Originally this post was going to present all the copper alloy jewelry recovered and conserved from the Cardiff Castle excavations. It was quickly realized that this would be a huge task based on the number of finds. As a result I have split the original post into three smaller posts, each looking at specific object types: bracelets, rings and other objects. This post covers mostly pendants as identified by archaeologists prior to conservation.
The conservation goal for these objects is simply to clean for identification and preservation, and report on any discoveries. The cleaning was conducted under a binocular microscope using a fresh scalpel blade to remove soil and copper corrosion, such as malachite (a copper carbonate) and extruded cuprite (a copper oxide), to reveal the ‘original’ surface of the object. Ethanol impregnated cotton swabs were used to remove loose material from the surface. The objects were coated with two coats of 10% (v/v) Incralac in toluene after cleaning.
It should be noted that the simple acts of using a scalpel blade and cotton swabs on the archaeological materials is grounded on a foundation of understanding regarding the technology, chemistry and corrosion processes of, in this case, copper alloys. Please do not use any information contained within these blog posts as a guide on how to clean archaeological objects unless the user has a similar level of understanding, training and experience.
Objects SF 0064, SF 0285 and SF 0412 were all identified as either a pendant (or fragment thereof) or as a possible pendant. All of the objects were covered in a mixture of soil and corrosion materials. These were removed to reveal as much of the original surface as possible. In post treatment objects SF 0064 and 0412 have a similar surface composed of a friable patinated surface underlain by a thin green corrosion crust which overlays the core metal. Object SF 0064 has less patinated surface remaining then SF 0412.
Object SF 0285 was cleaned to reveal a heavily warted surface and the presence of three holes as evident in the photographs. There was some question as to whether this actually represents a pendant and it is quite possible that it may be an attachment plate due to the presence of holes and the size of the object. The warts were retained as removal could result in a heavily pitted surface or even a fragmented object.
The final object in this blog entry object assemblage is a copper alloy pin (SF 0289). This pin actually should not be placed into this particular post as it would have had function beyond simple adornment. However I have included it here because it probably was worn on the person, serves as a segue for the next post and, finally, it had to be presented somewhere. This pin is special to me as I did not realize until I was nearly finished cleaning it that the head is actually a dollop of greenish-blue glass about the size of the small marble (I assumed it was all copper alloy). The moment of discovery was an influx of strong emotion with the initial instinct to tell someone followed by the realization that I would have to acknowledge that I did not identify it prior to cleaning (a point made moot now by presenting this to the world). The photograph of the cleaned object shows it in reflected light (as are most of the objects in this blog series) and transmitted light. The transmitted light photograph was produced by placing the object on top of a light table. One can see the under-glass extension of the copper-alloy pin into the glass by the refracted silhouette in the transmitted light photograph. The silhouette of which I refer is the dark blob in the glass slightly off-centered of the pin shaft. The long narrow shadow next to it is actually cleavage between two lobes in the glass surface. This may represent where the glass was starting to overlap itself as it was applied.
We delve into the mass of brooches in the next few blog posts. Below are thumbnails of the photographs provided in this post. Clicking on them will take the reader to a higher resolution image. Finally, be safe and use critical thinking during the Covid-19 lock-down.
Originally this post was going to present all the copper alloy jewelry recovered and conserved from the Cardiff Castle excavations. It was quickly realized that this would be a huge task based on the number of finds. As a result I have split the original post into three smaller posts, each looking at specific object types: bracelets, rings and other objects. This post covers finger rings which were identified as rings or finger rings by archaeologists prior to conservation. Most if not all of these are probably finger rings with the possible exceptions of SF 0348 (see below).
The finger rings recovered from excavations and conserved can fit into four broad categories based on style and object condition. It should be noted that these styles were constructed for the purpose of this blog and are not a reflection of convention in archaeology. I include object condition as a criterion for the simple reason that an incomplete finger ring may preclude it from a category that it might well fit into if complete.
The conservation goal for these objects is simply to clean for identification and preservation, and report on any discoveries. The cleaning was conducted under a binocular microscope using a fresh scalpel blade to remove soil and copper corrosion, such as malachite (a copper carbonate) and extruded cuprite (a copper oxide), to reveal the ‘original’ surface of the object. Ethanol impregnated cotton swabs were used to remove loose material from the surface. The objects were coated with two coats of 10% (v/v) Incralac in toluene after cleaning.
It should be noted that the simple acts of using a scalpel blade and cotton swabs on the archaeological materials is grounded on a foundation of understanding regarding the technology, chemistry and corrosion processes of, in this case, copper alloys. Please do not use any information contained within these blog posts as a guide on how to clean archaeological objects unless the user has a similar level of understanding, training and experience.
The Objects
Round Cross-Section Finger Rings
SF 0061, SF 0348, SF 0538, SF 0576, SF 0974
Five finger rings, either complete or fragmented (SF 0061 and SF 0348), fit into this category. All of the rings were covered in loose soil with some minor quantities of harder concreted soil and all exhibited some form of corrosion deposits on the surface. Upon cleaning, ring SF 0348 exhibited a dark green patinated surface with some pitting. The inner diameter of the ring is 30 mm (my wedding band is 19 mm inner diameter). This makes for the biggest ring in those presented here. It could be a large finger ring or possibly a harness ring. Ring is SF 0538 had similar patination and pitting to SF 0348. The surface of SF 0576 revealed similar pitting but lighter green colour. SF 0061 was similar to this but the colour is more buff. The surface of SF 0974 showed some areas of a green patinated surface but a lighter powdery textured surface underlying it was more extensive. This is not powder on the surface but rather a powdery or silty textured surface. This surface was noted on some of the bracelets mentioned in the previous post. It is not suspected of being ‘bronze disease’ as no additional ‘powder’ was produced during the several months that it was in our care. The object will be monitored to ensure that the object is not actively corroding.
Penannular Finger Rings
SF 0217, SF 0335
Two rings fit into the penannular (meaning the form of a ring with a small break in the circumference) category. These rings were covered in loose soil from the burial matrix. Ring SF 0217 contained a plug of dirt within the ring. This incited a slight feeling of apprehension as it gave the appearance that the plug of soil was supporting the entire structure of the ring. The condition of the uncleaned ring also fed into this fear as it was mottled light and dark green mixed with the soil on the surface. As it turns out, this was completely misleading as the ring was structurally sound (meaning it was not about to collapse) when the soil was removed. The surface consisted of a dark green patina broken by corrosion warts. The warts were retained as removal would most likely reveal pitting below. The ring is a highly eccentric ellipse in cross-section. Highfalutin words meaning generally rectangular with convex surfaces. Cleaning of Ring SF 0335 revealed a greenish-brown patinated surface with some warts. The ring is generally round in cross-section but were tapered to a point at the termini.
Strap Finger Ring
SF 0600
The ring had fragmented into three pieces. Each fragment was covered in soil from the burial matrix. One of the fragments also exhibited thick copper corrosion deposits. Cleaning revealed that two of the fragments could be joined. These were adhered using 40% (w/v) Paraloid B-72 in acetone. The surface is pitted with corrosion warts. The warts were retained due to the fragile nature of the ring body out of fear that the ring could fragment more. The ring is rectangular in cross-section with convex surfaces. This ring is similar to SF 0217 discussed above but was relegated to its own category due to it lacking termini.
Coil Finger Ring
SF 0155
This ring was covered in a layer of thick soil. Cleaning the surface revealed a coil ring composed of a single copper alloy wire coiled into a ring (as the name suggests). The surface appears to be copper carbonates overlying the copper alloy core metal. This has the same texture as mentioned above and in the last post regarding the ‘powdery-textured’ surface. This is not suspected of being ‘bronze disease’.
The following thumbnails are linked to higher resolution photographs.
The next post will cover other copper alloy materials possibly identified as personal adornment that were recovered from the Cardiff Castle excavations.
Originally this post was going to present all the copper alloy jewelry recovered and conserved from the Cardiff Castle excavations. It was quickly realized that this would be a huge task based on the number of finds. As a result I have split the original post into three smaller posts, each looking at specific object types: bracelets, rings and other objects. This post covers bracelets which were identified as such by archaeologists prior to conservation.
The bracelets recovered from excavations and conserved can fit into six broad categories based on style and object condition. It should be noted that these styles were constructed for the purpose of this blog and are not a reflection of convention in archaeology. I include object condition as a criterion for the simple reason that a fragment of a bracelet may preclude it from a category that it might well fit into if complete. One work around is to just include it into said category but this would be based on an assumption made on the part of this conservator. Such assumptions in archaeology in the past have taken years to correct after they were taken as fact. It seems a better solution to report on the existing characteristics of an object without relying on assumptions which may well prove false.
The conservation goal for these objects is simply to clean for identification and preservation, and report on any discoveries. The cleaning was conducted under a binocular microscope using a fresh scalpel blade to remove soil and copper corrosion, such as malachite (a copper carbonate) and extruded cuprite (a copper oxide), to reveal the ‘original’ surface of the object. Ethanol impregnated cotton swabs were used to remove loose material from the surface. The objects were coated with two coats of 10% (v/v) Incralac in toluene after cleaning.
It should be noted that the simple acts of using a scalpel blade and cotton swabs on the archaeological materials is grounded on a foundation of understanding regarding the technology, chemistry and corrosion processes of, in this case, copper alloys. Please do not use any information contained within these blog posts as a guide on how to clean archaeological objects unless the user has a similar level of understanding, training and experience.
The Objects
Side-Notched Penannular Bracelet
SF 0316
The bracelet was recovered from excavations in two pieces and was coated in loose soil and a harder concreted soil. Removal of the surface dirt revealed small localized areas of copper corrosion and large expanses of ‘original’ surface. Some dense corrosion material was retained as removal could have further damaged the object. The bracelet fragments were adhered together using a quick curing epoxy. Removal of most of the corrosion and soil material revealed a penannular (meaning the form of a ring with a small break in the circumference) bracelet with notches along the sides. The bracelet is oviod in cross-section with the longer convex surface on the exterior (top).
Top-Hatched Penannular Bracelet Fragment
SF 0415
This bracelet fragment was covered in loose surface soil and some small localized areas of dense copper corrosion. Removal of this material revealed a sub-rectangular cross-section with rounded edges. One terminus flattened out into a pancake-like feature. The other terminus represents a break-point in the bracelet. The top of the bracelet exhibits parallel hatching.
Side-Notched Bracelet Fragment
SF 0202
The bracelet remains had fragmented into five pieces. Each fragment was covered in a thick layer of soil from the burial matrix. Several of the fragments also exhibited thick copper corrosion deposits. Removal of these layers revealed a nice bronze coloured surface. The surface also revealed several pits. Removal of most of the thick copper corrosion products could have easily caused the bronze coloured surface to fragment and separate from the main body. The corrosion products were retained in these areas. Examination of the surface revealed notches along the side edges of the bracelet.
Top-Notched Bracelet Fragment
SF 0193
This is a small (< 2cm) bracelet fragment covered in loose soil material. The soil was removed to reveal the ‘original’ surface. The bracelet is ovoid in cross-section with the long curve on the exterior/top. The top is notched with broad ‘V’ cuts producing a series of facets along the upper surface. The bracelet is sub-rectangular in cross-section.
Some of the surface had fragmented while in deposition revealing a lighter green almost powdery textured surface. This ‘powder’ was not noted on the patinated surface. The ‘powder’ MAY be the first indication that the object is actively corroding. However, the lack of ‘powder’ on the surface does not support the presence of the feared ‘bronze disease’ which can completely reduce a copper alloy object to powder in a short period of time if the conditions are right. Only time and observation will reveal if the object is actually actively corroding.
Cable-Style Bracelets
SF 1037; SF 0265; SF 0118; SF 0102
Cable-style bracelets are produced by twisting copper alloy wires into a band or cable. The individual wires are relatively weak but the cable as a whole imparts a strength to the entire structure. From a conservation point-of-view these bracelets are relatively weaker than other bracelet types based on the wiring. This served as a guide while working on the objects.
The bracelets were covered in loose soil and light green copper corrosion products where the patinated surface was missing (similar to SF 0193 discussed above). Two of the fragments reveal structures that could represent termini. Bracelet SF 0265 reveals a copper alloy wire that bends outward and over the bracelet body. This could represent a terminus or a torsion break. Bracelet SF 0118 appears to have two wire bands that encircle the bracelet (in effect tying it off) and more probably represent a terminus of a penannular bracelet. Bracelet SF 0102 represents a bracelet fragment that has been partially bent over on itself.
Strap-Bracelet
SF 0984
This bracelet fragment was coated in loose surface soil and dense copper corrosion products. Cleaning revealed continued dense copper corrosion products, warts and pitting in the ‘original’ surface. The bracelet is sub-rectangular in cross-section with a short lobe on each upper edge of the bracelet exterior/top (see the figure). Much of the dense corrosion on the surface was retained as removal could have further damaged the object.
Thumbnails are provided here. Clicking on them will open a larger resolution image for closer inspection.
The next post will cover the copper alloy rings recovered from the Cardiff Castle excavations.
After a long delay the blog starts up again with a focus on several copper alloy finds. Over the next several posts we will be looking at the conservation of copper alloy objects through categorical groups of finds (e.g. brooches, toilet kits, etc…) and individual objects (e.g. bell, oil lamp). This post will serve as a conservation introduction to the non-coin copper alloy finds. Click on the links to read previous blog posts regarding coins (link 1), portable X-ray Fluorescence analysis (pXRF; link 2) and X-radiography (link 3 and link 4).
As with the coins, the copper alloy objects were cleaned with the goal of identification and, if possible, revealing the original surface. The objects entered the lab covered in surface obscuring soil and corrosion crusts. Dirt and corrosion were removed using scalpels, wooden skewers, needles, soft and glass bristle brushes and Industrial Methylated Spirits (IMS) impregnated cotton wool swabs. Cleaning was conducted under the stereo-microscope for better control. Many of the objects had a light to moderate coating of surface dirt which was easily removed through light brushing and swabbing with IMS while other objects were encrusted with thick corrosion deposits and required more intensive mechanical cleaning (e.g. scalpeling). Most of the copper alloys were somewhere in between. Intact pale green to dark green, and in some cases light buff to reddish-brown, patinated surfaces were visible beneath these corrosion deposits.
For this post I have selected four non-related objects as a preface of more to come. All of the objects were recovered from archaeological excavations. They are designated by small finds numbers (e.g. SF 0000). Time period/cultural attribution was determined prior to entering the conservation lab and most likely is associated with known excavation strata and/or through object form and style. While most of the objects are of Roman affiliation, some medieval objects were also considered significant and warranted further investigation. Thumbnails at the bottom of the page will direct you to a larger image.
SF 0150 (Fig. 1) Pre-Conservation ID: Medieval Knob
Medieval knob with gilding
This object is identified as a medieval knob. The object was covered in thick copper corrosion and soil from the burial matrix. It could readily be identified as a copper alloy by the green colouration in the soil surrounding it. Removal of the copper corrosion material, including some corrosion warts, revealed a gilded surface covering the entire object. PXRF confirmed the presence of gold (i.e. gilding).
This object was cleaned to greater degree than most of the objects in the assemblage. Removal of the soil revealed large green corrosion warts and tiny areas of gilded surface barely perceptible under the microscope. Cuprite (Cu2O, dark red to purple crystal-like corrosion) was overlying some of the gilded areas. Cuprite forms at grain boundaries in the metal and the exposure of it can mean that a copper alloy object was over-cleaned. However, there was still the question of the tiny gold areas that appeared to underlay this cuprite. Further investigation using a scalpel to flick off edges of some cuprite areas revealed that the gilded surface did, indeed, underlay it. Removal of this secondary cuprite (cuprite deposits above the original surface) revealed large expanses of gilded surface. As a result, the object was cleaned to a gilded surface, a green patinated surface at the gilded level or to a cuprite surface at the gilded level. Some green corrosion warts are still visible on the finished object. These were left in place out of fear that they would reveal corrosion pits that penetrated into the object if removed.
SF 0533 (Fig. 2) Pre-Conservation ID: Roman Button-and-loop fastener
Roman button-and-loop fastener with possible enamel
The button-and-loop fastener might have been used for many things including clothing, straps (horse furniture) and I have also seen a similar object in a presentation where it was used with Roman armour. The object entered the conservation lab covered in light soil and loose green corrosion. Removal of these products revealed a surface with many corrosion warts. The warts were retained out of fear that removal would expose corrosion pits.
This button-and-loop fastener closely resembles Wild’s (1970) type VIa (enameled head) or VIb (plain head) in that it has a rectangular head and triangular loop. There is a single chevron off-centered on the exterior of the head (labelled as “Obverse” in the photograph) that was first thought to be a casting fault. Closer examination of the chevron reveals what may be remnant red enamel flakes still attached. This was observed during cleaning but was mistaken for cuprite which appears as a darker red intermixed with green corrosion crust on the same surface.
Reference:
Wild, J.P. 1970 Button-and-Loop Fasteners in the Roman Provinces. Britannia, Vol. 1, pp. 137-155
SF 0607 (Fig. 3) Pre-Conservation ID: Roman Angled Bar
Anthropomorphized (human arm) fitting
This object was covered in soil and a light green corrosion. Removal of the soil and corrosion revealed an anthropomorphized attachment in the form of a human arm. The object’s surface is covered in dense corrosion warts but there are small areas of brassy and dark green patination. Warts were retained out of fear that they would exposed corrosion pits in the surface of the object. An attachment hole was revealed near the upper arm terminus. Despite the heavily warted surface, the ‘finger-tips’ and a possible bulge representing the base of the thumb are visible on the finished object. Other attachment holes may be present underlying the thick corrosion. This object may represent an anthropomorphic decorative fitting for some other object such as a piece of furniture.
SF 0951 (Fig. 4) Pre-conservation ID: Roman, uncertain, weight/fitting
A possible finial representing a peacock
The object was covered in light green corrosion and loose soil. Removal of the soil revealed large warts with a darker green patinated surface between. Moulded lines representing the edges of wings are evident on both sides of the finished object. Side A was treated to more intensive mechanical cleaning in the hope that it would reveal additional detail. However, this was not the case. This object could represent a peacock. The object may be a finial for a pole or some other object as evinced by the fitting hole in its base.
These thumbnails will direct you to a larger image.
Join us for the next post where we will be looking at copper alloy jewelry.
Portable X-ray fluorescence (pXRF) is an analytical technique using an instrument that can detect and identify most elements within a material. This makes it useful in instances where an investigator is trying to determine the material makeup of an analyte (solid, liquid and/or gas). In archaeology and conservation the analyte will most commonly be an object of some sort.
Portable X-ray fluorescence analyzer connected to an X-radiation protection cabinet. Object in cabinet is an Egyptian faience bowl. The larger laptop collects the data and displays the spectra. The smaller laptop is for recording all of the measurement parameters.
How does the instrument work? Well, it is complicated but here a very basic explanation. The pXRF sends out photons (packets of energy with no mass) from a source material inside the instrument (in our case, rhodium). These photons can eject electrons within the object causing a separate photon to be ejected from the object. These photons will have a specific energy related directly to the element from which they originated. If the generated photon strikes the detector inside the pXRF it will register at that certain energy level (kilo-electron volts (keV)) and the identity of the element from which it was generated will be known. The instrument readout is usually in the form of a spectrum where the individually identified elements will be represented by a peak at specific energies. For instance, X-ray fluorescence of a solid gold object will return a peak at the gold energy levels. An object composed of a copper alloy with silver will return a spectrum with peaks at copper, silver, tin, antimony and zinc depending on the type of copper alloy.
The setup parameters of the pXRF are important for analyses. Voltage, for example, can have a major impact on the elements the pXRF will detect and their depth of detection (object density and other parameters will also affect depth of detection and X-ray penetration). Different voltages are optimal for different elements. For the life of the instrument we restrict our measurements between 1 and 40 kilovolts (kV). A selection of 40 kV will reveal a great number of elements and will detect them from deeper (microns to millimeters) within the object but is not really optimized for any specific group of elements. A selection of 8 kV will detect very few elements at much shallower depths (microns) and is optimal for much lighter (lower molecular weight) elements. A typical full analysis of an object will be conducted at two or three voltages to produce optimal results for a wide variety of elements expected within an object.
The pXRF is a wonderful tool but it is only going to record that at which it is directed. This places a lot of responsibility in the hands of the investigator. Can pXRF be used to analyze an object just excavated from an archaeological site? The simple answer is ‘yes’ but unless the object is cleaned the soil will show up in the spectrum as well. The depth of penetration depends on the setup parameters of the instrument but also the density of the object being analyzed. So, if we have a suspect gold coin that has a surface obscuring concretion (for whatever reason), we can still analyze the object. The return of a gold peak (amongst the peaks from the concretion elements) in the spectrum may warrant further investigation of this coin. However, if it is a thick or relatively dense concretion we may have no return for gold leaving us to conduct investigative cleaning on the object until we can make a presence/absence confirmation.
With this background information, it should be obvious how pXRF can help in the identification of archaeological coins which can be composed mostly of copper, gold, silver, lead and even iron. But the makeup of coins, even of the same base metal, can change overtime with the ebbs and flows of the economy of a civilization; times of prosperity may be reflected in more precious metals in a coin whereas a decline may reveal less.
Now for some Hands-on experience
Something atypical about this typical Cardiff Castle Roman coin
The reader will see spectra of 40 kV measurements of the coins in this blog post. We, however, used both 40 kV (to identify most elements) and 15 kV (optimal for lighter elements) measurements with associated change to other setup parameters which are provided on the figures but will not address here for simplification. We have selected a single coin to illustrate these voltages.
Coin SF077 is a typical Roman copper alloy coin recovered from Cardiff Castle (…or is it?). Two or more measurements are necessary when analyzing objects. Measurements at 40 kV (Spectrum S1 (logarithmic scale to show more clarity in the comparison)) reveal a great number of elements present in the coin. The 15 kV measurement are incapable of returning peaks much greater than 15 kilo-electron volts (keV – the X axis) but do provide greater clarity of the spectra below ~ 10 keV.
SF0077: The coin was identified prior to conservation as a possible Roman irregular denarius (third century?).
S1: Spectra of coin SF0077 at 40 kV (red) and 15 kV (blue).
Spectrum S2a and S2b are the same spectrum measured using 15 kV (only difference are the associated labels with the peaks in S2b). For simplicity we will look at the elements above 5 keV. We can see that there is a high peak for copper (Cu) and small peaks for iron (Fe) and bromide (Br). The presence of bromide provides me with a clue as to what to expect with a higher measurement. Read on…
S2: 15 kV pXRF measurement of coin SF077 showing a large peak for copper (Cu) and small peaks for iron (Fe) and bromide (Br).
Armed with the information provided above, we know that a single measurement at 15 kV is not enough to determine all the elements within this coin. Therefore we conducted another measurement at 40 kV (Spectrum S3a and S3b). Now we see the addition of another element – silver (Ag). This was suspected due to the presence of bromide which is a constituent of silver bromide, a silver corrosion product. Bromide is usually not found on copper alloy coins containing no silver. The rhodium (Rh) and palladium (Pd) peaks are associated with the instrument and will almost always be present in the spectra.
S3: 40 kV pXRF measurement of coin SF077 showing a medium sized peak for silver (Ag). Rhodium (Rh) and palladium (Pd) are from the instrument.
So, perhaps this coin is not so typical of the Roman copper alloy coins found on the site. There is not enough silver to call this a silver coin. However, there may be enough to have given the coin a silver finish. As a comparison, here are the spectra (S4a and S4b) of a silver coin where we can see a much higher ratio of silver (Ag) to copper (Cu).
The coin was confirmed to be a silver Roman Republic coin after conservation.
S4: 40 kV pXRF measurement of silver coin SF2008. Silver (Ag) peak is much larger and the copper (Cu) is much smaller compared to the spectrum for SF0077 (S3).
The exercise above was just to illustrate a point; that one measurement is usually not enough for a fair assessment of the elemental makeup of the coin. In all fairness, we would be very interested in the elements below 5 keV in the 15 kV measurements. Additionally, we can expand the spectrum to see additional elements (Spectrum S5a and S5b) as can be seen in the logarithmically scaled spectrum S1.
S5: Expanding the 15 kV spectrum for SF0077 shows a greater number of elements than spectrum S2.
A sign of declining times?
Four coins (SF0333, SF0438, SF2019 and SF2021) recovered from the Cardiff Castle excavations are yellowish brown in colour. Three of these were identified as 4th century coins. The forth was not assigned to any century. The colour of these coins is different from the green, silver and black of most of the coins recovered from the excavations. They were analyzed and compared to four green copper alloy coins (SF0142, SF0376, SF0339 and SF0496) identified as the 3rd century or before to determine the reason for the colour differentiation. The spectra for the green copper alloy coins (Spectra S6a and S6b) exhibit a high peak for copper (Cu), medium peak for iron (Fe) and low peak for lead (Pb). The spectra for the yellowish brown coins were adjusted to represent the same scale (y-axis) as the green copper alloy coin spectra. These spectra (S7a and S7b) exhibited low peaks for copper (Cu) and iron (Fe), and medium peaks for lead (Pb). The spectra for two of the comparative coins are presented to show the difference (Spectra S8a and S8b).
The coins were identified prior to conservation as fourth century.
The coins were identified prior to conservation as uncertain (SF2019) and mid fourth century.
The coins were identifed prior to conservation as Victorinus/Virtus Aug (SF0376; radiate; AD 269-271) and uncertain (SF0496).
The coins were identified prior to conservation as TetricusII/Spes Publica (SF0142; AD 273-274) and Domitian (SF0339; AD 81-96).
S6: 40 kV spectra of four green copper alloy coins. The spectra are generally the same.
S7: 40 kV pXRF spectra of the four yellowish brown coins. The spectra are generally the same.
S8: Comparison between a green copper alloy coin (blue) and a yellowish brown coin (red). The yellowish brown coin have a much larger quantity of lead (Pb) while the green coins are mostly composed of copper (Cu).
So, the yellow coins appear to have a greater concentration of lead and a much lower peak for copper whereas the green coins have high peaks for copper and very low peaks for lead. Why is there such a difference in the composition of the coins? Are they lead tokens, fakes, forgeries or true coins with a clue to the economic times in which they were produced? They probably represent the latter. The fourth century, and especially the late forth century, was a time of upheaval. Roman Britain had to contend with invasions from Ireland and Scotland while with reduced resources due to campaigns and insurrections on the continent. Greater quantities of lead were introduced to some coins to extend general copper supplies.
In conclusion, we have seen how pXRF can identify the various elements within an object and how this can be used to inform on the greater economic situation at the time in this scenario. PXRF will be used again on some other objects recovered from Cardiff Castle.
We will now close this chapter on coins. The next blog post will be on the conservation of the general copper alloy objects.
Note: The coin captions will change as I get post-conservation identifications from a GGAT coin expert. All the coin posts will be amended to reflect those changes.
