and no fears need be entertained of its having an unfavorable
influence on the calcination; for when carefully separated
from the finest class, a heap formed entirely of ragging will
give reasonably good results. The extreme outside edge of
the ore, when all is in place, should not entirely cover the
external border of wood. At least a foot of uncovered fuel
should project beyond the layer of ragging, both to prevent the
ore from sliding off its bed as well as to insure a thorough
kindling of the outer covering of mineral. The amount of
wood required properly to burn a heap of 240 tons of ore will
vary greatly with the composition of the latter, standing in
direct proportion to its sulphur contents, and especially to the
amount of bisulphides present, but may, on the average, be
estimated at 12 cords, or one cord of wood to 20 tons of
ore. In smaller heaps, this proportion must be considerably
increased.
The fine ore that is to form the external layer, and on which
depends largely the success of the process, is seldom placed
upon the heap until after it is fired. Perhaps the most
judicious practice is to cover the sides of the pile with a very
thin layer, scattering it evenly with a shovel, and leaving the
upper surface, as well as a space eighteen inches broad at the
bottom, uncovered ; for if the fine ore is thrown carelessly upon
the lower circumference of the pile, the draught is decidedly
hampered and the fire stifled before getting fairly under way.
