f60 Dr Colquhoun on the Aasaij 



lime to flux their natural silica and clay, and it is necessary 

 therefore in assaying them to add a certain amount of lime, 

 ranging from 2 or 3 to 10 per cent, according to circumstan- 

 ces. If, however, the lime be superabundant, the flux to 

 be chosen is silica, which should be exhibited in a proportion 

 not under 6 and not exceeding 15 per cent., and varied with- 

 in these limits according to the excess of the lime, and also 

 according to the quantity of silica already present in the ore. 

 Should the ore contain little natural clay, the flux will be im- 

 proved by the addition of from 6 to 10 per cent, of pure clay. 



It may be remarked upon the whole of the cases just consi- 

 dered, that, in whatever state the siliceous, calcareous, or ar- 

 gillaceous ore may occur naturally, it should be the aim of the 

 smelter to prepare a mass for his furnace, the general propor- 

 tions of which should be somewhat approximating to 1 part 

 by weight of lime, 1 of clay, and 2 of silica. This is the mix- 

 ture which will most easily run into a glass, and which will 

 yield the most perfect and unreserved return of metal after 

 fusion. 



Magnesian ores generally prove the most refractory. If 

 the excess of magnesia be considerable, the ore must be fluxed 

 with a mixture of nearly equal parts of silica and lime, (from 

 6 to 12 per cent, of each,) together with a small amount of 

 clay. 



If an ore is extremely poor in metal, it may be inadviseable 

 to supply its deficient earth as a flux for those other earthly 

 ingredients which are already present in excess, on account 

 of the inconveniences which arise from having the earthy 

 scoria very abundant in proportion to the metallic product. 

 In such a case as this, recourse should be had to a more 

 powerful solvent, which, although used in small quantity on- 

 ly, may nevertheless flux the ore in the ordinary furnace heat. 

 Fluor spar and calcined borax are the safest and the most 

 efficacious substances for this purpose. If either be taken 

 singly, an amount varying according to circumstances from 

 12 to 25 per cent, will form the flux ; if they be taken jointly, 

 each may be in the proportion of about 10 per cent, of the 

 quantity of ironstone. Indeed it may be considered as a rule 

 of general application, that whenever an ore is found to be 



