A Closer Look 2: pXRF of Roman Coins

pXRF of coins? What does that mean?

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.

 

 

 

 

 

A Closer Look 1: X-radiography of Coins

Background: X-radiographs and Coins

A selection of objects were X-rayed to provide additional information prior to their conservation. Some of the objects were readily identifiable as coins prior to X-radiography. These coins were selected because information was required about their condition to help inform on their conservation. Coins are very useful in providing context to archaeological stratigraphy and features (see the blog post on coins). Some of the objects were completely encrusted in corrosion materials and X-radiography was required to determine if they were coins (priority for cleaning) or another object type. X-radiography of these objects also showed contrast in coins that were composed of a copper alloy from those that were later determined to have a high silver content.

This blog post is mostly pictorial and expands on the prior coins and on the radiography of iron blog posts. Please refer to them for an introduction regarding the coins recovered from the Cardiff Castle excavations and for the theoretical background of X-radiography.

Results

The X-radiographs will be presented with all coins identified that are referred to in this post. X-Radiographs are presented in pairs (unless indicated) with the first image representing a laser scan and the second a photograph of the X-radiograph (see the radiography of iron post for camera setup and parameters). Photographs of the selected coins from the X-radiographs will be presented below the X-radiographs with the pretreated representations on the left and the post conservation photographs on the right. Below these photographs are thumbnails which can be clicked on to provide a larger image of the individual X-radiographs and the coins. The first grouping of images (J844) will have descriptions interspersed to help the reader grow accustomed to this pictorial review. The remainder of image groupings will have descriptions and results presented below the thumbnails for each grouping.

X-Radiograph J844

The X-radiograph on top is the laser-scanned image. Below that is the photographed image. This is the only X-radiograph in which all the coins are selected for this blog post. Coloured squares and text are provided on the images to help identify each coin for the reader. The radiograph is divided into six separate X-ray parameter settings (parameters are in white (for example, KV 110; Mins: 1)). In other words, we have six groupings of coins on each radiograph above. This also means we have six x-radiographs of each coin using separate parameters. For example, follow coin SF0551 (in green) across the screen. The first image was taken using KV 110 for 1 minute (KV 110, Mins: 1). The third image of the same coin was taken using KV 110 for 5 minutes. The final two images of the coins are at the bottom of the X-radiograph and are in rows, not in columns as before, so they could fit on the X-ray plate. All rows and columns are separated by a relatively thick white field and/or post-processed white line. So, in effect what we are seeing is an increasing number of X-ray photons penetrating the object as we go across the X-radiograph from left to right. We take several X-radiographs using different parameters to capture as much information as possible as determined by density. For example, the human profile and the lettering on the coins are thicker areas of the coins and are more dense. They will block more X-ray photons from hitting the film than the general body of the coin. Therefore, they will show in an X-radiograph. If you do not see lettering (SF0858), it was either rubbed off in antiquity or never existed (less probable as most coins will have some form of legend).

These are the coins represented in the X-radiographs. They may not be presented in the same order as on the X-radiograph but each coin number is included for identification. On the left side are the pretreatment images. It was in this form that the coins were X-rayed. The images on the right are the post treatment images. These are provided for comparison with the pretreatment images and the X-radiographs.

The thumbnail images represent those above. These can be clicked on to provide larger images. As a result of the X-radiography of these coins we can make the following assumptions:

•  all of the objects are most likely coins
•  all of the coins are composed of a material similar in density
•  all of the coins are probably composed of the same general material
•  coin SF0551 has a shallow crack near the top which may be of concern during conservation

The X-radiograph will not tell us the material of which the coins are composed. However, the assumption of copper alloy can be made based on the corrosion materials and the final colour of the object. Interestingly, the shallow crack evident in the X-radiograph of coin SF0551 does not show up in the final post conservation image of the coin. It is, however, still present in the object.

X-Radiograph J826

 

This X-radiograph shows several objects X-rayed using four parameter settings (KV90 at 1-4 minutes of exposure). Coin SF0077 was X-radiographed to determine its condition prior to conservation. The X-radiograph shows a legend around the edge and a portrait in the center of a presumably flat field. The coin appears intact with no crack nor weak areas present. Remembering that an X-radiograph of a coin exhibits both surfaces, one can see the standing figure on the coin reverse as the vaguely bright band running down through the portrait in the X-radiograph. Look for this with other X-radiographs in this post.

The X-radiograph also shows annotations in the form of hand-drawn segments (divided by X-ray parameters used) and coin identification numbers. As discussed above, this plate represents digitalization through photography. Photography was conducted using transmitted light (i.e. lightbox) and a combination of transmitted and reflected light (i.e. lightbox and copy stand lights). The use of reflected light allows annotations made on the developed X-ray film to be captured in the digitalization. It is my opinion that the use of transmitted light photography of X-ray film shows more detail than reflected light or a combination of both. However, in this case, the combination of lighting techniques provided a superior digital image.

X-Radiograph J827

Two objects (SF0060 and SF0093) are highlighted for this X-radiograph. Object SF0060 is obviously a coin with legend and portrait on a less dense field evident in the X-radiograph. Object SF0093 was less obvious. The most defining feature was not a portrait but a series of curvy lines and a backward letter ‘S’. This made the object rather intriguing. Cleaning of the object revealed that it was, indeed, a coin. In hindsight there is a portrait present but at the time I was unsure if this was another feature associated with the curvy lines. The figure on the reverse of the coin is oriented upside down with its legs pointing toward the top and slightly left of the top of the coin. Based on the backwards ‘S’, the coin was X-rayed with the obverse facing up. Cleaning of the coin revealed that the obverse had been abraded/corroded away either during burial or during use in antiquity.

X-Radiograph J840

The X-radiographs show four bright objects amongst the other objects of varying brightness. Comparative brightness in an X-radiograph is a qualitative measure of relative density. In other words, assuming measurable thickness of the objects are relatively the same and all objects were X-rays using the same parameters, the brightness can only be a factor of object density which means a different material. In this case, the denser material was silver. While the other objects could have small amounts or no amount of silver, these four objects had significant amounts of silver. Cleaning revealed all to be silver coins of which coin F2008 exhibited the best portrait and reverse device.  Objects SF0314 and SF0315 appeared coin-like in the X-radiograph with intact portrait and legend. Object SF0242 was less obvious and cleaning would reveal it to be a coin as well.

X-Radiograph J842

 

Like X-radiograph J840, this X-radiograph also shows a brighter object (SF0534) which cleaning would reveal a silver coin although all surface relief was abraded away during deposition or in antiquity. The post treatment photograph shows this coin to be black. This is a silver tarnish on the coin surface which was not removed during conservation. Objects SF0322 and SF0553 are both obviously coins with portrait and legend readily evident in the X-radiograph.

Object SF0279 was originally identified as a disc or possible coin. The X-radiography of the object revealed features inconsistent with coinage and more consistent with buttonage…, I mean, buttons. Cleaning revealed this to be a button cap. The cap was fragile so some soil was retained for object stability. This highlights a conservator’s role as not only a cleaner and stabilizer of objects, but also an identifier of objects. Conservators make discoveries.

Object SF0504 was originally identified as a possible coin fused though corrosion products to a ring. The X-radiograph supports that part of this object is a coin with a portrait fairly visible in the 2 minute exposure. The object was rotated in subsequent X-ray events to capture different perspectives. The ‘ring’ portion of the object did not ‘materialize’. The X-radiograph showed very little dense material remaining in the ring-like structure. Cleaning did not reveal any difference between the ring-like structure and the other non-corrosion surface deposits on the coin. This object shows the need for both the laser-scanned and photographed digital images. While the photographed images may show more interesting details, the laser-scanned images show more information (see X-radiography of iron blog post for more discussion).

X-Radiography J843

Three digital image of the X-radiographs are presented. The first is the transmitted/reflected light photographic image which shows the annotations made to the developed X-ray film. This is presented so that the reader can identify the coin numbers of the other two digital X-radiograph images as colour coding all the coins selected in the images would have been convoluted and confusing. The objects have been X-rayed at KV 100 for 3 minutes and 5 minutes. This is where you become the conservator. What can you determine from these images?

Join me for my next post. We will be looking at the instrumental analysis of a selection of coins recovered from Cardiff Castle.

 

Find a Penny, Pick it Up: Conservation of Coins Recovered from Excavations at Cardiff Castle

Introduction

Have you ever found a coin lying on the ground? Not only were you lucky and somewhat enriched by your experience, but the coin actually provided information about the date of its deposition. Coins are considered very useful in archaeology. The information contained on them can supply archaeologists with a date, or the latest date of which a soil deposition can be attributed. This we call the terminus post quem. For example, if you find an AD 1432 coin in a thin sandy soil layer. The deposition of that coin cannot be before AD 1432. Therefore, a relative date of AD 1432 or after can be applied to the deposition of the sandy soil.

This excavation photograph is not of Cardiff Castle but it does provide another example of terminus post quem. This newspaper (dated 1994) at the bottom of this filled-in trench postdates the clay layer beneath and is concurrent or predates the soil strata above it. This provides us with a ‘time after which’ reference point. Coins provide a similar reference point. (Photograph courtesy of Chris Wilkins)

Understanding the importance of coins in archaeology, one can understand why coins from archaeological contexts need to be cleaned and conserved. In this post I will present the ‘what, why and how’ in the conservation of coins recovered from excavations at Cardiff Castle. The results at the end of the post are shown as a series of photographs that can be enlarged by clicking on their captions.

What we did

Almost 200 coins from the Cardiff Castle excavations were brought into the conservation lab. These coins mostly represented the Roman period but there were a few from later periods as well. Most of the coins were composed of a copper alloy (copper mixed with another metal, e.g. bronze (copper and tin)). Some of them were silver. The coins were cleaned for identification, examined for active corrosion and prepared for long-term storage. The coins have not been cleaned to museum exhibit quality.

Why we did it

Many of the coins were covered in a light dusting of soil and were readily identifiable. Some of copper alloy coins were completely covered in a thick layer of soil mixed with copper corrosion making visual identification impossible. Likewise, the silver coins were covered in a thick corrosion product on which soil deposits could also be encountered. The corrosion crust/soil deposits on these latter copper alloy and silver coins needed to be removed so the coins could be identified and any possible deterrents to long-term storage rectified (e.g. active corrosion).

SF0130 (Roman): Pretreatment photograph showing light dusting on the surface.

SF0496 (Roman): Pretreatment photograph showing thick corrosion/soil crust on surface

 

Common copper corrosion can consist of copper carbonates, sulphates and sulphides taking the forms of a nice patina or a crust overlying the surface depending on the speed of the corrosion. As conservators we usually clean down to a patinated surface which we view as the ‘original surface’ of the object. The patinated surface is also a corrosion stabilized surface meaning that corrosion will be inhibited as long as it is not penetrated. Below the crust and patinated surface is a red copper oxide overlying the metallic body core. Ideally one would not want to see these layers as they could represent over-cleaning of the object and would endanger the object to further corrosion.

The most detrimental copper corrosion product is an active corrosion called ‘bronze disease’. It takes the form of a light green powder composed of copper chloride hydroxide crystals. It is most readily identifiable on the surface of an object. However, these crystals can occur below the surface of the object causing it to break apart. If unchecked, it can result in the complete dissolution of the object. It is important to monitor for ‘bronze disease’ after archaeological recovery, during long-term storage and after conservation which may have inadvertently exposed cracks in the patinated surface allowing the ingress of moisture and oxygen (required for corrosion to occur).

The silver coins had a relatively thick layer of silver bromide giving the appearance of a dense brown to purple sandy concretion. Beneath this layer is what I call the silver corrosion trickster. It is a waxy silver chloride (horn silver) and can polish to a nice silver finish although being a corrosion product. I call it the trickster because it can fool unsuspecting lab personnel into thinking they have sliced into the coin. Below the horn silver is a black tarnish layer. This is the same tarnish one may find on their fine silverware at home. Removal of this layer reveals the actual silver surface of the object.

SF0021 (Medieval): Pretreatment photograph showing silver bromide crust on surface. The thin crust reveals details beneath.

The objects had to be prepared for long-term storage. The object housing (e.g. bag, box) should convey useful information indicating, in the least, what the object is and the project with which it is associated. More information is always useful. The object becomes nothing more than an interesting trinket without the information tying it into a site (provenance) and a specific context within the site (provenience). Long-term storage of these objects require a safe housing (e.g. bag or box) for the object and its associated information. A barrier inhibiting the ingress of oxygen and moisture (required for corrosion to occur) can be applied to the object if the environmental parameters of the storage facility cannot be controlled or are unknown.

How we did it

Photographs and notes were generated throughout the process. The importance of documentation cannot be stressed enough. It provides the provenance, provenience and a record of the conservation treatment for each object. These will be important to future researchers.

Copper alloy coins coated in a light dusting of soil required light brushing and the rolling application of an ethanol impregnated cotton swab. The ethanol helps to loosen any concreted soil. Coins with thicker corrosion crust required mechanical removal using a scalpel blade in conjunction with the methods already mentioned.

Silver coins were a bit more difficult. The thick brown to purple silver bromide corrosion crust was dense. Experience and an understanding of the chemical nature of the corrosion dictated the application of ammonium thiosulphate and mechanical cleaning using a scalpel to ‘break it open’. Once the surface was penetrated, the scalpel was used in a flicking manner to ‘pop-off’ the remainder of that particular crust. This revealed horn silver, the silver corrosion trickster. The horn silver could have been scrapped away from the silver surface but this technique could also lead to scratching the silver surface. Instead, the horn silver was flicked off much in the same manner as the silver bromide layer above it to reveal the tarnished silver surface. The tarnish was mostly left intact as it is a stabilizing corrosion that partially covered the surface helping to reveal some of the surface details.

SF2008 (Roman): This coin has been partially cleaned to reveal the silver bromide crust (purple), horn silver (waxy black) and the silver surface.

Coins were prepared for long-term storage by apply a coating of 10% (v/v) Incralac in toluene. Coins were placed into polythene see-through bags on which provenance information was included. The coins were returned to GGAT in an appropriately sized Steward box. Recommended storage parameters (e.g. relative humidity (≤42%)) were provided as well.

What we found

The results are presented as mostly pictorial in this post. Click on the caption that appears beneath each photograph to see a larger version. The assigned dates for each coin were made prior to conservation. Some of these attributions will have changed after conservation but are currently awaiting confirmation. The first photograph shows all obverse sides of the coins at the same scale for size comparison. The remainder of the photographs show the coins in an unclean state, semi-clean state and a final state. Luckily we did not find ‘bronze disease’ on any of these objects. ‘Bronze disease’ has been retained in this post to stress the importance of quick action when it is encountered.

 

A selection of post-conservation coins recovered from the Cardiff Castle excavations.

Pre-conservation ID: SF0013 Copper Alloy, Valens/Securitas type (AD 364-378; SF0021 Silver, Medieval Short Cross 1b/London/Renaud (AD 1180s)

Pre-conservation ID: SF0028 Copper Alloy, Valens/Securitas type (AD 364-378); SF0033 Copper Alloy, Irregular radiate (AD275-285)

Pre-conservation ID: SF0038 Copper Alloy C I/Soli Invicto Comiti/Lyon (~AD 310); SF0039 Copper Alloy, C II/Gloria Exercitus 2/Tier (AD 330-335)

Pre-conservation ID: SF0040 Copper Alloy, Allectus/Virtus Aug/QC (AD 293-296); SF0067 Copper Alloy, Tetricus II/Spes Publica (AD 273-274)

Pre-conservation ID: SF0077 Copper Alloy, irreg denarius? (third century AD?); SF0130 Copper Alloy, C I/Soli Invicto Comiti/Lyon (~AD 315)

Pre-conservation ID: SF0186 Copper Alloy, Valentinian I/gloria type (AD 364-375); SF0201 Copper Alloy, Magnentius/Victories/Trier (AD 350-353)

Pre-conservation ID: SF0238 Copper Alloy, as: Domitian (AD 81-96), SF0267 Copper Alloy, as: Domitian COS XII (AD 81-96)

Pre-conservation ID: SF0496 Copper Alloy, as/dp: encrusted (?); SF0526 Copper Alloy, as/dp Vespasian (AD 69-79)

Pre-conservation ID: SF0553 Copper Alloy, encrusted (?); SF0724 Copper Alloy, as: Vespasian (AD 69-79)

Pre-conservation ID: SF0816 Copper Alloy, as: Vespasian (AD 69-79); SF0817 Copper Alloy, irregular radiate (AD 275-285)

Pre-conservation ID: SF0855 Copper Alloy, as: Domitian? (AD 81-96?); SF0917 Copper Alloy, as: Nero (AD65-68)

Pre-conservation ID: SF2008 Silver, Republican (before 27 BC)

 

The next post will examine the use of X-radiography in the examination of the coins. This combines the information from this post with a technique used in the previous post, Not Your Doctor’s X-Ray: X-Radiography of Iron from Excavations at Cardiff Castle.

 

 

 

Not Your Doctor’s X-Ray: X-Radiography of Iron from Excavations at Cardiff Castle

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.

General Theory

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.

Spalled Iron Objects

X-Radiography Methodology

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.

Our results

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.

Conservation of Select Artefacts from the Cardiff Castle Excavations

Welcome to the Glamorgan-Gwent Archaeological Trust (GGAT) conservation webpage and blog. Over the next several weeks I, and others, will be posting on the conservation cleaning and treatment of a variety of archaeological materials recovered from excavations at Cardiff Castle. The goal of these posts is to inform on the conservation of some of the materials excavated from the grounds of Cardiff Castle. With each post we will look at the conservation of selected artefacts or use of a specific conservation or analytical technique. Look for periodic conservation posts to have a peek at what we, as conservators, are doing to conserve artefacts and to expand on the story of Cardiff Castle and Cardiff as a city.

As background, GGAT contacted Cardiff Conservation Services at Cardiff University to work on a selection of artefacts excavated from the footprint of the current Interpretation Center within the walls of Cardiff Castle. The objects, most of which are Roman in origin, are varied and consist of iron, copper alloy, silver, stone and glass.

Most of the archaeologically recovered material consists of metal objects. This material is typically covered in corrosion and soil from the burial matrix. The corrosion and soil may require removal to expose the original surface details and reveal additional information, such as presence of decoration. Some objects may be broken and require re-adhering. Other objects could be actively corroding. Most objects will exhibit a combination of these conditions and possibly some conditions not yet mentioned. Other material types will present their own issues and will be covered in upcoming posts.

The conservation goal for the Cardiff Castle objects is stabilization in preparation for long-term storage. Stabilization of the objects will facilitate their preservation and allow future study and/or display of them. A secondary goal is to reveal additional information such as how were they constructed and the presence/absence of decoration.

Various techniques were used during the conservation of these objects. Photographs were taken before and after, and in many cases, during conservation. X-Radiography was used to identify heavily encrusted objects and to determine if further conservation is warranted. Cleaning using bench tools, and possibly chemicals, was required to reveal the original surface and additional information. X-Ray fluorescence and scanning electron microscopy was used to provide elemental composition which, in turn, provided additional information on technology.

From top and around clockwise: Pretreatment images of Roman maille, X-ray of coins, Roman intaglio, glass, back-scattered electron image of glass inlay in brooch, coin, X-ray of wood covered in calcium deposits, and brooch.

We will post at intervals of at least once every two weeks but posts will be made more frequently at times. Join us for our next conservation post as we report on the X-radiography of corrosion-encrusted iron objects.