In this post we will be examining a single enameled brooch (SF 0532) excavated from Cardiff Castle. This is one of a handful of enameled brooches of the 51 recovered from the excavations. We will take an in-depth look of this object to provide a sense of the amount of work that can go into a conservation related examination.
The object is a copper alloy T-shaped brooch that was covered in thick deposits of soil and copper corrosion. Most of the deposits were removed through mechanical and chemical cleaning to reveal 3 fragments: 2 pins fragments and the brooch body. The pin fragments were small and highly corroded.
The technology of the body consists of a bow that partially wraps around the wings (arms) at the head. The wings are a single hollow tube with a notch cut out on the reverse for the central pin. Cleaning of the wings revealed iron corrosion products within the pin notch and on both ends of the wings. This indicates an iron axis bar to which the copper alloy pin was connected. The bow has a broad flat raised center with 4 diamond-shaped lozenges used to hold the enamel. The edges of the bow are serrated to form a saw-tooth effect. The lower bow is slightly tapered ending in the rounded foot with a dark blue glass bead embedded in the center. The reverse of the bow is generally flat except for an incomplete pin catch-plate which extends from the lower bow reverse.
Humans are wonderful at pattern recognition. This can be useful but also misleading as sometimes we see something that may not actually exist. The four lozenges on the bow are similar in size, spacing and orientation. Below the fourth lozenges towards the foot is an area of deeper green of a similar size, spacing and orientation to the existing lozenges. Could this be a fifth lozenge? It could, however, the texture is similar to the copper corrosion products on the surface of the brooch. More investigation is required to determine if the fifth lozenge exists.
An x-ray was taken to further elucidate the mystery. The x-ray was developed, scanned and digitally enhanced. The brighter areas represent denser areas on the brooch. We can see that the wing tube is, indeed, hollow. We can see the stub of the pin at the head of the brooch. It is slightly off-centered indicating that the brooch was at a slightly oblique angle during the x-raying process. We can see a more clearly defined section of the saw tooth edge on the bow. We can see the pin catch-plate on the lower bow (brighter area near the bottom) and we can see the semi-rounded notch that holds the glass bead at the foot. We can also see four lozenges used to hold the enameling along the bow. The fourth lozenge is slightly obscured by the catch-plate and any evidence of a fifth lozenge is completely obscured.
So, does any enameling remain and what colour would it have been? Microscopic examination reveals the possibility of a pale green enamel remaining in at least two of the four lozenges. McIntosh (2007) writes that typical first century AD Romano-British enameled brooches consist of red enameling with blue and yellow used less frequently. The colour palette was expanded in the second century to include green, orange, black and white. Blue was the most dominate colour and would often be used in conjunction with red. By the third and fourth centuries blue and white were commonly used.
Now, the colour of the remaining enamel could, in fact, be a pale green just as it looks in the photographs. However, I know through my experience with Egyptian faience glazes that copper-based blue glaze colour can be reduced to a pale green if exposed to water which can leach elements (e.g. sodium) from these materials. The colour of the enamel remains mysterious… or pale green just as we see it.
While under the microscope I decided to further examine the potential fifth lozenge site. What I saw were two overlapping possibilities in its placement. Knowing human nature, these two overlapping potential placements may just be evidence that, as a human, I am forcing the existence of the fifth lozenge whose existence seems much clearer when viewed from further away but upon closer examination becomes mottled. I am not so sure this lozenge actually exists.
What I do know is that there is clearly a blue glass bead on the foot of the brooch. Maybe further examination of this glass bead can help solve the mystery of the enamel colour in the bow lozenges. Relying on the previous experience with Egyptian faience I was expecting the colourant of this bead to be cobalt. A quick qualitative analysis using portable x-ray fluorescence (pXRF) will clearly identify the cobalt. Now, in full disclosure, the pXRF analysis was covering the whole foot of the brooch, not just the bead. But even so, the cobalt present in the bead should still be obvious. Only, it wasn’t.
I compared two pXRF spectra: one from the beaded foot (green spectrum and hard to see behind the red spectrum) and one from the back of the bow with no enameling (red spectrum). The spectra comparison from the pXRF analysis showed the same elements although there was disparity in the photon counts (the heights of the peaks) that could easily be explained by air attenuation, or the blocking of photons due to air molecules between the brooch foot and the pXRF. In other words, the back of the brooch bow made greater contact between the brooch and the pXRF than the brooch foot. Regardless, there was no cobalt. Use of the pXRF was extremely easy but to use additional methods, such as the scanning electron microscope, would require research into enameling so that I knew what to expect.
McIntosh (2007) and work conducted by Bateson and Hedges (1975) on the analysis of Romano-British enameled brooches indicated that copper can be used as a colourant for reds, blues and greens, and although previous research (Harden (1956) and Forbes (1957)) said that cobalt wasn’t used in Roman glass, Bateson and Hedges (1975) identified it in nearly all the blue enamel they analyzed. I needed to take a closer look at the blue glass bead using the scanning electron microscope (SEM).
The SEM I had access to has three detectors: secondary electron imagery (SE), energy dispersive spectroscopy (EDS) and Backscattered Electron imagery (BSE). SE imagery produces the 3D images we are familiar with concerning SEM. This includes those 3D images of bees and fly eyes and other similar subjects that can easily be found in the internet. This would not be helpful to me. The other two methods would be helpful. BSE produces a 2D black and white image (slide 8). It will show contrast in heavier and lighter elements and can show the difference in density of materials imaged with higher density showing up as more white and lower density as more black. This method would help in determining areas of glass from areas of copper alloy and/or copper corrosion. In other words, this would help me in finding suitable areas to analyze using EDS. EDS will show me all the elements that are present in the sample site being analyzed and, in this way, is very similar to pXRF analysis. SEM-EDS, however, allows me to analyze a much smaller area than pXRF.
The four lozenges and the existing blue bead were imaged using BSE. The bead appears dark in comparison to the surrounding material. This is to be expected as glass is less dense than copper alloy. Additionally we see some higher density material on the bead surface. This is most likely deposited copper alloy corrosion material from the body of the brooch. BSE images of the four lozenges alternate between bright and dark. The darker images (with brighter areas along the edge) could represent an existing glaze similar to how the glass bead showed up darker in the previous image. The other two lozenges show a brighter area surrounded by slightly less dense material. This probably represent deteriorated glass where elements have been leached from the glass leaving an artificially elevated amount of lead than what is usually encountered with enamel. Lead is commonly found in all Roman enamel as its characteristic affect melting, colouring and its adherence to the metal substrate (the brooch). The brighter areas in these images can be a result of nearly all of the glass having dissolved and been carried away by moisture in the burial surroundings, lead being particularly stubborn to dissolution.
EDS provides us with a spectra showing the elemental peaks. Many of the peaks are labelled but great care should be taken as automatic peak identification is can commonly mislabel peaks. Slide 10 shows spectra 1-2 (glass bead), 4-5 (brooch body) corresponding to the spectra in slide 9. There is no indication of cobalt in this blue glass bead. This indicates that copper is responsible for the blue colour. This also indicates that cobalt cannot be used in a qualitative mode to determine the colour of an enamel simply because it is not present.
This slide shows a BSE image similar to slide 9 where five locations have been analyzed using EDS. The EDS result in the brighter glaze areas of the first and third lozenge (from the head) does reveal higher levels of lead suggesting, as previously hypothesized, that the enamel has deteriorated leaving an artificially high level of lead.
So, we have an enameled brooch with four definite lozenges and the possibility of a fifth. The blue colour of the glass bead is due to copper. Its darker colour may be due to the thickness of the bead. The alternating bright and dark lozenges in the BSE images could be evidence of alternating colours (and an alternating propensity to dissolution) or a differentiation in deposition moisture levels/flow across a small area that resulted in differentiated dissolution of the enamels.
Copper, used for green, blue and red, is the only colourant identified and is consistent in all the enamels. Unfortunately it is also present in the copper alloy brooch and the copper alloy corrosion overlying everything. In this case our tenacity has resulted in little demystifying of the enameled brooch. It has shown, however, the amount of work that can go into an object. There can be a lot of chasing down leads that lead to nowhere, determine what something can’t be, and/or determine what it actually is.
This final picture shows the brooch with two potential enamel colour patterns. I have also included the fifth lozenge which may not have existed. The first image shows the brooch as we see it. We have a pale green enamel in the lozenges with a blue bead at the foot. The second image shows the pattern with blue inlays similar to the foot bead colour. The colour could have been red but my argument against this is that we have red existing on another brooch (to be discussed in another post) and it seems that it would be just as resilient on this brooch. A similar argument can be made against the lozenge enamels being blue. The deep blue (where is the cobalt?) of the glass foot bead doesn’t appear to have diminished so why would the potential blue enamel along the bow? The only answer is that the blue bead was protected from moisture and certainly water flow for over 2000 years while the bow enamel wasn’t. Possible but unlikely. Ultimately the colour determination remains unconfirmed.
We will examine another enameled brooch in the next post. The thumbnails below can be clicked upon to bring up a larger higher resolution image. Be safe!!!
References Cited:
Bateson, J. D. and Hedges, R.E.M (1975) The Scientific Analysis of a Group of Roman-Age Enamelled Brooches. Archaeometry 17 (2) pp. 177-190
Forbes, R. J. (1957) Glass. Studies in Ancient Technology 5, pp. 110-231
Harden, D. B. (1956) Glass and Glazes. A History of Technology, ed. Singer et al., Oxford, pp. 311-346
McIntosh F. (2009) A Study into Romano-British Enamelling with a Particular Focus on Brooches. The School of Historical Studies postgraduate Forum E-Journal Edition 7, Newcastle University, pp. 1-18
Cardiff Castle Excavations 