HMS Datasheets

Even after investigation and publication, metallurgical samples offer a valuable resource for future researchers. Experience has shown that such material is too frequently lost. This data sheet provides guidance to ensure that samples are securely housed.

X-radiography is a rapid imaging technique which is particularly useful for examining archaeological metalwork. The process is similar to conventional medical radiography and the two-dimensional negative image is usually produced on photosensitive film. Although Xradiography cannot identify the nature of the metals and other materials under examination, these may often be inferred from the morphology and densities in the image, and can be determined by analytical techniques such as scanning electron microscopy (SEM) based microanalysis and X-ray fluorescence (XRF) analysis (see Datasheet 12).

The composition of artefacts or samples can be determined by chemical analysis. The method of analysis chosen depends mainly on the answers needed. Some types of chemical analysis are quantitative, providing precise information about composition in percentages or parts per million; others give qualitative results, identifying the main elements or compounds present, and provide a rough idea of relative concentrations. Some methods require samples which will be destroyed by the analysis (destructive analysis) but surface analysis can be performed without damage to the artefact. Other considerations are cost and availability of equipment.

Metallography requires the removal of small samples, which are then mounted in a resin or bakelite block, polished and etched in dilute acid, before examination under a metallurgical microscope. This reveals the crystal structure of the metal, from which an assessment of the type of alloy and its mechanical and heat treatment history can be made. Metallography thus provides a good measure of the quality of the metal and its suitability for a particular application. Scott (1991) provides a good introduction to the structure of metals, metallography and the phase diagrams which help explain the microstructures it reveals.

Both types of hammerscale generally survive well in archaeological deposits and often retain their original metallic-lustrous sheen. They are generally too small to be recognised by the excavator, but can be recovered from soil
samples as they are strongly magnetic. Sometimes thick deposits become concreted together with iron hydroxides and may be dismissed as iron-panning.

Identifying the nature and extent of metalworking processes on an archaeological site relies on adequate recovery and recording of the evidence of these activities. This sheet provides advice on how standard archaeological practice can be supplemented to achieve this.

About 1500 currency bars and other forms of trade iron have been found in Britain, including two doublepointed ingots of continental type. Detailed examination of most of the 400 surviving bars has shown that there are at least 20 distinctive types, which can be distinguished by their dimensions, the shape of the socket and, sometimes, by the welding of the tips. A new class of trade iron has been recognised, consisting of billets with one end forged either to a hook-shape or to a long point.

The smelting of iron by the direct process results in a bloom, which must be refined to produce forgeable iron stock. Blooms produced from bog ores in a non-tapping furnace would have had more entrapped slag and would have needed more refining than the cleaner blooms made from an ore which could be smelted by slag tapping. At least the initial stages of refining would be carried out at the smelting or primary production site, so that both smelting and refining residues can be found together.

Once refined the iron stock would be traded to the blacksmithing community and the smithing or forging of the iron would be carried out at a secondary production site, which could be a specialised workshop or a small forge attached to a settlement or a farm.

All the stages of refining and smithing produce broadly the same kinds of structural and residue evidence and there is little of it which is diagnostic of a specific stage of the process. The characterisation and interpretation of this kind of evidence depends mainly on the size and relative quantities of the different types of residue.

There are many variables in the smelting process, including the ores, the fuel and the clays used, the furnace construction, the method of blowing the furnace and the way in which the slag and bloom were removed. These result in a wide variety of slags and other residues. Although it is possible to define and describe some basic types of slag, in reality there is a more or less continuous transition from ore through slag to the finished iron. The materials to be described fall into four broad groups – the raw materials, the structural evidence, the waste products and the bloom.