Charcoal

Produced by partly oxidizing wood in a low-oxygen fire, charcoal has the virtue of burning hotter and more steadily than wood. Riddley lives in an Iron Age society and to smelt iron, you need a charcoal fire.

Although the chard coal berners are secretive, from what Riddley says they seem to use the same method as old Anglo-Saxon colliers, building a specially shaped covered woodpile or "clamp" (what Riddley calls a hart) for a slow controlled burn. EB

See also

Regia Anglorum: Charcoal Burning

Anglo-Saxon and Viking techniques