^66 Dr Colquhoiin on the Assay 



terials of the assay, so as to derange its composition and mo- 

 dify its results. 



When the moisture is completely evaporated, a cavity for 

 the reception of the assay-mixture is to be hollowed out in 

 the centre of the carbonaceous mass, and of such a size that 

 it shall be considerably larger than is strictly necessary for 

 containing the mixture. A quantity of charcoal should be 

 left to coat the sides of the crucible, not less than one-fourth 

 of an inch in thickness, and as nearly as may be, of equal 

 depth all around. Upon the bottom of the crucible, the char- 

 coal should not be less than an inch in thickness. The cavity 

 for receiving the assay should contain no angles or corners, 

 and be well rounded off at the bottom ; its interior should be 

 smooth ; and all the loosened charcoal must be carefully blown 

 out before the assay is introduced. 



When the crucible has been thus prepared, take 150 or 

 200 grains of the natural ore, or a proportionally smaller 

 quantity of ore that has been previously calcined : let the ore 

 and the flux be completely pulverized and thoroughly incor- 

 porated with each other ; of course the weight of the flux 

 which is employed in the assay must be ascertained with the 

 same accuracy as the weight of the ore. Place the mixture in the 

 cavity of the crucible, taking care to leave no detached por- 

 tion of it sticking to any of the upper parts of the sides ; be- 

 cause in that situation it would be left beyond the sphere of 

 the scoria, and consequently, even although reduced, its me- 

 tal would not be transmitted to the metallic button. Fill up 

 the cavity to the top with charcoal powder, and then cover up 

 the crucible with a well-adjusted lid ; or, a similar crucible, 

 somewhat smaller in size, may be inverted within it, instead of 

 an ordinary flat lid. Then let all the joinings be accurately 

 luted with fire-clay, leaving only a single small opening, for 

 the purpose of furnishing a vent to the elastic fluids which are 

 generated during the ignition. 



The crucible, with its enclosed assay, is now ready for the 

 furnace. It will be found advantageous to strew a little 

 pounded glass over the seat on which the crucible rests within 

 the furnace, in order that it may remain the more securely, at- 

 tached. It may be observed that, in most cases, for the sake 



