Kilian Anheuser and Peter Northover
In 43 AD a large part of the British mainland formally entered the Roman world after a century of economic and political contact following the incursions of Gaius Julius Caesar in 55/54 BC. The Roman army, bureaucracy and traders brought with them enhances access to the metallurgical skills and practices of classical Mediterranean civilisations. They were entering, though, a realm with its own metallurgical tradition and one which, in that same century, had produced some supreme works. The first part of thus paper will explore in tum this Celtic achievement and the impact and assimilation of Roman ideas, and then look at the work of the Anglo-Saxon settlers in the twilight of the Roman occupation, perhaps the first really large-scale users of gilding in Britain. The second part will describe a research programme on mercury amalgam gilding, a technique used in all three of these archaeological periods. It will provide the first fully quantitative data on process variables available to archaeometallurgists and conservators, and show what was required to achieve the different effects observed on archaeological metalwork.
The history of decorating metals with other metals in Britain goes back to the Early bronze Age at the end of the 3rd Millennium BC. The metals of choice were always gold and tin, never silver. Throughout the Bronze Age gold was always applied as foil, as far as can be seen either mechanically or with an adhesive and examples will be shown. Gold was used generally to ornament and highlight parts of an object rather than a whole one, while tin was used to cover a whole axe and thermally bonded either via wipe tinning or cementation. Contrasting colours of gold could also be used on the same object.
At the end of the Bronze Age, c. 700 BC gold disappears from the archaeological record save in Ireland and remote parts of Scotland It does not reappear until the 2nd century BC in the form of imported gold coins. Then, suddenly, perhaps around 80-70 BC, the Iceni, the ‘Celtic’ tribe settled in Norfolk, perhaps inspired by imported gold torcs, established a workshop to convert imported bullion into a long series of elaborate torcs, with analogues in silver, bronze and iron. During the 50s BC there was a massive influx of gold used by the Gauls to buy military assistance against Caesar, but with the Roman occupation of Gaul this ceased. For a period, there was a rapid debasement of both coinage and torcs. With the latter the object seems always to have been to make the torcs as golden as possible. To this end, mercury amalgam gilding on bronze as well, perhaps, as depletion gilding; similarly silver was plated onto copper. From about the 40s BC, though, the manufacture of torcs ceased and bullion became almost exclusively used for coinage, although there is a small number of silver and white gold ornaments and brooches, and some of these were gilded. However, whether officially or unofficially, plated coins were circulated and gold plated examples will be shown.
The Roman conquest paradoxically may initially have decreased the availability of gold as the Celtic coinage was called in, melted down and refined. The picture of gilding that we have in Roman Britain is surprisingly limited. The techniques were much the same as those available to the Celts and it is hard to say what influence there was in either direction. Evidence from other materials suggests that in occupied Britain Celtic technology was effectively submerged by 100 AD. Thereafter the application of gilding reflected Roman practice from small ornaments to large statues, with mercury gilding becoming the norm.
Towards the end of the Roman occupation Germanic settlers were invited to help stem incursions from their more violent neighbours to the north and west. These settlers brought their own thriving metallurgical tradition, and early Saxon cemeteries are characterised by a rich variety of brooches, very many of them thickly mercury gilded Tentative results suggest that they might have developed their own tradition whether originally influenced by Rome or not. Gilding is often thick and can be heavily burnished. Mercury contents are high and suggestive of low temperature or short firing times: low temperatures also characterise contemporary tin-plating. As a tailpiece we can see actual workshop remains of the amalgam gilding process from a Middle Saxon context in Southampton.
We have seen in this paper the early history of amalgam gilding in the west; by the 2nd or 3rd century AD it became the standard technique that it remained until the introduction of electroplating in the 19th century. Existing analyses showed that the gold amalgam paste, after firing, had a residual mercury content of 5-20% and either a porous or a solid microstructure. The presence of mercury has been taken as an indicator of amalgam gilding, its absence to signify leaf gilding. This paper reports results from fire-gilding experiments on copper and silver executed under controlled conditions of temperature and technique. It has been possible to reproduce both the structure and surface appearance of ancient gildings and these can be related to temperature, duration, and thickness of the gilding layer. Variations in the pre-treatment of the surface and the application of gold as leaf or foil on an amalgamated surface are explored. For examination of experimental products we have used optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction.