of the Argillaceous Iroii-Ore^ S67 



of convenience, the assay furnace is made large enough to 

 contain two or three crucibles at the same time. The fuel 

 which is best fitted for the use of the assay furnace is coke^ 

 because it affords both the most equable and the most intense 

 heat, and the crucibles should be kept completely buried in it 

 during the whole process. It is proper to apply the fire at 

 first in a very gradual manner, during the developement of 

 the gaseous matter arising out of the decomposition of the gum, 

 and the deoxidation of the ore, and therefore it should not be 

 allowed to advance beyond a dull redness during the first half 

 hour. By that time, the evolution of gaseous matter will have 

 terminated, and the temperature may then be raised to its 

 highest pitch, at which it should be permanently sustained for 

 upwards of half an hour. The reduction will then be com- 

 plete, so that the crucible may be withdrawn from the fur- 

 nace. It should immediately be tapped, for several times, 

 smartly against the floor, care being taken to preserve its per- 

 pendicular position, and this will have the effect of bringing 

 down any globules of reduced iron which may not previously 

 have descended to the metallic button. When this has been 

 done, the crucible may be set aside till it cools. 



When the flux is properly adjusted, the ordinary tempera- 

 ture of a good air furnace will be found sufficient for the re- 

 duction of the ore and the complete extrication of the reduced 

 metal. In a pretty extensive series of assays conducted in a 

 small portable furnace, in which no other fuel was employed 

 than common pit-coal, a failure from deficiency of heat was 

 rarely experienced, and in no instance where the flux had been 

 adjusted to the ore with even very moderate accuracy, ii tiami 



If the assay has been successful, the original mixture of ore 

 and flux will be found resolved into two separate bodies, the 

 one consisting of metal, lying at the bottom of the crucible in 

 the form of a flattened globule, and the other composed of all 

 the unvolatilizable part of the earthy ingredients resting in a 

 superincumbent mass above the metal, and termed the scoria. 

 These bodies should be carefully separated from each other, 

 and preserved for examination. The first step to be taken in 

 this examination is an accurate determination of their respec- 

 tive weights. The upper surface of the scoria will frequently 



