Most people are familiar with the use of X-radiography in hospitals. The same principles apply to the use of X-rays in a museum, though we may use longer exposures to penetrate metals and other dense materials.
X-ray is a useful tool for the conservator as it can help us discover ‘hidden’ constructions and mechanisms, which we otherwise couldn’t see. It can be used to consider the extent of deterioration of an object or to check whether a firearm is loaded. A major advantage of this technique is that it is entirely non-destructive.
X-rays form part of the same electromagnetic spectrum as radio waves, ultraviolet and visible light. However, their greater energy compared to visible light allows them to pass through much denser materials. Light can be stopped by a couple of pieces of paper, whereas X-rays, if their energy is sufficiently high, may penetrate several centimetres of steel.
The X-ray beam, however, will be partially absorbed by the object and the intensity of x-rays is reduced when passing through denser parts of an object.
Photographic film is sensitive to X-rays and a sheet of this is placed below the object being examined. After developing the film, the less exposed areas beneath the denser parts will appear lighter. The completed X-rays are scanned to produce digital images for use in reports and presentations.
The gun shown in the header image has suffered from insect damage, and the weakened wood of the stock has been filled with an adhesive to strengthen it. The adhesive has a higher density than the wood and so the extent of the woodworm damage is visible on the x-ray.
Uses of X-Radiography
- Prior to lending or exhibiting artefacts, X-radiography allows any objects that are in poor structural condition to be identified and withdrawn or replaced.
- An X-radiograph provides an important source of information during conservation treatment.
- Where artefacts from marine wrecks or archaeological sites are covered in thick concretions of soils and corrosion products, X-radiography can help identify the object, record it in case of future deterioration, and pinpoint areas that would benefit from selective investigative conservation.
- Internal features, such as firing mechanisms in guns and crossbows, may be examined without dismantling.
- Maker’s marks concealed by corrosion or other parts of the object (such as the grip of a sword) can be identified: Case study 12.
- Over-painted designs may become visible where dense compounds (such as lead oxides) are used as pigments, fillers or opacifiers. For example, changes in the heraldic designs on shields may provide evidence for changes of ownership or allegiance.
- Where edged weapons have been constructed by welding together different alloys, the weld lines may be visible.
- Evidence of re-used materials may become apparent
- Features, such as modern screw threads, welded repairs etc may point to inconsistencies with the apparent age of the object, or to recent repairs.
- For demonstrating and teaching purposes, X-radiographic plates provide an alternative, engaging, but easily understandable display.