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.
An anthracite occurring in connexion with the old volcanic rocks of Arthur's Seat,Edinburgh, which contains a large amount of sulphur in proportion to the Caking coals.