THE MANUFACTURE AND PROPERTIES OF COKE. 19
coal lying- on the bottom of a cold or hot oven being vmcoked or fused. The" coking process proceeds from the top of a charge. There are times when the heat of the crown of a very hot oven may fuse the top surface of the coal and form a thin crust or film which will prevent the usual freedom in the escape of gases. These being held back for a time, will deposit a soot or lampblack in the cells of the forming coke so as to result in giving black tops, or a black coke. As soon as the gases gain sufficient pressure to burst through the top crust or film, then the deposit of sooty matter ceases.
Stock coke is generally of a smaller* size than that conveyed directly from ovens to cars for shipment, for the reason that it is broken up by extra handling. It is called stock coke for the reason that it is coke that, for want of orders or cars to make shipment, has to be stored in large piles at the coke works — sometimes months and sometimes years. Lying thus it is subjected to rain, snow, dust, and smoke, collects excessive moisture, and becomes dirty. Sometimes, in order to keep the ovens going and save stocking, heavy charges are resorted to and the coal coked from ninety-six to one hundred and twenty hours. This process causes a loss of coke in the ovens.
The fixed carbon in coke used for furnace and foundry work g-enerally ranges from eighty to ninety per cent. vSometimes it is considerably under this, and occasionally it may exceed the highest limits by two to five per cent. Some of the carbon is lost by the process of coking. If cooled by water at the proper time the percentage lost is rarely very large. When more than from two to four per cent of carbon is lost,theethod usedt has proved an excellent fuel for this and kindred uses.''