Articles tagged with: x-ray

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.

Thinking of doing a work placement? My experience with CMAS

on Wednesday, 29 August 2018. Posted in Conservation, History Centre

I can’t believe that the first year of university is over! It goes so fast and with so much information it can be a bit overwhelming, but trust me, all that hard work and studying will pay off. The Conservation of Objects in Museums and Archaeology undergraduate course at Cardiff introduced me to a whole new world of practical science, as well as in-depth theory, of conservation materials and specialised equipment, such as x-ray and air abrasion machines. By the end of the year I felt pretty confident with the concept of conservation but was still nervous on how to actually apply the theory with real, archaeological objects; in a true work environment.  This is where a work placement comes in. My first-year placement was at the Wiltshire and Swindon History centre, as part of the Conservation & Museum Advisory Service (CMAS), a commercial business which deals with issues both in museums and in public collections.

Although it can seem daunting at first, this experience is essential for developing those practical skills and applying the theory with real, archaeological objects, as well as understanding the treatment of different materials and the ethical choices conservators must make; focusing on what’s best for the object and adjusting treatment plans with the client’s wishes accordingly. Keep in mind that work experience is for your benefit, so don’t panic when you have millions of doubts and questions because the people you work with are there to help you (even if you ask questions every 5 mins).

So anyway, onto the actual conservation, hooray!

First things first, you will need to assess the object just by looking at it and writing up a condition report, which simply states any observable issues with the object. The majority of my time was spent working with a Roman ceramic oil lamp in the shape of a foot! Quite a fun object from Chippenham Museum, but as you can see there is a bit of a messy application of adhesive around the centre of the lamp where it has broken in two and was re-joined.  There were also scratches, dust and cobwebs on the inside, layers of red dirt/soil on the surface as well as white flaking corrosion (see figures 1-4).

Before treatment:

Figures 1-4

Ok, so the lamp required a good clean and that adhesive definitely needed to come off. Ultimately, the decision was to completely remove the adhesive and undo the join so that I could re-attach the two pieces with a better, cleaner join.  In order to remove the adhesive, I needed to work out what solvent it was soluble in. For this, I took small samples of the adhesive from the lamp by slicing off some of the softer areas with a scalpel, under a microscope.  I then put the samples into a petri dish and tested them with different solvents (see figures 5 & 6).

Testing solvents on the adhesive:

Figures 5-6

After about 30 minutes, I could see which solvent made the adhesive go soft and rubbery. The process of removing the adhesive required quite a lot of patience as the it didn’t want to budge; a scalpel was used to remove larger chunks of the adhesive and a poultice was placed around the join. A poultice was a way of creating a solvent environment to help loosen the adhesive and separate the two pieces. 
 *Just to give you an idea of the tools used in this process, I’ve taken a couple of photos for reference.

Figure 7 From left to right – pin vice, plastic tweezers, scalpel, wooden stick and cotton wool

Figure 8

In conservation, we usually make or own cotton swabs by using a bamboo stick or cocktail sticks (depending on what you’re working on) instead of regular, pre-made cotton swabs.  Making your own means that when the cotton gets dirty it can be easily replaced and the size of the swab can be varied so you can get into the small nooks and crannies that need a good clean. It also means that we aren’t throwing away millions of cotton swabs and being more environmentally friendly.

After many tries, the poultice wasn’t loosening the adhesive, so I went in with the scalpel and pin vice to try and dig out some adhesive in the join. Another poultice was then left on for a couple of hours. When it was removed I was able to gently pry apart the two pieces (finally!) and clean the new surfaces (see figure 9).

Copper Conserved: Cholsey Excavations Unearth Unusual Finds in Agricultural Setting

on Tuesday, 28 November 2017. Posted in Conservation

CMAS are excited to be working on the conservation of two Roman copper alloy items recently excavated by Foundations Archaeology.

The site at Cholsey, South Oxfordshire is thought to be an Iron Age settlement which evolved into a Roman Villa site. The villa buildings have been preserved in situ, but excavations were carried out on almost 2 hectares of land surrounding them.

The excavations revealed numerous burials and enclosures including a number of impressive corn driers.

Interestingly the archaeologists propose that the site was a prosperous farm that evolved to a villa, unusual as villas were more commonly set up by representatives of the empire.

CMAS are conserving a copper alloy necklace with a circular pendant, possibly made from bone, and a large copper alloy bowl.

The bowl was found upturned within the base of a corn drier, on top of charcoal deposits.

The backfill from the demolition of the corn drier was deposited on top of the bowl. This shows that the bowl was deposited at the time that use of the drier ceased, possibly as a closing offering and that this was done at the same time as closure of the site, not at a later date.

From a block of soil...

on Tuesday, 05 July 2016. Posted in Archaeology, Conservation

The conservation team are celebrating this week as we have completed work on a beautiful and exciting project. Conservation of the stunning finds excavated from Bognor Regis by Thames Valley Archaeological Services in 2008 has come to fruition. The items form part of an unusual burial assemblage along with an iron ‘bed’ frame and sword and are thought to originate from the late Bronze Age/ early Iron Age.

Taking block of soil for x-ray

The finds first came to us in the unassuming form of a large soil block, this was too large to x-ray at our labs so was transported to a local hospital where x-rays revealed a large amount of intricate metal latticework and a helmet.

X-ray of soil block from hospital

The soil block was carefully excavated, layer by layer, revealing the spectacular nature of the copper alloy items held within. The helmet and latticework were extremely fragmented and fragile, the helmet was split in half and part of the lattice was adhered to the helmet with corrosion products.

A tale of two hoards...

on Saturday, 05 April 2014. Posted in Conservation

The conservation lab has been overtaken by the Bronze Age recently with two hoards totalling over 200 objects requiring x-ray, cleaning and stabilisation. Both the Hindon and Wardour hoards were found by a metal detectorist near Salisbury and have been declared as treasure. The hoards were excavated by archaeologists and information about how they were buried is being collected and analysed in the hope of gaining some clue as to why the objects were buried. Both hoards have been acquired by the Salisbury and South Wiltshire Museum and once the objects have been conserved they will be put on display in the renovated Wessex Gallery at the museum.

In January of this year the conservation team took microscopes, cleaning tools and brushes and a few objects from the Wardour hoard to the Salisbury and South Wiltshire Museum as part of the ‘Watch This Space Exhibition’ giving visitors and local school groups a chance to see behind-the-scenes conservation in action. In the public gallery, conservators were on hand to answer questions about the hoard and demonstrate the cleaning techniques we use. Using the microscopes, the conservators were able to see the objects under magnification and could carry out mechanical cleaning.

logos1

Accredited Archive Service