X-radiography in the conservation lab

on Monday, 16 March 2020. Posted in Conservation, History Centre

X-radiography in the conservation lab

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.

Left: X-ray machine Right: Objects placed on a cassette in the middle of x-ray machine

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.

Examples of photographic film showing lighter areas where the object has a higher density, and darker areas where the x-rays have penetrated material which is less dense.

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.

Iron sword hilt excavated from a site at Adanac Park, in Nursling, Southampton. Adanac Park is the site of a Bronze Age settlement and Iron Age Cemetery and the objects from the site needed to be fully investigated and recorded, so that its significance could be explained. As the wood and leather must be preserved, the characteristics of the material underneath could only be revealed through x-radiography. It is apparent by the x-ray that in addition to the hilt, the scabbard is also present - as a copper alloy ring around the sword.

Completing this process for archaeological finds such as bronzes, iron weapons and copper alloy coins, is particularly useful, as they are usually covered in thick layers of soil and corrosion products. Decoration, surface details and previous repairs can all be concealed, and the layers of soil and corrosion can hide many breaks or voids which could lead to the object breaking without warning.

Soil block thought to contain Saxon finds, and an x-ray of the same block showing the details of a brooch, and the pin on the reverse.

As well as being a useful process to help the conservators determine the best way to handle or clean an object, it is a fascinating method that allows us to reveal objects and details that may not have been uncovered for many centuries.

Nikki Simpson, Conservation Assistant

The Conservation and Museums Advisory Service (CMAS) aims to promote excellence in the care and use of collections by providing conservation advice and practical treatments to heritage organisations and the public. It also supports museums in Wiltshire to meet professional standards and become sustainable.

If you would like conservation advice about your own documents or objects, we hold a free ‘Conservation Surgery’ on the 2nd Thursday of every month (please book in advance through the WSHC helpdesk – 01249 705500).

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