Using PXRF Technology to Aid in the Recovery and Analysis of Human Remains
John D. Richards, University of Wisconsin-Milwaukee
Catherine R. Jones, University of Wisconsin-Milwaukee
ABSTRACT
Excavation and analysis of human remains from the Milwaukee County Institution
Grounds Cemetery (MCIG) provided an opportunity to test the effectiveness of portable
X-ray fluorescence (pXRF) as both a field and laboratory tool. During the fieldwork
portion of the project, excavations exposed soils that visual inspection suggested might
harbor a concentration of toxic materials. PXRF was used on site to determine the nature
of the potential toxins and determine the risk factor associated with continued excavation.
Subsequent laboratory analysis used the pXRF analyzer in two separate instances. First,
elemental composition of excavated soil samples was identified to determine background
levels of soil constituents that might produce diagenetic changes in human skeletal
remains. Second, the MCIG excavations recovered multiple instances of commingled
human remains representing multiple individuals. This paper reports the results of a pilot
study to use pXRF as an aid in identifying commingled bones from MCIG interments as
belonging to specific individuals.
DRAFT READING VERSION
Please do not cite without permission of authors
Presented in Symposium " People That No One Had Use for, Had Nothing to Give to, No
Place to Offer: The Milwaukee County Institution Grounds Poor Farm Cemetery", 80th
Annual Meeting of the Society for American Archaeology, San Francisco, CA, April 1519, 2015
Introduction
This paper reports a very preliminary study of the use of a portable X-ray
fluorescence (pXRF) analyzer as an aid to the excavation and analysis of human remains.
The use of handheld XRF analyzers in archaeology continues to increase as costs go
down and instrumentation improves (Liritzis and Zacharias 2011). Within the limits of
instrument design parameters, some modern handheld XRF units are now capable of
achieving results comparable to bench mounted laboratory units (Hunt and Speakman
2015). However, any number of cautionary tales exist pointing out the pitfalls of treating
these devices as the scientific equivalent of a point-and-shoot digital camera (e.g.,
Shackley 2010; Shugar and Mass 2012a, Richards 2015). As Shugar and Mass (2015b)
note the uncritical use of these instruments can easily produce bad data and spurious
interpretations. Recently, Speakman and Shackley (2013) have characterized this as “silo
science” that is neither reproducible nor comparable on an inter-laboratory basis. Lest we
be accused of building yet one more such silo, we wish to be very clear concerning the
goals of the study reported here. Specifically, the exercise was intended as a
methodological experiment to evaluate the applicability of pXRF technology within a
particular archaeological context; i.e, excavation and analysis of a late nineteenth and
early twentieth century pauper cemetery. Our goal was to generate information that
would allow us to design a comprehensive analytical protocol tailored to the specifics of
the MCIG sample. Thus, no claim is made that our results represent a useful data set
relating to the elemental composition of the artifacts or the human bone subjected to
analysis. However, we do think the results are informative regarding the potential
applicability of future, more intensive, detailed analyses of the materials presented here.
Richards and Jones-pXRF at MCIG
During the course of the MCIG project, the analyzer was used in the field to test soils
containing potentially contaminated deposits. Laboratory use included analysis of soils
and selected artifacts as well as analysis of human remains directed toward sorting out
commingled human bone.
Methods
The artifact analysis reported here used a Niton XLt analyzer utilizing factory
calibrations and operated in bulk soil mode. The instrument was controlled by a computer
and three readings of 180 second duration were recorded for each artifact. The Niton
analyzer returns elemental values in parts per million.
Soils were analyzed using a Bruker Tracer IIIv+ analyzer. The instrument was
operated at settings of 40Kv and 30 micro-amps with Bruker’s “green” beam filter” (6
mil Cu/1 mil Ti/12 mil Al) installed. Three readings of 180 seconds each were recorded
for each sample. The instrument was operated by hand in the field but was controlled by
a laptop computer in the laboratory. All soils were analyzed wet and un-processed.
Human bone was analyzed using the Bruker analyzer also. The instrument was
operated at 15Kv and 28 micro-amps under vacuum and without a filter. Three readings
of 180 seconds duration were recorded at three separate sites on each bone. Results were
exported to Microsoft Excel using Bruker’s S1PXRF and Artax software.
Soil and Artifact Analysis
A benefit of pXRF is the ability to conduct rapid field examinations of soils and
sediments as a component of field investigations. As a result, pXRF units have been used
by geologists as well as archaeologists in order to aid stratigraphic interpretation
(Colombo et al 2011), assist in site survey (Hayes 2013), provide a first approximation of
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Richards and Jones-pXRF at MCIG
soil composition (McLaren 2012; Zhu, Weindorf, and Zhang 2011), or as an initial test to
screen for the presence of heavy metals or toxic compounds (Radu and Diamond 2009).
During the 2013 MCIG excavations, an exposed coffin was observed associated with a
bright blue soil deposit (Fig. 1). Since blue soils do not occur naturally in southeastern
Wisconsin, excavators were reasonably concerned that they had encountered a pocket of
contaminated soil. Excavators at Dubuque’s Third Street Cemetery (Lillie and Mack
2015) also noted the presence of bluish soils but considered the coloration the result of
particular kinds of molds. However, at MCIG, the immediate area was cordoned off and
excavation of the associated burial was halted until the pXRF unit could be brought to the
site. Tests produced the spectrogram shown in the slide (Fig. 2) and the crew and
Principal Investigator breathed a sigh of relief to see that the deposit was not radioactive
or worse. However, it was clear that the deposit did contain elevated levels of arsenic so
excavators took special care removing the nearby burial (Lot# 10569).
Subsequent comparisons to adjacent soils as well as soils from other parts of the
cemetery suggests that low levels of arsenic are typical of many locations in the cemetery
but the 10569 deposit did not appear to have spread too far from its point of origin (Fig.
3). The genesis of this deposit remains unknown. It should be noted that the four soil
samples analyzed were coffin fill collected from the pelvic region of burials identified in
the field as female. Consequently, arsenic levels in these deposits may not be typical of
undisturbed cemetery soils.
Selected grave goods were analyzed in the laboratory as an aid to preliminary artifact
analysis. Items included an upper denture, a partial denture, two metal finger rings, and a
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piece of what appears to be slag. All were recovered from within excavated coffins. Only
the dentures are reported on here.
Late nineteenth century and early twentieth century dentures were manufactured from
a compound patented as Vulcanite by Charles Goodyear (Fig. 4). Basically a mixture of
natural rubber and sulfur, Vulcanite revolutionized denture production. When the
Goodyear Dental Vulcanite Company (the same firm still manufacturing tires) chose to
no longer enforce its patent, dentures became commonly available at affordable prices
(Wynbrant 2000). The MCIG specimens appear to be typical examples of vulcanite
dentures with an elemental composition including high relative levels of sulfur. Mercury
present in the dentures is likely the result of the use of vermillion to color the naturally
black vulcanite a more pleasing shade of reddish pink.
Human Remains
Previous studies have successfully tested the applicability of portable X-ray
fluorescence analysis (pXRF) on known single adult populations (Perrone et al. 2014;
Gonzalez-Rodriguez and Fowler 2013). Peronne et al. reported an analysis of 20 sets of
human remains consisting of forensic specimens recovered as in-flesh and skeletonized
surface finds as well as a single inhumation. The analysis was directed toward
determining if elemental variation was greater between individuals than within
individuals. Results suggest that little or no intra-skeletal variation was present in the
Perrone et al. sample. Peronne et al. conclude that pXRF may be a viable technique to aid
in re-associating individuals in small-scale commingling scenarios.
The Gonzalez-Rodriguez and Fowler study examined five medieval stone coffin
burials using pXRF to derive elemental ratios for 23 bones from each skeleton. The
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authors report a high success rate in correctly assigning individual bones to specific
burials. Results are suggested to support the use of the method as a screening tool prior to
confirming association by DNA analysis.
Both studies offer the hope that pXRF analysis can be used to confidently sort and reassociated commingled human bone. However, as Peronne et al. note, neither study was
designed to consider the potential effects of diagenetic processes on inhumed remains.
This is a potentially confounding issue as post-mortem diagenetic changes in elemental
concentrations can be profound (e.g. Price et al. 1992). In the case of the MCIG
Cemetery we suspect that significant diagenetic alteration of bone may have occurred as
a result of ground water infiltration, deposition of industrial and medical wastes,
compaction of overlying sediments, and episodic inundation and desiccation due to
changes in the local water table. Consequently, the present study was intended to
determine if a pXRF analysis of human bone would be an effective aid to the sorting of
commingled human bone in the diagenetically complex environment of the MCIG
Cemetery.
Data Set
The analysis was conducted on 15 skeletal lots from the 2013 excavations at the
cemetery (fig. 5). Burials included are unidentified and undated so may span the entire
period of cemetery use from 1882 to 1925. As noted in earlier presentations, a burial
register was kept for the cemetery but no map keyed to ledger entries has yet been found.
In addition, removal of grave tags, incomplete record keeping, episodic disturbances and
consequent reburial preclude association of burial locations with individuals listed in the
burial register. Biological analysis was conducted for all remains to allow estimation of
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an age range and sex in accordance with established methods (Buikstra and Ubelaker
1994; Spradley and Jantz 2011; White et al. 2012).
Sample Selection
During the 2013 excavations, portions of the cemetery were highly saturated with
ground water. As noted, this can have a significant diagenetic affect on human bone, so
lots were chosen that appeared to be relatively dry at the time of excavation. However,
excavations revealed also that the portion of the cemetery excavated in 2013 harbors a
perched water table and it is likely that fluctuating water levels may have inundated all
burials at one or more points in time.
Three control lots and five commingled burials consisting of adult and subadult
remains were chosen for analysis. Control lot 10293 represents a middle adult probable
male and lot 10737 consists of a middle adult female adult interment (Fig. 6). Both
burials were relatively complete and were recovered from dry sediments. These lots are
typical of the majority of burials recovered during the 2013 excavations at the site. The
third control lot consisted of a human skeleton previously purchased from a biological
supply house as a teaching aid. The five commingled sets of human remains include
mixed interments with established MNIs of between 2-6 individuals. These lots likely
result from the disposal of medical cadavers, and were selected for varying complexity of
commingling and potential for positive re-association.
The commingled burials of Lots 10097/10137 and Lots 10707/10881represent the
simpler commingled burials recovered from the site. These burials involved the interment
of one or two individuals whose skeletons remained discrete enough in situ to be quickly
and confidently re-associated in the field. These field associations were confirmed by
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laboratory analysis. While separated by analysts into individual skeletons, the remains
shared a diagenic environment that may have tended to obscure individual elemental
signatures. Lots 10097/10137 include a middle adult male west-east interment clearly
delineated from a second middle adult male east-west interment. Lots 10707 and 10881
consist of a young adult probable male extended interment pushed to the side of the
coffin to make space for a disarticulated juvenile adolescent and other miscellaneous
commingled bone.
The commingled burials of Lots 10342/10429/11021, Lots 10525/11052, and Lots
10669/11042/11043 represent the more complex commingled burials present at
the site (Fig. 8). These burials involved the interment of multiple individuals as
well as disassociated disarticulated bone and medical waste. Laboratory analysis
of the commingled lots separated some articulated bones into analytical units
termed Element Sets (ES). Lots 10342, 10429, and 11021 include two disparate
middle adult male torsos and two right leg inclusions. Lot 10525 consists of
disarticulated skeletal sections that could be matched to two included rib cages.
Lots 11042, and 11043 represent a middle adult male and juvenile adolescent
heavily mixed with commingled bone and medical waste and disturbed by
installation of a water pipe.
Of particular interest are two sets of subadult remains included in the commingled
lots chosen for analysis. Lot 10881 contains a full adolescent skeleton with missing
cranium and arms, while Lot 11042 contains a compatibly aged set of adolescent arms. If
these lots can be positively re-associated, it would indicate that bones from one individual
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were interred in multiple locations throughout the site. This would have significant
impact on future research with the commingled remains from the 2013 excavation.
Methods
Bones evaluated in the study include cranial, scapula, humerus, ulna, innominate,
femur, and tibia elements. Prior to analysis each bone was cleaned with water and airdried. This was followed by a second cleaning with 100% denatured alcohol. Each bone
was scanned at three sites for 180 seconds apiece and results averaged to produce the
final data set. Readings were analyzed as raw net intensity values and were not calibrated
to Bruker’s mudstone standard. Thus, results do not represent elemental concentrations in
parts per million. Net intensity values were averaged and subjected to principal
components analysis using the software package XLSTAT.
Results
The analyzer returned useful values on 14 elements including arsenic, calcium, copper,
iron, magnesium, manganese, nickel, phosphorus, rubidium, tin, strontium, zinc, and
zirconium. This resulted in a data set of 485 readings. Following removal of missing data,
reading sets were averaged to produce a final data set of 71 readings.
Control Lots
The two MCIG control lots appear to separate from one another (Fig.9). Lot 293 clusters
more closely but this may be an artifact of a greater number of data points. When the
reference skeleton is added to the analysis, there is a clear separation between the
reference skeleton data points and the MCIG samples (Fig 10). However, the MCIG
samples no longer separate out from one another but instead form two mixed clusters.
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This suggests that the influence of diagenesis increases as the number of data sets
increases.
Complete Data Set
When all MCIG samples are included in the analysis the only data set easily
distinguished from the main point cloud is the reference skeleton (Fig 11). Note that Lot
10293 continues to form a discrete set of data points although these are not clearly
separated from adjacent data points.
Lot 10881 and Lot 11042
The two subadult skeletal assemblages (Fig. 12) do not appear to group together although
they do form relatively discrete clusters. When evaluated against the entire data set (Fig.
13), including Lot 10707, recovered from the same coffin that harbored Lot 10881,
overlap decreases markedly suggesting that these remains represent two individuals.
Commingled Lots
Data points representing commingled Lots 10525 and 11052 (Fig. 14) are widely
distributed and do not appear to sort out from one another or from the main point cloud.
This appears to be true also for commingled Lots 10666, 11042, and 11043. Lots 10342,
10429, and 11021 cluster tightly and exhibit considerable overlap making confident reassociation difficult.
Elemental Ratios
Ratios calculated for this study included Sr/Ca and Zn/Fe. Lead was in low frequency in
all samples so a Pb/Ca ration was not calculated. When PCA scores for Lot 10293, Lot
10737, and the reference skeleton are plotted (Fig. 17), all three samples do seem to sort.
Curiously, the reference skeleton is less tightly clustered by this procedure. A biplot of
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the averaged PCA scores for the same ratios continues to distinguish the reference
skeleton as well as Lot 11052 but the remaining lots are more closely clustered (Fig. 18).
Conclusions
The very preliminary assessment presented here suggests that it is possible to distinguish
individual bone sets in the MCIG sample using pXRF. As in previous studies, intraskeletal variation does not appear to be a significant factor. Not surprisingly, the clearest
distinction is between the reference skeleton used as a control and the MCIG burials. This
is likely due to a lack of diagenetic effects on the control skeleton and/or its different
recovery context. This highlights the difficulty of deriving confident re-associations of
commingled remains in diagenetically complex environments. We are guardedly
optimistic that it may be possible to sort commingled remains in the MCIG sample by a
combination of pXRF and careful archaeological and osteological analysis. However, our
pilot study has taught us several important lessons. First, if low energy elements are
targeted under vacuum, the need to eliminate or minimize any air gap between the
analyzer lens and the surface of the analyzed bone may preclude analysis of small or
irregularly shaped elements thus reducing the potential pool of possible re-associations.
Second, confident re-associations using this method will require collection of a much
larger set of elemental readings per bone per commingled assemblage. Third, any future
study of the MCIG human remains must also target a more comprehensive suite of
elements with results calibrated to a specific reference standard. Finally, at a minimum of
30 minutes per analyzed bone, collection of a truly representative data set may take a
very long time indeed.
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Fig#
Slide#
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Analyzing contaminated soils on site
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Contaminated soil spectrogram
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MCIG soils compared
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Vulcanite dentures
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Skeletal assemblages analyzed in study
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Lots 10293 and 10737 in situ
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Lots 10097 and 10137; 10707 and 10881 in situ
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Lots 10669/11042/11043; Lots 10525/11052; and Lots
10342/10429/11021 in situ
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PCA of 10293 and 10737 control samples
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PCA of all control samples
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PCA of all data points
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PCA of Lots 10881 and 11042
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PCA of Lots 10881, 11042, and 10707
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PCA of Lots 10525 and 110515
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PCA of Lots 10666, 11042, and 11043
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PCA of Lots 10342, 10429, and 11021
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PCA of elemental ratios Sr/Ca and Zn/FE for control lots
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PCA of averaged elemental ratios Sr/Ca and Zn/FE for all lots
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