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Kent Archaeological Review extract

Science in Archaeology --
The use of x-rays in archaeology -- Part One.
by Dr Tony Cox.

One of the problems of working with archaeological finds is that they have been in the ground for a long time and as a result are often corroded. Consequently in any subsequent examination, extreme care needs to be exercised in their handling if accidental damage is to be avoided. With some finds it is useful to know the actual constituents as these can tell us much about the peoples who made them, but unfortunately, the very action of taking samples to get this information by conventional chemical analysis may damage the finds, while with such valuable items as gold torcs, etc., the removal of material for a destructive test will lead to much gnashing of teeth. Under these conditions, therefore, a non-destructive test is needed, which will provide the same information, but leave a find unmarked.

Many non-destructive test methods are available, but the one introduced in this brief note relies on utilising the properties of x-rays. To understand the nature of these invisible rays is difficult enough for those with experience of science, but for those who have not trod the scientific path then the very existence of rays, which can be manipulated to pass through the proverbial hoop is often received with silent scepticism. To help these, a visit to the local mass x-ray unit is recommended, where they will see and appreciate, in a rather personal way, the penetrating power of x-rays.

X-rays are generated when a beam of electrons (negatively charged atomic particles) are accelerated towards a target under a potential of 10-40,000 volts. A heated tungsten filament produces the electrons and typical target materials are cobalt, molybdenum and tungsten. The x-rays produced are described in terms of their wave lengths, some x-ray generators produce a continuous spectrum of x-rays with wave lengths varying from 0.1-100 A (1 centimetre = 100,000,000 A) whilst from others only x-rays in a very narrow range of wave lengths are obtained.

The short wave length x-rays are noted for their ability to pass through heavy sections of solid material, and as the intensity of the emergent beam is proportional to the section thickness, the rays lend themselves naturally to studying the surfaces and internal structures of finds. This particular way of using the rays is known as "x-ray radiography." In operation the x-ray beam is passed through the find and the intensity of the emergent beam recorded on photographic film.

PHOTO: X-ray of a Bronze Age axe head.

X-ray of the Bronze Age axe head found near Deal.

One very useful application of radiography is in studying the technology of the early smiths in their efforts to produce strong and reliable weapons. Swords were made by taking thin slices of steel, in some recorded instances as many as forty-three were used, wrapping them around each other, and then forging this bundle into a tough homogeneous blade. To detect these involved processes visually is difficult, especially after centuries of corrosion, but a radiograph of the sword quickly uncovers the secrets of the early sword smith.

An example of the use of radiography to Kentish finds is shown in the photograph which is of a Bronze Age axe head found near Seal.

(To be continued.)

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