Conservation considerations: A Roman Coin

By Natasha Waddell, MA Conservation Studies student, specialising in Metals

This blog post is being used as a tool for working through my thought and decision-making process when finalising my treatment proposal for an archaeological Roman coin. This is the first time I will be working on an object of archaeological context. As incredibly excited as I am, the whole process is brand new.

I will start off by discussing the context of the coin and will then walk you through my thought processes when analysing the condition of the artefact. This will guide my final reflection on a possible treatment proposal.

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Figure 1. Roman coin under 10x microscope with raking light, measuring about 17mm diameter
Figure 1. Roman coin under 10x microscope with raking light, measuring about 17mm diameter
Figure 1 Roman coin under 10x microscope with raking light, measuring about 17mm diameter
Figure 1. Roman coin under 10x microscope with raking light, measuring about 17mm diameter

Context

What is it and where was it found? 

This Roman coin, currently belonging to Worthing Museum, was excavated with a larger hoard of hundreds of Roman coins from a boggy landscape, capped with up to one meter of peat. Through correspondence with the Curator, they are thought to date to 4th Century AD.  

What is the material? 

Determining the exact material of this coin involves a lovely bit of analytical equipment: the XRF (X-Ray Fluorescence). This is a fantastic tool for conservators to determine what elements are present in a given sample. 

Figure 2 Here I am positioning the XRF equipment to get an accurate reading of the coin
Figure 2. Here I am positioning the XRF equipment to get an accurate reading of the coin
Figure 3 XRF Scan quantitative analysis spectrum of the Roman coin, concentration resulted in 53% Cu, 27% Pb & 5% Sn
Figure 3 XRF Scan quantitative analysis spectrum of the Roman coin, concentration resulted in 53% Cu, 27% Pb & 5% Sn

This funky looking spectrum above is a typical format of an XRF scan. With the help of a trained eye, we can plot what elements are evident and then receive a concentration of their presence. As a result, this spectrum tells us that the coin is made from bronze!

Condition investigation

Now that we know the material of the coin, we can investigate its condition.  

What corrosion are we seeing? 

As it is difficult to determine the exact material present at this stage without conducting interventive sampling and analysis, we can only infer during preliminary examination and research. Let’s take a look at some closer images:

Summary of assessment:

  • At this stage, there seems to be very little variation in copper corrosion, soiling and dirt remnants allowing the details to be highly identifiable
  • There is a relatively even layer of dark, grey sulphide corrosion across the coin surface
  • Preliminary investigation, through the use of light mechanical methods, revealed a white/green waxy substance below the sulphide layer

At first glance, it seems that the coin is in relatively good nick. However, research tells us that if nantokite is exposed to oxygen with high levels of moisture, there poses a risk of activating bronze disease (Scott, 2002, p.125). If this does take place, it could be detrimental to the integrity of the coin.

In short, bronze disease (not a disease, just to clarify) is a cyclical process caused by copper chloride corrosion and its interaction with moisture.

How clear are the details?

As the coin has an archaeological context, clarity of its details is significant for identification, dating and education. However, the aesthetics of the coin throws another spoon into the mix. Where clarity helps us to identify, aesthetics could also be dependent on the viewers’ relationship with the object. My conservation proposal could intend to clarify the image and reduce the corrosion layers but not so far as to reap the coin of its historical character. In the case of this coin, the details are already quite clear. However, the potential risk of bronze disease could cause significant loss to such fascinating details.

What is my aim for the treatment?

On reflection, inhibiting potential bronze disease will dominate my treatment proposal to avoid the risk of losing such clear details in the future. There is a classic treatment used by many metalwork conservators which involves the use of Benzotriazole (BTA). In essence, BTA has the ability to form a thin Copper-BTA film complex on the surface of the metal which removes the site of anodic corrosion; the site where the cyclical nature of bronze disease takes place:

With BTA being a widely known and used treatment, this will be an ideal practice to increase my skillset as well as providing protection for the coin.

There is one final consideration to make before this treatment can be decided. Before I begin the corrosion inhibition, it is essential to conduct an X-Ray of the artefact to discover if there are any voids within the object. Although I will not go into the details of the BTA treatment process, the method involves the use of a vacuum to ensure the chemical reaches all the nooks and crannies within the artefact to provide full protection. If any voids are present, these areas will sink and cause damage to the object.

In this case, there are no voids present! However, there is a considerable crack. Although this will not directly affect the BTA treatment, this is an area for caution during the mechanical cleaning phase so as not to cause further damage.

 

Conclusion

This blog post has been a great opportunity for me to synthesise my research and develop a grounded treatment proposal for this project. Alongside this, I hope such process can be used as a tool, employing these foundational questions and considerations to assist in the investigation phase of analysing objects.

Cited in this blog:

Scott, D. (2002) ‘Corrosion and Environment’ & ‘Chlorides and Basic Chlorides’ in Copper and Bronze in Art, Los Angeles: Getty Publications, pp.10-79 & 122-145.

Yun-Ho Lee, Min-Sung Hong, Sang-Jin Ko & Jung-Gu Kim (2021) ‘Effect of Benzotriazole on the Localized Corrosion of Copper Covered with Carbonaceous Residue’ in Materials 14, pp.1-15. https://doi.org/10.3390/ma14112722 (accessed 06/21/2021)