Lead . 
237 
from lead, or be capable of being, by a very slight opera- 
tion, reduced into lead again. 
It has been mentioned in the preceding essay, that red 
lead is made from litharge at Holywell : this red lead, 
which is made from litharge, is not perhaps, in all its pro- 
perties, of quite the same kind with that which is made di- 
rectly from lead ; at least I have been informed, that the 
makers of flint glass, who use much red lead in the com- 
position of that glass, are of opinion, that the litharge red 
lead does not flux so well as that which is made from the 
direct calcination of lead, as is practised in Derby- 
shire. There are in that county nine red lead mills or 
furnaces, all of which are much upon the same construc- 
tion. 
The furnace is very like a bakers oven, its vaulted 
roof is not a great distance from the bottom or floor, on 
each side of the furnace there are two party walls, rising 
from the floor of the furnace, but not reaching to the roof ; 
into the intervals, between these walls and the sides of the 
furnace, the pit- coal is put, the flame of which being 
drawn over die party walls, and striking upon the roof, is 
from thence reflected down upon the lead, which is placed 
in a cavity at the bottom, by which means the lead is soon 
melted. The surface of melted lead, when exposed to 
the open air, instantly becomes covered with a dusky 
pellicle ; and this pellicle being removed another is form- 
ed, and thus by removing the pellicle, as fast as it forms, 
the greatest part of the lead is changed into a yellowish 
green powder. This yellowish powder is then ground 
very fine in a mill, and being washed, in order to separate 
it from such parts of the lead as are still in their metallic 
state, it becomes of an uniform yellow colour, and, when 
it is dried to a proper consistency, it is thrown back again 
into the furnace, and being constantly stirred, so that all 
its parts may be exposed to the action cf the flame of the 
