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Metal, soil and animals - some experiments with radiography

Another day with Dana Goodburn-Brown, Archaeological Conservator.

A Roman-age boar's head placed within a burial. The entire offering has been lifted as a soil block. The mandible is visible, along with the original wood (below and above mandible).

We spent our time looking at some specimens that Dana had brought along to investigate with radiography. Some resulted in success, others failure! The first was a boar's head from a Roman burial, lifted as a soil block and wrapped in plaster cast. Although the bone was clearly visible with the naked eye, efforts to show it radiographically had limited success. After some test images we took some of the plaster away (being careful not to disturb the wood or bone), but the density of the bone was far less than that of the soil or plaster to show well.

Our first x-ray, with the soil block placed upon the detector and the x-ray tube overhead (100cm). A reasonable exposure of 105 kV / 5 mAs. The plaster underneath the specimen proved too difficult an obstacle to overcome. With a bit of imagination you can just about see the bottom incisor (follow up from the RT marker and a little to the left).

Dana was keen to see what ability our clinical digital radiography equipment had when imaging metal findings. From what I've gleamed, she usually uses equipment from a non-destructive testing facility with equipment and settings quite different to our own. The first test object was a Roman horse bridle, heavily corroded and contaminated with soil.

The Roman horse bridle was imaged 'as is', upon the plastic tub lid with cling film covering.

The resultant image, using 70 kV / 5.6 mAs (100cm). Of all the remnants, a ring is clearly visualised in the middle. It will be interesting to learn what these are when finally conserved by Dana.

As with previous imaging of non-biological (or non-orthodox) specimens, I decided to simply have a marker on the Left or Right of the item so that we knew the orientation of the image. This was useful when imaging the boar's head, as I didn't initially know which way was 'up'. As can be seen in the image above, I've also decided to annotate with exposure factors directly on the image. The equipment we are using only allows for anatomical descriptions, making it comical and somewhat frustrating when trying to decide if a horse bridge is more like a wrist or skull x-ray. Perhaps I shall investigate adding another option - archaeological specimen - to the optional choices. When imaging human remains it is sometimes useful to have the correct terminology though, especially as these show on the screen when scrolling through datasets of imaging.

A variety of Anglo Saxon metal work (iron, copper, possibly gold). I am not an advocate of imaging a large assemblage on one image, but this was a test scenario.

We moved onto a range of different metals (as can be seen above). The iron objects required a greater kV (i.e. greater kinetic energy) to visualise different areas of metal thickness. The initial low energy exposures resulted in quantum mottle within the centre of the iron objects. Quantum mottle occurs when not enough x-rays hit the detector/receptor and the resultant image is poor (within that specific region of the 'film'). We increased the kV accordingly, and despite the increased passage of x-rays onto the detector we were still able to visualise the details of the thin copper. This was primarily due to the ability of re-processing digital images, changing the contrast and brightness. There is a limit to how much of the fine detail can be retrieved through processing the images and I would advocate using bespoke exposure values for each item.

A 'goose' bone.

One of our less-than-scientific specimens was a 'goose' bone (part of a pelvis) that was found on the beach in Folkstone. Ellie Williams suggested that the bone matrix was far too heavy to be that of a bird, more likely to be a goat or sheep. We took two images from different angles (AP/LAT), our imaging was to be used by a student. It was interesting to experiment with non-human remains and possibly someone could identify the species from the photos and x-rays in the future.

The 'goose' bone x-ray.

Images: All images were taken by the author. With kind permission of Dana Goodburn-Brown and Canterbury Christ Church University.

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