330 ABSTRACTS : CERAMICS 



prevent its decreasing much below its maximum until after the forma- 

 tion of plane cracks has begun. This can be accomplished by increas- 

 ing the cooling rate at the proper time. 



The cooling rate can be reduced (i) by heating the surroundings of 

 the melt, i. e., placing it in a heated kiln; and (2) by surrounding it 

 with an envelope of some such insulating material as sand or kieselguhr. 

 Variations of these two methods are described and time-temperature 

 data given. H. S. R. 



CERAMICS. — Thermocouple installation in annealing kilns for optical 

 glass. E. D. Williamson and H. S. Roberts. Bull. Amer. Inst. 

 Min. Met. Eng. 1445-1453. Aug., 1919. (Geophysical Lab. 

 Papers on Optical Glass, No, 15.) 



During the war-time rush to prepare the glass necessary for the needs 

 of the Army and Navy, the problem of the temperature control of the 

 annealing kilns became most serious. This paper gives a short account 

 of the system evolved by the members of the Laboratory staff who 

 were cooperating with the Pittsburgh Plate Glass Company at its 

 Charleroi plant. The points which are most specifically treated are: 

 (i) The advantages of the thermocouple over other devices for this 

 type of work. (2) Choice of material for thermocouples. (3) Choice 

 of measuring instrument. (4) Arrangement of leads and other apparatus. 

 (5) The general problem of annealing optical glass. E- D. W. 



CERAMICS. — The volatilization of lead oxide from lead silicate melts. 

 Olaf Andersen. Journ. Amer. Ceram. Soc. 2: 784-789. Oct., 

 1919. (Geophysical Lab. Papers on Optical Glass, No. 19.) 

 Experiments were made on the amount of PbO volatilized from the 

 surface of lead silicate glasses at temperatures from 900° to 1400° C. 

 It was found that the volatilization from an unstirred glass in 15 min- 

 utes took place at practically the same rate as from a stirred glass 

 heated for a longer period. The rate of volatilization falls off consid- 

 erably during a long heating if the glass is not stirred, as a film con- 

 siderably lower in PbO forms on the surface of the melt, into which 

 PbO must diffuse from below before it can escape. The rate of vola- 

 tilization of PbO at the temperatures usually employed in optical glass 

 manufacture would seem from these experiments to be small enough 

 so that variations in refractive index due to volatilization are not to 

 be expected if the procedure is reasonably constant from melt to melt, 

 but large enough to cause considerable variations in index if the melt- 

 ing schedule is changed. O. A. 



