6 MASS. EXPERIMENT STATION BULLETIN 341 



eight minutes, and as the usual processing period in a boiling water bath is 

 longer than ten minutes, jars processed with the bails tightened have as good 

 a chance of venting as have jars processed partially sealed. This is further 

 substantiated by the experimental evidence obtained in making a comparison 

 of the vacuums formed in jars so processed. 



Table 1. — Time of Venting (Pressure Release) of Fully and Partially 

 Sealed Jars Filled with Water and Processed in a Water Bath at 



180° F. 



Time of Venting 



Filling Temperature 

 70° F. 



Fully Partially 

 sealed jars sealed jars 



Filling Temperature 

 170° F. 



Fully Partially 

 sealed jars sealrd jars 



Minutes Number Number Number Number 



1 92 



2 64 



3 10 



4 18 



5 10 



6 2 



7 4 



8 



Total 200 200 200 



200 



PRESSURES DEVELOPED IN GLASS JARS DURING THERMAL TREATMENT 



Method of Determining Pressure 



A consideration of the safety of the fully sealed method entails a study of 

 the internal pressures developed in glass jars during processing. To insure 

 safety to the manipulator, the glass jar must be able to withstand the highest 

 pressure that may develop within it. 



In order to study the internal pressures developed during processing in a 

 boiling water bath and in a steam pressure retort, it was necessary to connect 

 the inside of a jar with an open U-tube manometer containing mercury. Holes 

 drilled in glass covers caused these covers to crack as soon as they were placed 

 in hot water. The best means was found to be the use of an aluminum cover 

 duplicating exactly the measurements of an ordinary glass cover. Aluminum 

 was chosen as the metal because of its rust resistance and light weight. A one- 

 eighth inch brass tube, 2.5 inches long, was threaded into a hole in this alumi- 

 num cap and was connected to the U-tube manometer by means of 8 mm. 

 pressure tubing. (Plate II shows this pressure determination apparatus.) For 

 steam pressure retort determinations the safety valve in the cover of the retort 

 was removed and the pressure tubing attached to a brass tube inserted in the 

 hole. All connections were sealed with a mixture of litharge, glycerin, and 



