VSF 2010 Report - Nabo

detail has been paid to the study of slag, of which only a short summary has been given here
(Bachman 1982; McDonnell 1989: 373; Paynter 2006; Rostoker and Bronson 1990: 91-4;
Scott 1990: 155-8; Tholander 1989; Tylecote 1980: 223-4, 1987: 291-324).
The ‘bloom’ of iron that is formed at the bottom of the furnace is often rich in slag
during bloomery smelting. In order to remove the majority of this slag, the bloom is usually
hammered on an anvil in a process known as ‘primary smithing’. ‘Secondary smithing’ is
where the metallic iron bar, or billet is formed into a finished object, and is also the process
by which iron objects are reworked, modified or repaired. Slag that is expelled from the iron
bar, or object, accumulates in the hearth to form ‘smithing hearth bottoms’ (SHB’s), also
referred to in shape as ‘plano-convex bottoms’. During the secondary smithing stage
hammerscale is formed whereby flakes of oxidised iron and spheroidal droplets (that solidify
in the air before landing) detach from the surface of the object being worked during striking.
Methodology
The archaeometallurgical residues collected were first examined visually, before samples
were selected for further chemical and microscope analysis. The methods employed for this
study are described in detail below. All abbreviations, symbols and acronyms used throughout
the report in the text and illustrations are listed in Appendix 1.
Visual assessment
The maximum and minimum dimensions for each classification type were recorded, as well
as the frequency 14 (where possible) and the total weight (g). The classifications used for this
assessment are described and justified in the following section.
Sample selection
Five samples were selected from the assemblage representative of the different slag
morphologies identified. Two smelting slag fragments were sampled, along with one sample
of tapped slag, flowed slag, hearth bottom, and undiagnostic slag. All samples analysed in this
report derive from the most productive slag context W319 (VSF05) and correspond with
Finds Number 113. Rather than compare different samples from different archaeological
contexts, it was decided to gain a representative selection from one context where the slag
residues are assumed to be related.
Preparation and analytical methods
Each sample was obtained using a water-lubricated rotary silicon carbide (SiC) grinder cut-off
machine. The sample was ground using 120 grade SiC abrasive paper prior to being mounted
in a two-component epoxy resin. The resin block was ground using SiC abrasive grade paper
(grade 120-4000) and then flatly polished using diamond paste mediums 6 and 0.25µm.
A metallographic study was performed on each sample using a metallographic optical
microscope in plane polarised light (PPL) to assess the microstructure and distinguish
different zones of interest. All micrographs are provided with scale bare (in micrometers) and
microscope magnification (100X, 200X). Each sample was carbon-coated for chemical
analysis. The bulk composition of each sample was determined based on five area analyses
14 For some contexts, a count is provided for ‘hammerscale’. This count represents the number of slag spheres that were
identified, and do not represent the total count of the magnetic residues collected.
100