HORSEPOWER
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Interactions between China, Mongolia and the steppe 2000-0 BCE
Behind the scenes in the scientific research labs of the British Museum
A rare and sumptuous gold dagger hilt from the Eastern Zhou period (c. 6th-5th c. BC) in the BM collection (reg. no. 1937,0416.218) (Figure 1) was studied in the department of Scientific Research of the British Museum. It was analysed using microscopy (optical/digital microscopy and scanning electron microscopy) and X-radiography to understand its manufacturing technology, and X-ray fluorescence (XRF) to identify its alloy composition.
Figure 1. Chinese gold hilt reg. no. 1937,0416.218) © Trustees of the British Museum
Scientific techniques (Figures 2-4)
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The use of microscopy allows scientists to look closely at and identify surfaces features and textures of objects, which could be characteristic of specific metalworking tools and technologies. Optical/digital microscopy generates close-up images in colour, as it uses light as a source, while scanning electron microscopy (SEM) creates images in black and white, as its excitation source is made
Figure 2. Variable Pressure Scanning Electron Microscope in the Department of Scientific Research of the British Museum © Trustees of the British Museum
of a beam of electrons. SEM produces different types of images from two different types of electrons released by the excited artefact/specimen: secondary electrons (SE), which yield information about the topography, and back-scattered electrons (BSE), which create the image contrast through the differences in mean atomic weights of the elements present (the denser the element, the paler/brighter it appears in the BSE image).
Figure 3. X-radiography facilities in the Department of Scientific Research of the British Museum: exposure room where the radiograph is acquired (above) and control room where the instrument is operated (right)
Figure 4. Scanning X-ray Fluorescence in the Department of Scientific Research of the British Museum
Application to an artefact
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This extraordinary openwork gold hilt has been produced by casting, which is achieved by pouring molten metal into a mould with the negative shape of the desired piece to be made and letting the metal solidify in it. Close observation of its features and design details revealed that it was made of two identical sides, cast individually and separately into identical moulds, and soldered together along their long edge (Figure 5). SEM imaging shows clear details of the fine creatures’ body features and their surface texture: these features which have been cast and not reworked post-casting, include very fine lines and dots (Figure 6). Several areas show clear traces of wear while others appear untouched (Figure 6). Several slightly deeper areas might have contained coloured inlays of stones or organic materials, which would have created a stunning contrast with the gold; these are now all lost.
Figure 5. Close up image of the gold hilt showing the join along the long edges of the halves soldered together (several areas of the join are indicated by the arrows) © Trustees of the British Museum
Figure 6. SEM BSE images of: Top: detail of the heads and parts of the bodies of two of the four creatures from one side (note some of the shallow cells making the ears for instance, which might have contained inlays now lost) (magnification x7, width of view about 18 mm); and Bottom: close up view of the decoration made of dots and lines (note the rough texture of the surface, especially in recessed areas where there is no wear, characteristic of casting) (magnification x80, width of view about 1.6 mm). All images © Trustees of the British Museum.
Macro-observation, X-radiography and XRF mapping confirmed that it has been repaired in several damaged areas: a darker grey colour visible macroscopically, clear fractures visible on the radiographs and a different elemental distribution between the golden areas, made of gold alloyed with some silver and a little copper, and the repaired areas, composed of an alloy of tin and lead (with possibly a little copper) (Figure 7).
Figure 7. X-radiograph (top left), close up photo (bottom left) and XRF compositional elemental maps for gold and silver (bottom right) and for tin and lead (top right), highlighting the various fractured areas of the hilt which have been repaired.