coal has formed. Coking coal is graded according to its ash percentage-by-weight after burning:

* Steel Grade I (Ash content not exceeding 15%)
* Steel Grade II (15-18%)
* Washery Grade I (18-21%)
* Washery Grade II (21-24%)
* Washery Grade III (24-28%)
* Washery Grade IV (28-35%)

The "hearth" process

The "hearth" process of coke-making, using lump coal, was akin to that of charcoal-burning; instead of a heap of prepared wood, covered with twigs, leaves, and earth, there was a heap of coal, covered

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