Under the microscope: the Oxus treasure and Scythian gold
As part of the research supporting the BP exhibition Scythians: warriors of ancient Siberia, I studied eight remarkable gold objects from the Oxus treasure. This treasure consists of about 180 gold and silver objects dated from c. 500–300 BC. Many objects in the Oxus treasure are of the so-called Achaemenid Court style (the ruling dynasty in Persia c. 550–330 BC), but a few are associated with the Scythian-style art of western Siberia. They include a pair of bracelets with terminals in the shape of winged beasts with long snouts, a finger ring with a winged lion, an aigrette in the shape of a lion-griffin, a possible bow-case attachment with a bird’s head and three roundels. The roundels depict a demon’s face, a lion’s face and boars and ibex heads. I also investigated a related gold clothing appliqué depicting two archers, which was found in the 19th century in a Scythian burial mound in the northern Black Sea region.
Using non-invasive scientific techniques – mainly optical microscopy and scanning electron microscopy (SEM) – I captured images at low and high magnifications. This enabled me to identify and record physical features, tool marks and surface textures, which are characteristic of the techniques used by goldsmiths to manufacture and decorate objects. Optical microscopy allows overall observation of the construction methods and decoration of the gold, while the SEM enables more detailed investigation using two types of black and white images: secondary electron images (SE) and backscattered electron images (BSE). SE images give a general view of the surface of an object while in BSE images, subtle differences in surface topography (e.g. tool marks) and composition are accentuated. This is because the contrast in a BSE image is based on the difference between densities: the heavier an element, the brighter it appears on the image. The SEM is also equipped with energy dispersive X-ray spectroscopy, which identifies the chemical composition of alloys.
The objects were probably made by different craftsmen and are not necessarily even from the same area, although they are all broadly dated to c. 500–300 BC. From our research, we found that the goldsmiths worked gold sheets and wires by hand to create seven of the nine objects studied. The sheets were hammered into the desired shapes and thicknesses from small cast ingots. Further work on the front and the back created the various 3D designs from the flat sheet. The goldsmiths had to go through cycles of hammering and annealing in order to achieve the desired deformation of the metal. Annealing the metal releases the internal stress produced by hammering: the metal sheet was heated to several hundred degrees in order to soften it and allow further deformation and shaping without cracking. Solid wires, such as the tail of the lion-griffin aigrette and the attachment loops on the back of the roundels, were also hammered into a circular section from a small square ingot. The wire tail of the aigrette is a nice example of the choices made by the goldsmith – the leaf-like terminal is shaped directly from the wire itself rather than as a separate piece attached by soldering.
The goldsmiths could use a variety of techniques and tools to deform the gold sheet by hand: they could work the gold sheet into relief from the back (a technique called repoussé) or from the front (a technique called chasing). Both techniques are often combined on one object and most gold objects studied here nicely illustrate this. Chasing is also applied for the decoration by gently hammering blunt-edged punches of various shapes along the gold surface. The punches move and push the metal in order to trace outlines and produce decorative patterns. Evidence of ‘punching’ is also frequently seen on these Scythian-style objects, to produce smaller decorative embellishments. A specially shaped punch was impressed directly into the front of the metal sheet in order to produce a single design, which was often repeated. The most common examples of punched motifs are lines of dots, and these occur on the finger ring and lion-griffin aigrette. All of these hand-working techniques were widely known and used by goldsmiths in c. 500 BC.
The two exceptions are the matching solid gold bracelets: they were created using a technique known as lost wax casting. This involves making a wax model with all the necessary details, including the high relief features of the eyes, ears, deep grooves and inlay cells. The wax model is then encased in clay, creating a mould which is the exact negative of the original wax model. Then the mould is heated in order to harden the clay and allow the wax to melt out. The mould is then inverted and the molten gold alloy is poured into it. After the metal has cooled, the mould is broken, revealing the cast object. The goldsmith would then most likely further accentuate the outlines and give finer definition to the designs by chasing and punching the metal directly.
This group of objects shows a wide range of alloy compositions, from high-purity gold to high-silver electrum – a naturally occurring gold-silver alloy. The copper content varies from naturally occurring levels in unrefined gold to alloys made with silver-containing gold. Copper makes gold harder and stronger and therefore easier to work and shape. Ancient gold sources exploited from early times are generally gold particles deposited by water movement and found in river beds – this is called alluvial gold. These native gold particles are not pure gold, and usually include a proportion of silver, which is the case for most of the objects analysed here. Another feature of native alluvial gold deposits is the presence of tiny, hard Platinum Group Elements (PGE) inclusions, which have been seen on the surfaces of the objects studied.
The wide variety of techniques used to make these objects was commonly in use in the first millennium BC. From the earlier scientific research I carried out on the Oxus Treasure, we know that chasing, punching and repoussé were the main techniques used to produce gold objects of Achaemenid style. It appears from the present study that the type of objects investigated were manufactured using the same methods, despite being of different style. Does this then mean that an Achaemenid goldsmith created objects in a Scythian style or that Scythian goldsmiths used the same techniques for similar types of objects? We may never know but it is only by continually asking questions and testing them with scientific research like this that we can better understand the development of ancient crafts.
The BP exhibition Scythians: warriors of ancient Siberia is on at the British Museum from 14 September 2017 to 14 January 2018.
Supported by BP.