208 
Lead. 
inattention to the fire will set the whole contents of the 
hearth into a solid mass, or cause it to boil and flow down 
! in a liquid state on the work-stone — the lead flows very 
hot, and the hearth appears hot and foul. The addition 
of lime is necessary to correct this defect in the ore, which 
combining with the fluid scorise, solidifies, and thus as- 
sists its collecting in masses : care should be taken not 
to add more lime than is absolutely necessary for the pur- 
pose intended, as all extraneous matter thrown in with the 
ore lessens the produce of lead. 
The substances which are found to render the ores of 
lead refractory, when mixed with them, are cawk, (sul- 
phate of barytes ,) blackjack, (blende or sulphur et of zinc,) 
sulphur, (iron pyrites ,) and silver, or copper, when they 
are contained in the ore in larger proportions than usual. 
I have always considered that these substances render 
an ore refractory by the extra quantity of sulphur they 
bring with them. I do not think the earth or metals alone 
would produce any visible effect in the smelting ; and I 
am almost confirmed in this opinion, by repeatedly ob- 
serving the effect produced by roasting the ore previous to 
smelting ; it works more pleasantly, requires less lime and 
fuel, and gives a better produce. 
The quality of the coals materially affects the working 
of the hearth and the produce of lead ; those which are 
free from sulphur, and which leave but little residuum 
after combustion, are the best fitted for smelting. 
The lead which is separated directly from the ore, is 
called ore lead, or Common lead, to distinguish it from 
that which is the result of a subsequent process. 
Slag-hearth Smelting . 
The slags or scoriae separated in the process of ore- 
hearth smelting, consist of the infusible part of the ore, the 
ashes of the coals, peats, &c. semi-vitrified and aggluti- 
