliminary survey indicated that definite ad- 

 vantage would be realized, development for manufacture was under- 

 taken. The part, shown in Fig. 2, consists of a thick-walled tube with 

 a spiral groove on the outer surface in which a resistance filament is 

 placed. Tests first were made on pressing blanks from sodium silicate 

 and powdered slate mixtures. These parts adhered to the die, were 

 difficult to dry, and were very weak in the fired state. Further work 

 was done with talc and sodium silicate mixtures which were stronger 

 but still had the objectionable feature of adhering to the dies. Addi- 



