I have been working as a Conservation Assistant at the WSHC for nearly a year, and a large part of my role involves administration and financial processing. However, one of the more interesting and slightly unusual aspects of the job is the hands-on work I get to do as part of the Conservation and Museums Advisory Service here at the centre, and I have recently undertaken training to use the x-ray machine which we have in the lab. This is a complex process which requires stringent processes, records and maintenance checks to ensure the machine is used safely and functioning correctly. Regular use of the equipment is key to building experience and a ‘feel’ for the items being investigated and how to get the strongest images.
How it works
Inside the x-ray machine is an x-ray tube. A heated filament called a cathode sits inside the tube and accelerates high energy electrons at a metal target anode, usually made of tungsten, as the electrons strike the anode they interact with the atoms. In this process, which is called Bremsstrahlung (braking radiation), the electrons lose much of their energy and a photon x-ray is produced. X-rays are electromagnetic radiation of a short wavelength and high frequency invisible to the human eye, but possible to record on photosensitive film, known as x-radiographs.
The object to be x-rayed is placed on top of a cassette which holds the photosensitive film. On exposure the x-rays will penetrate through the object leaving the image captured on the film. The x-rays are partially absorbed, “attenuated”, by the denser materials such as bone or metal and pass more easily through soft material such as soil or skin. Therefore, the strength of the x-ray (KV) and the length of time the object is exposed for is adjusted for the type of material, size and condition of the item.
The photographic films then require a wet, chemical process similar to that used for black and white photographic film with a developer solution to reveal the image, followed by a fixer to secure the image and a wash to remove all chemical residues. This process takes around an hour and half and must be carried out in the dark room, with only red light, which can be a little disorientating at first!
Digital or computed x-radiography is well established and allows greater speed in reviewing and manipulating images. CMAS are actively working to move to digital, so watch this space!
How X-radiographs are used in conservation
X-radiography produces images on a 1:1 scale which allow the conservators to investigate the structure, manufacture or identity of an object. Small dark bubbles can indicate casting processes, the distinctive herringbone structure of pattern welding or wave formations of damascening are often clearly visible in x-rays, even when the surface of a blade is severely deformed.
Moreover, x-rays pass more easily through deteriorated materials and voids, these areas will appear darker grey or black compared to the brighter white of more stable areas. This can show up pitting, cracks and breaks assisting in the accurate assessment of the condition of items and their longer-term conservation requirements.
X-radiography is a non-destructive way of retrieving and revealing information and so it is commonly used in the primary stages of investigation.
Conservation has been undertaken on a rare Visigoth Brooch here at the Wiltshire and Swindon History Centre. We were honoured with a visit by the finder of the artefact Matt Smith, who came for a tour of our facilities and to view the work being undertaken.
Thought to be only the second of its kind found in the country the iron and copper alloy brooch has been identified as a late 5th, early 6th-century AD type, predominantly found in southern France and central Spain. The brooch was uncovered during excavations undertaken by Operation Nightingale and Wessex Archaeology at Barrow Clump on Salisbury Plain.
The brooch formed part of the grave goods associated with one of the female burials on the site, and Matt’s first solo grave excavation. Significantly, well preserved organics remain on the surface of the object with the weave of the fabric visible through the microscope.
The brooch arrived at the conservation labs after x-radiography revealed the decorative copper alloy inlay. Still covered in corrosion products and soil from the burial environment, clues to the presence of preserved organics were just showing through the soil covering. Cleaning started slowly with scalpels and pins under the microscope to remove the soft chalky soil and reveal the extent of the organics.