2017 AGM The metallurgy of our portable heritage

This one-day meeting was organised jointly with the Portable Antiquities Scheme (www.finds.org.uk) and was held at the Institute of Archaeology, UCL on a glorious sunny June day.

As we’ve come to expect from HMS meetings, there was a packed programme with eleven papers on a wide variety of topics ranging in date from the Bronze Age to post medieval times. The involvement of the PAS meant that the majority of the papers dealt with metals and metalworking in Britain, but two of the speakers had travelled especially from abroad to present their papers. Takahiko Kutsuna spoke about the change of gold production from gold dust to gold ore in Japan. It is thought gold mining started in Yamanashi prefecture, central Japan, in the 16th century and two mine sites there were excavated in the 1990s. More recently scientific examination of ceramic finds has identified gold on them, demonstrating their use in smelting gold ores. Cupellation has not been identified from physical remains but its use is clearly shown on the Sado gold and silver mine picture scrolls that survive from the Edo period (1603-1868). Diya Mukherjee’s approach was rather different as she spoke about understanding lost wax casting through an ethnographic study of present-day craftsmen, showing a film of them at work. This technique has been used in the Indian sub-continent since at least the 3rd millennium BCE and is still in use today.

The Bronze Age was well represented with three papers. Miriam Andrews explained her work measuring the use intensity of Bronze Age palstave axes. Replicas were subjected to systematic wear by repetitive wood-cutting and were re-sharpened at optimum intervals. The incremental increase in hardness of the axe blades was due to both use and sharpening, and the results of the project can be used to estimate the degree of use of prehistoric axes and the number of times they have been sharpened. Harriet White described a Middle Bronze Age four-flange twisted gold bar torc, discovered by a metal detectorist near Reach, East Cambridgeshire in 2015. It was 1.265m long, weighed 732g and is one of the largest found in Britain, Ireland and the near Continent. Its composition was determined for the Treasure process and a technical examination investigated manufacturing methods and wear, comparing it with other bar torcs. Tim Young described his attempts to determine the composition of Late Bronze Age ingots from St Michael’s Mount, Cornwall. pXRF was used on external surfaces while some pieces were sliced to expose a fresh cross section. The coarse dendritic microstructure and the strong weathering of the external surface meant the analytical data proved extremely problematic; the most useful were those obtained with the pXRF on a cut surface, with multiple analysed areas, each of approximately 8 x 10mm. Representative elemental analyses were not obtainable from drilled samples. Although the analyses tentatively suggested a Welsh Borders source for the copper, the most important outcome was that unless previous analyses of ingots were made on large areas of fresh cross sections, they should be considered of dubious utility. These problems affect essentially the whole corpus of British Bronze Age copper ingots.

Matt Phelps described the scientific examination of precious metal jewellery from a mid-1st-century AD hoard from Colchester. The results demonstrate the application of a wide range of production methods including diffusion bonding, hard soldering, wire production by hammering and rolling, details on the setting of emeralds within the gold rings and information on the fabrication of the silver medallion. The jewellery was of a high-grade gold alloy typical of Roman compositions and much purer than Iron Age gold coinage. Eleanor Blakelock’s talk also focussed on gold: her analysis of objects from the Middle Saxon Staffordshire Hoard. They had found quite varied compositions with up to 30% silver and a few % copper in the gold. More surprising was the repeated observation that the surface 10-15μm of many objects was depleted in these elements, systematically giving the gold the appearance of a purer alloy than that actually used to make them.

Justine Bayley noted there was little technical innovation in the processes used to manufacture base metal objects during the 1st millennium AD. Evidence for the processes comes from part-made and finished objects as well as from the tools and debris that can be found in abandoned workshops; things such as scrap and waste metal, moulds and crucibles. She provided insights into the ways craftsmen worked in the past and how they made the metal objects archaeometallurgists study today. Kevin Leahy also dealt with metalworking processes, but by reviewing some of the finds of various dates reported to the Portable Antiquities Scheme. The earliest examples were a mould for a Bronze Age palstave and a Celtic coin die. Other, early medieval, dies showed how pressblech work and backing sheets for interlace were produced. Mis-cast objects and lead patterns for making clay moulds were also illustrated.

Alex Bliss described a class of finds that is virtually unknown from archaeological excavations. These are medieval jettons (accounting tokens), mainly of English origin, which were converted for use as brooches. The necessary fittings were riveted onto the obverse and the reverse was gilded; they are found mainly in East Anglia. Their manufacture, use and possible social significance to the people who wore them were all discussed. The final two papers concerned post-medieval objects. Ann-Marie Carey showed how she and her colleagues had reconstructed two objects from the 17th-century Cheapside Hoard by producing detailed laser scans of them. The scent bottle made of gold and precious stones was recreated from its many component pieces to understand the manufacturing processes involved, while for the Ferlite watch just the casing and the internal bell were recreated. This meant the bell could be struck; its peal was clear and beautifully pitched, an unusual association of sound with a museum artefact.

John Davis talked about the metallurgy of portable sundials, part of an ongoing study of medieval scientific instruments. The alloy compositions are being measured by XRF but copper-alloys suffer from the well-known problem of ‘dezincification’ which can seriously distort XRF results as it is a surface-sensitive technique. Experiments are being carried out to quantify and profile this loss of zinc. Eventually, it is hoped that some light may be shone on the locations of the workshops producing these early ‘mathematical instruments’ and on their sources of materials and the techniques employed.