Spinel-rich slag and slag inclusions from a bloomery smelting and smithing experiment with sideritic ore.
Sarah Paynter, Peter Crew, Eleanor Blakelock and Gareth Hatton.
Historical Metallurgy Journal Volume 49, Issue 2
This paper describes the scientific investigation of the raw materials, iron products, slags and slag inclusions from a bloomery smelting and bloom forging experiment. The smelt used a high alumina sideritic ore, a clay-built shaft furnace and a blowing rate of around 200 litres per minute. The smelting slag produced was viscous and frothy, most similar in composition, microstructure and morphology to slag from medieval bloomeries smelting similar ores. Unusually, the slag contained magnetite rather than the typical wüstite, in the form of dendrites which were often nucleated on euhedral spinel crystals. The distinctive microstructure and composition of the smelting slag allowed inclusions of this slag to be differentiated from others which were formed and modified during smithing from bloom to bar. The silica to alumina and the silica to lime ratios in the bloom and bar slag inclusions were lower than in the smelting slag. A series of materials balance calculations indicates the influence of different proportions of furnace lining, fuel ash and ore on the smelting slags and the inclusions.