‘With its iron-rich bedrock, and with a flat, poorly draining and acidic hinterland, Tiree is likely to have been quite well-supplied with iron in the late prehistoric and Early Medieval periods. Many forms of the Lewisian gneiss complex have a high iron content, concentrated in its darker bands. This is slowly leached from the surface of the rock by water in a process of chemical weathering. Iron-oxidising bacteria then feed on this iron-rich water. In doing so, they produce nodules containing iron oxyhydroxide, which collect in the banks of small streams on the sliabh. This is bog iron. These flat landscapes cut by streams lined with darkly stained rocks, and pools of rust-coloured water with their characteristic iridescent film, were highly prized in late prehistoric and Early Medieval times. They are very common on Tiree. A plausible derivation of the Tiree primary settlement name Ruaig is ON *Rauðavík ‘the red inlet’ from the iron-rich water in its hinterland (see Gazetteer). The modern name of the moorland east of this settlement is G An Sliabh Dearg ‘the red moor’. Bog iron is a renewable resource, and an area can be re-harvested approximately every generation. Smelting and smithing were extremely fuel-intensive, however, and iron production and processing came at some environmental cost. Tiree’s woodlands are likely to have been much reduced by the Early Medieval period (see section 5.6.5). Most of the charcoal needed to create the high temperatures demanded must have been made by the controlled burning of peat over several days inside a turfcovered clamp. Bog iron nodules were first roasted to create surface cracks. Smelting was done in a small furnace called a bloomery. This was made from clay that was tempered with dung. After lighting a fire inside the furnace to dry the clay, the bog iron and peat-charcoal were loaded in equal proportions. After setting light to the mixture, the furnace was then pumped from below with bellows. Liquid waste slag collected at the base and could be drained through a small hole, leaving the sponge-like iron-rich bloom to be picked out of another hole with tongs. The bloom then had to be beaten and re-heated to purify it. Objects made with bog iron have a characteristic sheen due to the high silica content and are relatively rust-resistant. Bloomeries, with their unmistakable conical heaps of slag, were usually sited on the sliabh near the raw materials, and slag and pottery moulds have been found recently at Baca Charachain, a deflated dune in Balevullin.’ Holliday, J. (2021) Longships on the Sand, pp. 200–1.