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12 Apr 2019

Smelting moments on Ben Lomond

Written by Derek Alexander
Excavating one of the bloomery mounds on Ben Lomond
Excavating one of the bloomery mounds on Ben Lomond
Volunteers on a recent Trust Thistle Camp learned first-hand just how hard it would have been to make iron in the past.

One of the archaeological sites on the Ardess Hidden History Trail, in the woodland on the lower slopes of Ben Lomond, has long intrigued property manager Alasdair Eckersall. A prominent green mound above the treeline (site 6 on the trail) has produced quantities of iron slag and is described as a bloomery mound (a mound of waste material produced by iron smelting). As part of our Thistle Camp, we excavated small archaeological trial trenches across the mound and also built a replica furnace.

Geophysical survey over the bloomery mounds
Geophysical survey over the bloomery mounds

Before excavation, a geophysical survey of the site was conducted by staff and students from the University of Glasgow’s Archaeology Department. This located the concentrations of burnt and magnetic material, allowing the trenches to be positioned accurately. One trench investigated the main bloomery mound, which was shown to consist of a deep deposit of waste material rich in charcoal and iron slag. The second trench located a substantial feature that might be the flue or bowl of an iron smelting furnace. Another small trench over a shallow hollow close to the site uncovered traces of burning and might represent where the iron ore, dug up locally from the soil, was roasted prior to smelting.

Large lump of iron slag from the excavation
Large lump of iron slag from the excavation
Quote
“Although limited, the archaeological excavations have given us a better understanding of the site and samples of charcoal will hopefully provide radiocarbon dates.”
Derek Alexander, Head of Archaeological Services

Excavations on similar sites in Scotland have shown bloomery mounds to date from the 14th–16th century, but using technology similar to that used in smelting dating back to the Iron Age. Before the introduction of large-scale blast furnaces in the 18th century most iron making would have been on a domestic scale, local to the sources of both iron ore and charcoal. The woodlands along the slopes of Ben Lomond would have been ideal for producing the large quantities of charcoal required for the smelting process.

To better understand both the process of smelting and how the archaeological remains were formed, the Thistle Camp volunteers undertook the experimental construction of a replica furnace. 

Alasdair Eckersall overseeing the replica iron smelting furnace
Alasdair Eckersall overseeing the replica iron smelting furnace

Iron-rich soil was dug up locally and then roasted, to dry out the iron ore and break it down. Two sets of bellows were constructed from wood and leather (in this case a couple of old sofas!). A small bowl furnace was made from local clay and then fired with the appropriate mix of Ben Lomond charcoal and the iron ore. Unfortunately, the weather was very wet and the clay of the furnace remained damp. Six hours of pumping the bellows by hand only produced some small quantities of slag and no iron bloom!

The replica iron smelting furnace with its two sets of bellows
The replica iron smelting furnace with its two sets of bellows

This initial foray into the ancient technology of iron smelting has only whetted our appetite for more. Next time we’ll get it right but it’s certainly given us a greater appreciation of just how much effort in the past had to go in to producing something that we so easily take for granted.

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