VSF 2010 Report - Nabo

Smithing Hearth Bottom
During the process of smithing, slag may accumulate in the bottom of a hearth. Forming the
bottom of the heart, smithing hearth bottoms (SHB’s) typically have a convex base and are
circular to oval shape. The upper surface, the ‘top’ of an SHB may be planar (plano-convex
SHB) to concave, a depression caused by the air blast (concavo-convex SHB). The upper
surface may also be irregular. For the purpose of this study, a diagnostic SHB (or fragment
thereof) should exhibit a planar-to-concavo top, defined by a ‘rim’ which follows into a
convex bottom. Sometimes referred to as hearth cakes, SHB’s can range in size and weight
depending on the amount of slag that has accumulated. Primary smithing (bloom-refining),
often generates larger SHB’s due to the slag-rich nature of the bloom being worked,
compared to secondary smithing (manufacture, repair, re-working). SHB’s can be magnetic
due to the fragments of iron and hammer scale that has been incorporated into the slag. The
base may include impressions of charcoal, remnants of hearth lining, and sometimes other
material (sand, stones) adhering to the surface. SHB’s and furnace bottoms can be mistaken
for one another, and so careful attention needs to be paid to the diagnostic features.
Hammerscale
Iron oxides and microslags generated from smithing are known as hammerscale. They can be
identified by their shape and size. There are two types of hammerscale: platy (sometimes
termed ‘flake’), and spheroidal. During smithing, the iron can be heated in the reducing or
oxidising part of the hearth. When the iron oxidises, it forms a thin scale of iron oxide at the
surface. Upon being struck by a hammer, the iron oxide skin will usually break away from the
metallic iron substrate to produce platy hammerscale. Platy hammerscale is often very small
and magnetic, with flakes rarely being greater than 5mm in length. Platy hammerscale may be
associated with primary and secondary smithing. Spheroidal hammerscale (microslag) is often
associated with primary smithing. As a bloom is refined, the iron is consolidated as it
becomes depleted in slag. The entrapped slag inclusions are extruded from the iron during
smithing, squeezed out in a liquated state during the high temperature process. The extruded
slag cools upon contact with the air and solidifies into a spheroidal globule. The spheroidal
globules formed from primary smithing may contain a central void of vesicle that forms as the
rapid cooling of liquated slag rapidly cools in combination with the process of any escaping
gases. Spheroidal hammerscale may also form from any fluxes that have been utilized during
the smithing process, which liquate and are expelled from the surface of the worked iron
along with the departing iron oxide fragments. This hammerscale consists of an iron oxide
core subsumed within a globular spheroid of slag, and may result from primary or secondary
smithing. For the purposes of this report, the category ‘magnetic residues’ is employed, in
which hammerscale is included. Due to their microscopic nature, it is difficult to assess
whether all magnetic residues represent hammerscale, and so the term ‘magnetic residues’
will be employed.
Hammerscale is often missed during excavation due to its size. Only by employing
certain excavation strategies can it be systematically recovered. The Society for Historical
Metallurgy provide the most up-to-date methods dedicated to the recovery of metallurgical
remains in archaeology (Bayley et al. 2001; Bayley et al. 2008:29; Dungworth & Paynter
2006: 7, 13; McDonnell & Starley 2002; Starley 1995, 2002). Mapping the distribution of
hammerscale may yield information on the location of the hearth and anvil using during
smithing.
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