For efficiency the operation must be conducted with small quantities; caking may be prevented by mixing the substance with sand or powdered pumice, or, better, with iron filings, which also renders the decomposition more regular by increasing the conductivity of the mass.
The property of caking or yielding a coherent coke is usually absent, and the ash is often very high.
Very important distinctions-those of caking or non-caking-are founded on the behaviour of coals when subjected to the process of coking.
The caking property is best developed in coals low in oxygen with 25 to 30% of volatile matters.
As a matter of experience, it is found that caking coals lose that property when exposed to the action of the air for a lengthened period, or by heating to about 300° C., and that the dust or slack of non-caking coal may, in some instances, be converted into a coherent coke by exposing it suddenly to a very high temperature, or compressing it strongly before charging it into the oven.
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