The science of lead smelting

Lead smelting took place in Marske until at least the eighteenth century – the 1759 map of Clints shows conspicuous smelt mills at the appropriately named Orgate.  The map shows that ore was brought over by packhorse from Arkengarthdale¹ and processed into lead metal before being taken to markets via Stockton or Darlington². 

Smelting is the process through which this is achieved – a process that involves some of the chemical components being in molten form.  The principal lead ore in Swaledale (and indeed world-wide) is galena, or lead sulphide (PbS) – a silvery coloured mineral which has a cubic habit.  The technology to produce lead ore dates back to Roman times and folklore records that a Roman lead ingot was found at Hurst³. Chemically the process of turning galena to lead metal involves two stages. The best results (i.e. lead metal with fewer impurities from ores with modest concentrations of lead) require some control of those processes. 

Before any attempt to create metallic lead could begin the ore and rock would be crushed, and then processed as far as possible to concentrate the galena.  Typically the galena was concentrated in the crushed material by rinsing it with water to wash off the lighter weight components (“dressing”).  The more effectively this could be done the fewer packhorses would be needed to carry the ore from Arkengarthdale to the smelting sites near Orgate!  For example the lead metal transported from Swaledale in the nineteenth century for example equated to 200 outbound packhorse journeys per day (see pages on Marske’s Temperance Hotel). Transit of unsmelted ores would have required even more packhorse trips – which may explain why the smelting of ores from Arkengarthdale near Marske didn’t persist.

The first stage of the smelting process turns lead sulphide (galena) into lead oxide.  This is achieved through “roasting” – a metallurgical term for the heating of an ore in a gas (in this case the oxygen in air). Roasting galena at a temperature of around 1000⁰C leads to the following chemical reaction⁴. 

2 PbS + 3 O₂ -> 2 PbO + 2 SO₂

The second stage in the smelting is to create metallic lead from the lead oxide.  Chemical reactions that remove oxygen are called “reduction” reactions (the opposite is oxidation).  The less reactive the metal the easier it is to reduce⁵ – this accounts for the fact that the history of lead production goes back many millennia – whereas the smelting of more reactive aluminium, for example, had to wait until the nineteenth century⁶.  In the case of lead its oxide can be removed by reactions with carbon or carbon monoxide at around 1200⁰C.  This produces molten lead through the reactions below⁴,⁷.

2 PbO + C -> 2 Pb + CO₂

PbO + CO -> Pb + CO₂

Lead metal melts at the compatively low temperature of 327⁰C⁸.  Hence, at the temperatures at which these reactions take place lead will be molten. It would quickly percolate through solid materials in the furnance to pool at the bottom where it could be collected once everything had cooled down.

In fact, at the temperatures of these reactions some of the lead would be a vapour, which together with the release of sulphur dioxide from the roasting process, means that smelting produced a wholly noxious combination of gases.  For these reasons whilst the internet has recipes for experimental hobbyists to produce copper from ore, doing the same from lead falls into the category of “don’t do this at home”!  Unfortunately the work of the Orgate smelters in the eighteenth century is very likely to have shortened their lives. 

As noted the smelting process required temperatures of over 1000⁰C to be generated.  This is challenging as most wood combusts at between 300⁰C and 600⁰C – temperatures where the volatile organic compounds that fuel combustion burn off⁹.  Charcoal however burns at temperatures exceeding 1100⁰C¹⁰.  Charcoal is wood that has been heated in a low oxygen environment, allowing the volatile compounds to be removed without fully combusting the carbon-rich “wood”.  As has already been noted the reduction of lead oxide to form lead is also aided by a low oxygen environment.  Hence the wood fires that were used to smelt lead would have also turned some of the wood fuel into charcoal – creating a higher termperaure in the furnace.  It is also possible that charcoal was created separately as a fuel and then fed into the furnaces that smelted the lead ore.  Either way a lot of wood was needed to smelt lead ore. The Marske, Clints and Orgate areas, then as now, had a plentiful supply of wood – partly on account of being on steep ground that was not suitable for grazing.

Finally most chemical reactions work in both directions.  In the case of the roasting of galena (lead sulphide) if there is either a deficit of oxygen during combustion, or if the sulphur dioxide is allowed to build up, then the reaction may not run to completion.  (For this reason the industrial scale smelt mills, such as those in use at Marrick in the nineteenth century, had elaborate chimneys both to vent fumes and draw in oxygen). If the chemical reactions do not run to completion then impurities may remain in the final lead metal.  Furthermore the original lead ore itself would also have contained impurities including the rock that was crushed alongside the galena, and probably traces of zinc and copper ores.  By mixing the ore with crushed limestone (as a “flux”) parallel reactions took place that removed some of these impurities¹¹.  Calcium oxide, in the form of lime, was often used for this purpose, since it could react with the carbon dioxide and sulphur dioxide produced during roasting and smelting to prevent them impeding the primary lead smelting reactions above.  Again there was no shortage of limestone near Orgate and Clints for this purpose – including that that had accumulated as large loose blocks on the slopes of Clints Scar.

Industrial scale smelt mills, such as those in use at Marrick in the nineteenth century, had elaborate chimneys to vent funes and draw in oxygen

In this way ores from Arkengarthdale or Hurst were smelted in the Clints and Orgate areas near Marske in the eighteenth century. However as the most significant sources for lead ores were west of Marske it is likely that as production volumes grew in the nineteenth century, perhaps coupled with the greater use of coal from the Tan Hill area, that it became more economical to continue smelting closer to the source of the majority of the Swaledale ores. Hence the Marske area remained dominated by farming throughout the nineteenth century as is amply evidenced by comparing the Marske and Hurst census returns from those years.

1. Richardson, R. 1759. A Plan of the Clints Estate. Coloured estate plan in North Yorkshire County Archives, Ref ZAZ(M)3.[↩][↩]
2. Wright, G.N..  1985. Roads and Trackways of The Yorkshire Dales.  Moorland Publishing Company.[↩]
3. Insert ref[↩]
4. Wikipedia.  Lead(II) oxide.  Accessed 2023.[↩][↩]
5. BBC Bitesize.  The reactivity of metals.   Accessed 2023[↩]
6. Philadelphia Science History Institute Website.  Aluminium.  Accessed 2023.[↩]
7. Essential Chemical Industry Online.  Lead.  Accessed 2023.[↩]
8. Royal Society of Chemistry.  Lead.  Accessed 2023.[↩]
9. Engineering toolbox website.  Wood – combustion heat values.  Accessed 2023.[↩]
10. Wikipedia.  Charcoal  Accessed 2023.[↩]
11. Wikipedia.  Smelting.  Accessed 2023.[↩]