How are X-rays used in archaeological conservation? This blog post demonstrates one use of X-rays and covers the initial investigative X-radiography of iron objects recovered from excavations at Cardiff Castle. It presents a very basic theory behind archaeological X-radiography and reports its use on specific iron artefacts from an assemblage of 2000+ iron objects recovered from the excavations. The use of X-radiography on copper alloys will be presented in a later blog post. Future posts will reveal another use of X-rays in the form of X-ray fluorescence and scanning electron microscopy.
The following general theory of X-radiography may get a little thick and you should feel free to skip to the next paragraph. The general theory behind X-radiography is that a steady stream of high energy X-rays are emitted from an X-ray source and directed towards the material to be X-rayed. As the X-rays interact with the material, some of them are absorbed while others with higher energy pass through striking a form of detector (e.g. film or scanning bed). Our system uses film. A greyscale two-dimensional representation of the three-dimensional object is presented when the film is developed. The brightness of the image indicates X-ray absorption and object density. For example, a brighter area on the film indicates more X-rays were absorbed by a greater object density in that area, i.e. there were less X-rays available to impact the film. Darker images reveal areas of lower density or absence of object, i.e. more X-rays were available to impact the film turning it darker grey or black. This may seem counter-intuitive at first but with continued examination of X-rays it will become second nature.
A developed X-ray film is called a radiograph. In archaeological conservation, radiographs are used to determine object shape, method of assembly, presence of decoration, hidden damage and structural weakness, and extent of decay. X-Radiography is also used as an initial investigative technique to determine objects that require further cleaning.
The Iron Collection
Over 2000 individual iron objects were presented for initial investigation using X-radiography. The assemblage consisted of many readily identifiable objects such as nails. Many other objects were covered in dense corrosion crusts and visual identification was difficult to impossible. Some of the iron material had spalled during storage resulting in fragmentation. In other words, these objects had broken apart since they were excavated.
X-radiography was used to determine what the objects might be, the condition they might be in and the significance of the object and whether it required cleaning. The iron artefacts were X-rayed in batches composed of objects of similar thickness/density. Many of the objects that had spalled during storage were individually wrapped in tissue allowing the fragments to be X-rayed in place (in-situ) as long as the tissue was not removed. This facilitated the possibility of using the radiograph to visualize the original shape of the object. Annotations were made on the surface of the X-radiographs after they were developed. The annotations contain information regarding the project name, the x-ray parameters used and the archaeological context for object groups or special finds numbers for individual objects.
For this post, the results will be presented in pictures with captions. Select objects in the images will be referenced in the captions. No picture can show the detail and contrast of the original radiograph. For this reason, the radiographs from the project were digitally recorded using both a dedicated digital X-ray laser scanner and a DSLR camera. The digital scanner was set to best image quality and highest resolution. The DSLR images were taken on a Nikon 3100 using ISO 100, f-stop 5.6 and shutter speed of 1/6 seconds with 18 mm lens on a copy stand. RAW images were produced and post-processed to increase contrast. Comparing the two sets of x-ray images, the DLSR have greater contrast more closely resembling the original film but at the loss of some visual information. The DLSR does allow recordation of annotations made on film surface. The laser scanned images, on the other hand, retain all the visual information but with less contrast. Surface annotations on the film do not scan well. Both sets will be represented in the displayed images below with the DSLR image above the digitally scanned image.
J714: Object ‘A’ is a bent iron strap with a possible nail remaining in situ. Corrosion covers the nail body. The left edge of the strap reveals a partial hole possibly meant for a second nail. The object itself appears to be intact although relatively thin when compared to the denser areas of object ‘B’. Object ‘B’ is a horse shoe.
J750: Objects ‘A’ and ‘B’ are nails possibly retaining their original embedded orientation in an object, or have been concreted together with iron corrosion. ‘A’ consists of two nails and ‘B’ of three or more nails. The structural density of the nails is less than some of the other objects on this radiograph. This is evident in our ability to ‘see-through’ the bodies of the nails. We do not know, however, their actual mechanical strength. Most observations made in archaeological radiography are relative.
J796: Object ‘A’ is a tool bit. Object ‘B’ is an iron strap end and ‘C’ may be a file. The tool bit (A) appears to be spalling as there are voids evident between the core body and the outer edges. A relatively severe crack in the body two-thirds of the way toward the right may indicate a structural weakness in the object. Extra care should be taken when handling this object. The strap end (B) appears relatively thinner than the tool bit. The bright spot in the center represents a denser material. It may be the remnant of an iron nail. The file (C) reveals great structural weakness and lower density. The tip (far right) is susceptible to breaking off of the object. The left-most portion of the object reveals greater density probably due to thickness of the object. This is where it would connect to a handle.
J798: It is difficult to determine what object ‘A’ is based on the DSLR image (top). The scanned image readily reveals that it is a saw fragment with the saw teeth pointed towards the top of the images. The radiographs show that the teeth are relatively weak when compared to the rest of the saw blade body.
J800: This series of radiographs is similar to the last in that ‘A’ also represents a saw fragment. This saw blade is lower in density and relatively weak (the edge opposite the teeth in particular) when compared to some of the other objects in this radiograph. A size comparison of the saw blade teeth in these images and those of the previous radiographs (J798) suggests that the fragments come from two different saws.
J805: The labelled objects consist of keys (A and B), wire (C) and possibly nails (D) similar to those in radiograph J750. The bow is intact for key ‘A’ but is partially missing in key ‘B’. Both keys reveal a partially hollow stem where they would fit over a shaft in the lock mechanism. The ward (the part that interacts with the lock mechanism) is missing on both of the keys. The wire (C) is tangled and partially twisted. The possible nails (D) are oriented to appear to be pointed toward or away from the observer of the radiograph.
J816: The selected objects are a key (A) and a possible knife blade (B). The key is intact but the stem is bent in two areas. The bow of the key is generally intact but the radiograph reveals a weak area covered in corrosion crust. The ward is intact but is relatively weaker than the stem. The knife blade (B) is missing the proximal end where the handle would be fitted. The blade, being thin, is less dense and relatively weaker than other objects in the radiograph. The bright spot on the knife is a denser material. This bright spot probably represents something that has been concreted into the corrosion crust on the knife and is not a part of the actual knife. Remember, a radiograph is a representation of a 3D object in a 2D image.
J817: The selected object (A) is also a knife blade (compare with J816). This blade is complete with the tang where the handle would be fitted. The radiograph reveals, however, that there are major cracks in the object. If this object was cleaned there would be a high probability that the blade would be in pieces. Note the two long stick-like objects to the right of the knife. The radiograph reveals that these two objects have spalled. This is evident in that there appear to be spaces between many of the smaller fragments and the main body. They appear to be intact because the objects were not removed from the tissue in which they were wrapped prior to the X-raying process.
In conclusion, we have seen here how the use of X-rays have informed us of the identity and condition of the objects without spending the time and money in cleaning them. If we chose to clean them, we would know what to expect such as areas of low density and structural weakness. We have also seen the difference between digitally laser scanned radiographs and photographed radiographs. Having the original radiographs in hand is the best way to interpret them. But using both scanned and photographed digital copies can provide a similar level of revelation.
The next blog post will examine the conservation of ~200 Roman coins recovered from the Cardiff Castle excavations.