268 BELL SYSTEM TECHNICAL JOURNAL 



Red Lead 40 



Pearl Ash 6 



Sodium Nitrate 9 



White Arsenic Oxide 6 



Flint 31 



Borax 8 



100 



In practice constituents of this enamel were first thoroughly melted 

 to a homogeneous glass, giving on cooling a glass magma having high 

 opalescence. The dead white opacity of this enamel could only be 

 developed by slow cooling through the range necessary to precipitate 

 the arsenic compounds. Manufacturing considerations, such as the 

 necessity of an enclosed room for commercial smelting to avoid con- 

 tamination as well as to avoid the possible health hazards involved in 

 the smelting of arsenic-lead combinations, led us to purchase the 

 required enamel. The fact that a suitable composition had been 

 developed and was available for manufacture if necessary was an 

 advantage from a supply standpoint. 



Various enameling procedures were considered. In order to cover 

 the vertical surface of the collar satisfactorily, it was essential that 

 this surface be coated either by dipping or by spraying. It was equally 

 important to apply the enamel coating to the flat surface of the plate 

 by dusting on a thick coat of dry powdered enamel. This dust coat 

 was necessary because of the thickness of enamel required on the flat 

 portion to strengthen the number plate and also to obtain the desired 

 quality of finish on the surface bearing the numerals and characters. 

 From an economic standpoint, it was also imperative that only one 

 enamel fire be used. Initially, efforts were made to dust enamel on a 

 blank already completely coated with a thin coat of enamel slip con- 

 sisting of finely divided enamel frit suspended in water by means of 

 clay or bentonite. It was found on firing that the added refractoriness 

 of the enamel slip containing the clay or bentonite resulted in a 

 roughened fired surface over the dusted area. This was caused by the 

 formation of gases in the decomposition of the clay or bentonite while 

 the enamel was in a viscous state. It was therefore necessary to 

 protect the flat portion of the plates by templates during spraying. 

 As this was costly, a study was made of other means of floating the 

 enamel frit for collar application. 



In order to overcome these process difficulties, it was desirable to 

 find a material which would (1) satisfactorily hold the heavy lead 

 enamel particles in suspension and prevent packing, (2) not attack the 

 enamel or impair its durability, (3) decompose before the enamel 

 started to fuse, and (4) be inexpensive. Soluble alginates appeared to 



