of the Argillaceous Iron-Ore. 266 



it is decomposed by the heat, leaving a quantity of an impal- 

 pable carbonaceous powder intimately diffused throughout the 

 mixture. The best state in which the assay mixture can be 

 introduced into the crucible is that of a fine, dry, and tho- 

 roughly blended powder, and the aid of any adventitious ce- 

 menting matter is altogether unnecessary. 



The crucibles that are to be employed in the assay obvious- 

 ly require to be highly refractory, and not easily susceptible of 

 cracking, through exposure to any sudden change of tempera- 

 ture. The best are certainly those which are formed either of 

 Stourbridge clay, or of a similarly constituted material. But 

 the common Hessian crucibles, if they be genuine, will be found 

 very suitable in all cases where the temperature of the assay 

 furnace is not raised to an extreme degree of intensity. In the 

 course of numerous experiments on the assaying of iron-ores, 

 during which these Hessian crucibles were almost invariably 

 employed, there did not occur a single failure in consequence 

 of their either cracking or melting. 



Before introducing the assay-mixture into the crucible, it is 

 necessary to line it interiorly with a coating of charcoal, which 

 in the high temperature of the furnace, deoxidates the oxide of 

 iron and reduces it to the metallic state. This coating may be 

 applied in the following manner. 



Take a quantity of pulverized wood charcoal, and sift it 

 pretty fine with a wire-cloth sieve. Then form a gum water, 

 composed of 2 parts of gum arabic and 1 part of gum traga- 

 canth, with 100 parts of water, and with this liquor make the 

 charcoal powder into a thick dryish paste. Fill the crucibles 

 with this paste, observing the precaution of introducing it in- 

 to them in small successive quantities, so as to admit of its be- 

 ing more firmly rammed together into a solid, compact mass. 

 Then reduce the crucibles and their contents to a state of 

 thorough dryness, by keeping them in a warm situation for 

 six or eight days. 



It is not uncommon to diffuse a portion of clay through the 

 gum water, with a view to increase the adhesion of the char* 

 coal powder to the sides of the crucible. This practice ought 

 not, however, to be imitated ; for it is unnecessary, and will 

 evidently cause a portion of clay to intermingle with the ma* 



