Slag Hearth

A secondary smelting hearth, built alongside a lead smelt mill's main ore hearth, used to re-smelt the slag the ore hearth produced and recover the lead still trapped within it.

Historical usage

Ore hearth smelt mills, introduced to England from the Mendips around 1577, remained the normal way of smelting lead until the 18th century, when they were increasingly replaced by the reverberatory ('cupola') furnace. Larger 18th- and 19th-century smelt mill complexes commonly combined several ore and slag hearths with roasting furnaces to prepare ore, refining furnaces to extract silver by cupellation, and reducing furnaces to recover lead from the cupellation residue, all connected by increasingly elaborate flues and condensers built to trap lead fume that would otherwise have escaped up the chimney.

How it worked

An ore hearth's slag still contained recoverable lead; feeding it into a separate slag hearth and re-smelting it at a higher temperature -- fired with charcoal or, later, coke -- extracted a further yield rather than discarding the slag as waste.

Why it was used

Reprocessing slag through a dedicated hearth substantially increased the total lead a smelt mill could recover from a given quantity of ore, at the cost of additional fuel and labour.

Images

Mines associated with this term

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this might plausibly apply.

• Old Gang Mines — North Yorkshire (surviving physical example)
  The mill building survives to roof height in places and contains the remains of a wheelhouse, a slag hearth and three ore hearths.
• Grassington Moor Mines — North Yorkshire (well documented example)
  The 1792 Cupola smelt mill combined two reverberatory furnaces, a roasting furnace and a slag hearth.

Sources

1. (primary) her.derbyshire.gov.uk (Designation DDR297) — Derbyshire Historic Environment Record
2. (secondary) Smeltmill — Wikipedia

Record created: 31 August 2026 · Last researched: 31 August 2026