TEST FOR DETERMINATION OF CALCIUM IN LEAD 



437 



not critical in shape or size. The one used in this development is an iron 

 plate f in. x 4 in. x 8 in. with a tapered depression milled in its surface. 

 The test ingot is about 4 in. long, f in. wide, ^ in. thick at the casting end 

 and f in. thick at the other end. It weighs 80 to 100 grams. The effect of 

 mold tempera(ure has been studied and is not critical in the range from room 

 temperature to that reached by the mold as a result of casting into it at 

 intervals of a few minutes. 



Control of the atmosphere over the surface has been found necessary to 

 insure reproducible results. For this reason the cellophane enclosed 

 casting chamber shown in Fig. 2 was devised. There is a door on the right 

 hand side for insertion of the crucible and a rubber inlet tube on the left 

 for entry of the gas mixture. It has been found by experiment that both 

 moisture and the carbon dioxide content of the air influence the results 

 obtained. Therefore, both are removed chemically and then re-introduced 



Fig. 2 — Apparatus used in casting lead-calcium ingots under controlled atmospheres 



in metered quantities to the air passing into the casting chamber. By 

 means of appropriate flow meters, dry air could be mixed in definite pro- 

 portions with air saturated with moisture to produce the desired humidity. 

 Another and perhaps simpler system is to pass the dry air over certain salt 

 solutions of known vapor pressure. Carbon dioxide is conveniently made 

 available by placing sohd carbon dioxide (dry ice) in a Dewar flask having a 

 stopper with two exit tubes. By means of an escape valve on one tube any 

 desired CO2 pressure can be built up in the flask to force the gas through a 

 flow meter and into the air line leading to the casting chamber. One 

 satisfactory arrangement of apparatus for controlling the composition of 

 the atmosphere is shown in Fig. 3. 



The surface appearances shown in Fig. 1 were obtained by casting in an 

 atmosphere having 50 per cent relative humidity and 0.02 per cent carbon 

 dioxide. Increasing the carbon dioxide or decreasing the moisture content 



