108 
Iron* 
most exposed to air. In this progress the fire of each vent 
meets, and, when united, rise gradually, and burst forth 
on all sides at once. 
If the coal contains pyrites^, the combustion is allowed 
to continue a considerable time after the disappearance of 
smoke ; the sulphur then becomes disengaged, and part 
of it is found in flowers upon the surface of the heap. If 
the coal is free from this hurtful mixture, the fire is cover- 
ed up in a short time after the smoke disappears ; begin- 
ning at the foundation, and proceeding gradually to the top. 
The length of time necessary to produce good coaks 
depends upon the nature of the coal to be coaked, and 
the state of the weather. In 50, 60 to 70 hours the fire 
is generally completely covered over with the ashes of 
char formerly made : the coaks, thus entirely secluded 
from air, soon cool, and in 12 or 14 days may be drawn 
and wheeled to the furnace. 
It is with pit-coal, in a great measure, as with wood, In 
the process of charring. Coals do not always afford a 
weight of coaks In proportion to the quantity of carbon In 
the coal. It depends more upon the capability of the coal 
to resist the waste occasioned by combustion, supported 
by external air, than upon the real quantity of carbon in- 
herent in the coal ; as may be seen by comparing the loss 
of weight which coals undergo in the large way, and the 
results obtained from the same coals, by burning them un- 
exposed to external air in the small way. 
The loss sustained in coaking coals fit for the purpose 
of manufacturing, or smelting iron, is found to be nearly 
as follows : 
2240 pounds of free coals - yield 700lbs. coaks, loss 1540lbs, 
2240 - - of splint and free coal mixt 840lbs. coaks, loss 1400lbs, 
2240 - of splint slightly mixt yield lOOOlbs. coaks, loss 1240lbs. 
3240 - - of pure splint - - 1100 - - loss USOlbs,* 
* Iron combined with sulphur. T. €,■ 
