Historic iron ore smelting in Scotland

From very early days there was iron ore smelting in Scotland, but initially on a very small scale and in a very crude form. The smelting ‘furnace' was a rough hearth or fireplace in the open air, and the fuel was wood or charcoal, and the iron ore was bog iron ore. This is an ore which forms in bogs by the precipitation of iron from groundwater with the aid of bacterial action. In this ore the iron is loosely bonded to oxygen so it is fairly easily smelted to release iron metal. Interestingly, bog iron is still being formed today, and it has been shown that a deposit can regrow within 20 years after harvesting. So it was a renewable resource! These primitive hearth furnaces were called ‘bloomeries' and they operated throughout Scotland, including Argyll. Their historic age is often difficult to ascertain, but many have been assigned to the 13th to 16th centuries. MacAdam (1886*) lists many bloomery sites in this area including Easarchan, Garvalhill, Drimdarroch, Leanach, Leak, Esmore, Feorline (all at Stralachlan), and Strachurbeg and Phuill at Strachur. (For more details, consult the original report, cited below.) MacAdam examined a deposit of bog iron ore in the Kilfinan / Otter Ferry area.

During the smelting process, molten iron is produced and a surface slag of impurities is scooped off and discarded.

plate 11

PLATE 11 SLAG FROM IRON ORE SMELTING A. Heavy iron tap slag from primitive smelter or bloomery in Loch Tay area.
Note that it is metallic in appearance and heavy, and probably has a high iron content
B. Glassy slag from historic Bonawe iron works, Taynuilt

Sample 11A (Plate 11, sample A) is heavy iron tap slag from Loch Tay area, exact source uncertain. Slag from primitive furnaces often contained a high proportion of iron, and so were often thrown back into the furnace for a second round of smelting.

As the need for iron grew, so the furnaces developed into blast furnaces capable of high sustained temperatues which produced a purer iron product and a much lighter, glassy slag containing very little iron. A furnace at Bonawe, Taynuilt opened in 1730. It used iron ore from the north of England and charcoal from the woods around Bonawe, as it was cheaper to transport heavy, compact ore to Scotland than to transport lighter and bulkier wood to England. This smelting operation was destructive of the local forests despite efforts to make it more sustainable by coppicing of the trees. Eventually with the advent of the Industrial Reveolution and the use of coal for smelting, the furnace at Bonawe fell into disuse and closed in1866. A similar furnace opened on Loch Fyne in 1754 at Goatfield (the name was later changed to Furnace).

Sample 11B is slag from Bonawe furnace. Note how glassy and lightweight this is compared to sample 11A, and there is probably very little iron remaining in the Bonawe slag.

*MacAdam W I, 1886: Notes on the Ancient Iron Industry of Scotland. Proceedings of the Royal Society

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