Mrass* 
ff 
upper opening or smaller end of the cone : over this open- 
ing is a perforated cover made of fire-bricks and clay, and 
kept together with bars of iron so as to lit closely. This 
cover serves to regulate the heat in the following manner 
the draught of air is formed through an under- ground 
vault to the ash-hole, thence through the grate and round 
the crucibles, and through the smaller upper opening into 
an area where the workmen stand, which is covered by a 
large dome and a chimney to convey the smoke into the 
outer air. When the draught is the strongest, and the 
heat is required of the greatest intensity, the cover is en- 
tirely removed and the flame then draws through the up- 
per opening of the furnace to a considerable height into 
the outer brick dome ; when the heat is to be lessened the 
cover is put on, which intercepts more or less of the 
draught from the furnace, as more or fewer of the holes 
of the cover are left unstopped. 
The crucibles are charged with the mixed calamine 
and charcoal, together with copper clippings and refuse 
bits of various kinds, and sometimes brass clippings also, 
most of which are previously melted and run into a small 
sunk cistern of water through a kind of cullender, which 
divides the metal into globules, like shot. Powdered 
charcoal is put over all, and the crucibles are then cover-, 
ed and luted up with a mixture of clay or loam and horse^ 
dung/ 
The time required for heating the crucibles and com- 
pleating the process varies considerably in different works, 
being determined by custom, by the quantity of materials, 
the size of the crucibles, and especially the nature of the 
calamine. In the great way from ten to twenty-four hours 
are required. At Holywell, in Flintshire, about twenty- 
four hours are taken. 
During the process, and especially towards the latter 
end, part of the reduced zinc which escapes absorption fay 
