270 BELL SYSTEM TECHNICAL JOURNAL 



which the red numerals were printed and dusted. By using a special 

 black powder which would give an intense black in combination with 

 a thin film of red powder, one firing for both colors was possible. This 

 method required close control of temperature and humidity of the air 

 in the room which was therefore air conditioned. 



Even under good conditions considerable difficulty was experienced 

 at times with the adherence of the powder to unprinted areas. In 

 addition, the application of the powder and the brushing operation 

 required the installation of a special well exhausted unit and involved 

 some problems in the recovery of the ceramic dust which were quite 

 expensive. Efforts were therefore made to incorporate the glass 

 powder directly in the printing vehicle. The development of a 



Fig. 6 — Iron base number plate. 



ceramic printing ink covered tests on various printing vehicles to deter- 

 mine what vehicle or mixture of vehicles would be most suitable. 

 Difficulty was encountered in incorporating a sufficient amount of 

 inert, finely pulverized, intensely colored glasses into a vehicle and 

 still retaining the properties essential for offset printing by transferring 

 an impression from an electrotype plate to a vitreous enamel. This 

 problem was finally solved by the use of a relatively large percentage 

 of uncalcined ceramic material in combination with light and heavy 



