The maximum rate of combustion may be as much as so lb of coal per square foot of grate per hour, and in exceptional cases even a greater rate than this has been maintained.
The element is melted, either by the heat of its own combustion or other means, and runs off from the earthy residue.
The first cell is heated and the products of combustion are led into the second cell where they give up part of their heat to the contained ore, so that by the time the first cell is exhausted the mass in the second cell is at a sufficiently high temperature to ignite spontaneously when air is admitted.