1. Journal of Archaeological Science 1983, 10, 385-395
A Spectroscopic Study of the Nimrud Ivories
G. V. Robins,” C. de1Re,a*N. J. Seeley,”
A. G. Davi? and J. A.-A. Hawar?
Electron spin resonance and infrared spectroscopic techniques have been used to
examine the thermal and environmental deterioration of ivory from Nimrud, Iraq.
It has been possible to ascertain the extent of ancient pyrolysis of the ivory with the
former technique, and infrared spectroscopy hasallowed an insight into the deterio-
ration of the protein matrix of the ivory both in relation to, and independent from,
thermal breakdown. Comparative spectroscopic studies of both ancient and modern
ivory are also reported. The structural information obtained is discussedin the light
of previously reported studies based upon chemical analyses.
Keywords: IVORY, BONE, IRAQ, NIMRUD, ELECTRON SPIN RESONANCE,
INFRARED, SPECTROSCOPY, HEAT, PYROLYSIS.
Introduction
Ivory artifacts are often associated with ancient sites and as such they are extensively
documented in the art historical literature (for example, Mallowan, 1978). Scientific
studies of ancient ivory artifacts are less common, however, and those reported concen-
trate upon gross compositional analyses as an aid to assessing deterioration with the aim
of determining their age in relation to environmental history, and thence to establish
suitable conservation strategies (Baer et al., 1971, 1978; Baer & Indictor, 1974; Taylor,
1955). A complicating factor in assessing the deterioration of ivory is its thermal history:
any ancient heating or burning can both influence and determine deterioration mechan-
isms apart from producing a range of colours in the ancient material which may compete
with discolouration induced by non-thermal deterioration. In assessing the deterioration
of ancient ivory artifacts it is necessary, therefore, to establish whether or not the ivory
had been burnt, and we have developed criteria to measure the extent of pyrolysis with
the use of electron spin resonance (ESR) spectroscopy and linked this to changes in the
infrared (IR) spectra of ancient and modern ivories when subjected to pyrolysis.
The availability of a large quantity of uncatalogued miscellaneous fragments of
variously coloured ivory from Nimrud, together with our interest in the study of burnt
archaeological materials, prompted us to undertake this spectroscopic study and compare
the Nimrud ivories with modern pyrolysed ivory.
%stitute of Archaeology, University of London, 31-34 Gordon Square,
London WCIH OPY, England.
*Department of Chemistry, University College London, 20 Gordon Street,
London WClH OAJ, England.
*Present address: Conservation Department, University Museum, 33rd and
Spruce Streets, Philadelphia PA19104, U.S.A.
385
03054403/83/040385 + I I $03.00/O (Q 1983 Academic Press Inc. (London) Limited

2. 386 G. V. ROBINS ET AL.
The Composition and Deterioration of Ivory
The term “ivory” is strictly applicable to the large upper incisors of the elephant, but it
is also used more widely to describe any large mammalian teeth (Miles & Boyde, 1961).
The physiological structure of ivory is described extensively in the literature (Miles &
Bourne, 1967) although it may be pointed out that each type of ivory has characteristic
growth and development patterns which permit identification even when deterioration
is far advanced.
Ivory may be thought of as interpenetrating organic and inorganic matrices, and the
latter accounts for some 55% of the dry weight of fresh ivory and consists essentially of
a hydroxyphosphate lattice with interstitial calcium and magnesium ions. Small amounts
of other anions such as carbonate and fluoride are also present in deteriorated ivory and
bone and their presence has been utilized in dating and other studies. (Baer et al. 1978).
The organic matrix is largely proteinaceous, consisting of collagen with smaller traces
of mucopolysaccharides and elastin, and overall in fresh ivory the water content averages
10%. There have been a number of morphological studies of ivory and we refer the
reader at this point to some pertinent references in the literature (for example, Lazzari,
1976).
Deterioration of archaeological and historical ivory occurs mainly from the loss of
collagen by hydrolysis in damp environments although in acidic soils decomposition of
the inorganic matrix can occur in conjunction with the absorption of soluble salts.
Removal of the collagen eventually leads to the development of a crumbly chalk-like
consistency, a condition exacerbated by acidic attack, and such deterioration presents
many problems of handling and treatment in the conservation of heavily deteriorated
artifacts.
The Nimrud ivories exhibit most of these compositional changes but added to these
are the effects of burning when the citadel was fired by the Medes in 612 BC.
Sample Preparation
From the fragments available for study six samples were selected from among the com-
monest colours: two each of cream, black and blue-grey. Five further small samples of
the cream coloured ivory were selected at the same time and these were subjected to con-
trolled pyrolysis at 100 “C increments between 300 “C and 700 “C using the conditions
described below. Finally a sample of modern elephant ivory was cut into small pieces and
these were subjected to pyrolysis at 100 “C increments between 100 “C and 1000 “C. The
ancient and modern comparative samples were heated in silica crucibles in a limited supply
of air in a pre-heated Carbolite furnace for 30 minutes total heating time, at the various
selected temperatures (each individual sample, irrespective of previous thermal history,
being heated directly from ambient to its one required temperature). Our choice of heating
regime was determined by our previous experience with flint (Robins et al., 1978) where
heating times of this length were sufficient to induce thermal changes that were being
studied. Preliminary examination of bone and ivory samples showed that similar regimes
could be used to study these materials with ESR spectroscopy (Griffiths et al., 1981).
All samples were lightly crushed after cooling to be in the optimum form for spectro-
scopic examination. Sample details, including colour, are shown in Table 1.
Ashing experiments were made on a variety of archaeological, historical and modern
ivory and the procedure employed involved heating a weighed sample in a silica crucible
placed in a Carbolite furnace preset to 1000 “C. The samples were heated for one hour,
cooled in a desiccator and then weighed. This procedure was repeated (normally four
times) until constant weight of the residue was obtained and the ash weight then calcu-
lated.

3. STUDY OF NIMRUD IVORIES
Table 1. Description of samples of ancient and modern ivory.
387
Sample and heating
temperature (“C)
Code Colour
Group A Nimrud ivory fragments
as supplied
Group B Nimrud ivory fragments
heated
300
400
500
600
700
Group C Modern ivory fragments
Unheated
Heated
loo
200
2
500
600
700
800
1000
NI/4]
NJ/61
NI/2
NI/5
NI/l 1
NI/3J
black
blue-grey
BNI/
BNI/I
BNI/II
BNI/III
BNI/IV
BNI/V
cream-brown
brown
brown-black
black
blue-grey
MI(a) white
MI(b) white
MI(c) white-cream
MI(d) cream-brown
MI(e) brown
MI(f) brown-black
MI@ black
MI(h) blue-grey
MI(i) blue-grey
MI(j) white
Table 2. Ash analyses on ancient and modern ivories
Sample description Colour Percentage ash
1. Modern elephant ivory white
(unburnt) white
2. 17th century ivory-
City of London (unburnt)
brown
3. Nimrud, Iraq
(burnt)
white 82.20, 87.16,
white 77.89, 82.53
brown 81.10
black 77.83, 88.47, 90.61
black/blue 86.78
4. Acem Hiiyiik, Anatolia black/grey 78.43*
(burnt) blue/grey 9260*
5. Hasanlu, Syria grey 87.15, 87.55*
(burnt) black 87.48. 77.69*
6. Fossilized unburnt ivory
mammoth (Siberia)
mastodon (N. America)
not stated 54.24*
not stated 56.04*
52.35, 52.78
53.32, 55.13*
58.82
* Baer & Indictor (1974). Other results-this work.

4. 388 G. V. ROBINS ET AL.
White Yellow Brown Black Blue Grey White
0 200 400 600 BOO 1000
Temperature PC)
Figure 1. Comparative data for ash content and colour of ancient and modern
ivories (Baer et al., 1971).
Ash Analyses
Baer et al. (1971, 1978) and Baer & Indictor (1974) have reported an extensive series of
ashing experiments, coupled with elemental analyses, on various samples of ancient and
modern ivory, and we show these data for ancient ivory, in comparison with our own
analyses of ancient and modern ivories in Table 2. An increasing ash residue is seen as a
measure of progressive deterioration as the organic matrix is removed, although this
analysis will not distinguish between thermal and other modes of decay: supporting
evidence for thermal decay is adduced from the colour of the ivory sample.
The colour/ash residue relationship demonstrated by Baer et a/. (197 1, 1978) and Baer
& Indictor (1974) for modern pyrolysed ivory is illustrated graphically in Figure 1 where
comparative data largely drawn from Table 2 in respect of ancient ivory are overlaid
against the colour/ash residue axes. It seems that the relationship between these two
properties of ancient ivory is at best a complex one, and at worst indefinable. Thus, the
white Nimrud ivories analysed both by Baer and colleagues and ourselves show a wide
range of ash values which actually fall within the overall range for the black Nimrud
samples. Similarly, the Acem Hiiyiik and Hasanlu ivories show a wide ash range within
a limited colour range of black to blue-grey. Interestingly, the brown City of London
ivories show an ash content close to that of modern elephant ivory which suggests that
little deterioration has occurred since its burial in wet acid soil several centuries ago.
The superimposed data in Figure 1 highlight the difficulty of drawing any real con-
clusions from colour and external appearance since the relationship between deteriora-
tion and composition is influenced by a multitude of factors of which burial conditions
may not necessarily be a major one. We may cite here the analyses of fossil mammoth
and mastodon bone shown in Table 2 where ash contents close to that of modern ivory
are indicated, with the implication of a matching organic content: long environmental
exposure and fossilization do not seem to have brought about compositional changes
detectable by simple ash analyses,

5. STUDY OF NIMRUD IVORIES 389
Spectroscopic Investigations
A number of spectroscopic techniques have been used in archaeological investigations
and while some of these applications are destructive of the sample, others are essentially
non-destructive and the sample is, in principle, recoverable. ESR and IR spectroscopy
both come into this latter category, and whilst the information reported in this paper is
essentially qualitative, we are developing this actively into a quantitative form which will
enhance the usefulness of these analytical methods.
Electron spin resonance spectroscopy
Electron Spin Resonance is a relatively new addition to the range of analytical techniques
available to the archaeologist. Its use has been reported in dating cave deposits by
quantitative measurement of trapped electron populations accumulated through archaeo-
logical time (Ikeya, 1978) and more recently in the dating of bone. (Griffiths et al, 1981).
The principles employed in ESR dating have much in common with the related and
complementary technique of thermoluminescence (TL) dating, although ESR does offer
some distinct and practical advantages (Ikeya, 1978).
We have already reported (Robins et al., 1978) the extension of this spectroscopic
technique to the identification of flints subjected to ancient heating-either deliberately
through heat treatment of preforms or accidentally through their incorporation into a
hearth site-through the detection of radical carbon produced during the pyrolysis of
traces of organic matter occluded in the flint at diagenesis. This organic matter, on pyro-
lysis in the archaeological heating process, gives rise to radical carbon which is readily
detectable with ESR as a strong, sharp signal shown in Figure 2 where spectral para-
meters are also given. We have found that heating of flint produces this signal when
temperatures above 350 “C are reached and we have since examined a wide range of
organic materials to show the generality of this effect: some of these studies will be
reported shortly and this study of ivory constitutes the first application of this discovery
of the radical carbon heat marker to an archaeological problem involving thermal history.
Figure 2. The ESR spectrum of radical carbon (g=2.0030, AHms=3.5 G).

6. 390 G. V. ROBINS ET AL.
The diagnostic value of the radical carbon ESR signal as a thermal marker is enhanced
by its almost infinite stability (Robins et al., 1978) and by the observation that the in-
tensity of the signal is related to the level of heating, a point discussed below.
Electron Spin Resonance spectra of the samples shown in Table 1 were measured on
a Varian E4 spectrometer at ambient temperatures and compared quantitatively for
signal strength by measuring signal areas produced by known amounts of powdered
samples mounted in capillary tubes and placed in the spectrometer cavity. Sample weights
in the region 8-l 1 mg were found to be optimum for the experiment.
It was apparent that radical carbon signals were present in the modern samples that
had been heated above 200 “C and that above this temperature the signal strength grew
rapidly, reaching a maximum c. 550 “C after which it declined although it was still
detectable above 800 “C, a temperature at which it is difficult to identify the presence of
residual organic matter by gross chemical means. The results of these measurements are
shown cumulatively in Figure 3 where colours for modern ivory are also given against
Brown Black Blue Grey White
Temperature PC)
Figure 3. The development of radical carbon signal intensity in pyrolysed
ivory.
the corresponding temperature. The generation temperature for the radical carbon signal
in ivory, c. 200 “C, is considerably lower than that for flint although both of the sample
types reach maximum intensity of signal c. 550 “C. It may be inferred that the similarity
in maxima reflects the identity of the radical carbon species formed through both routes
and that the difference in generation temperatures is a consequence of the differing
chemical species undergoing pyrolysis as the heating proceeds. We are now exploring
other parameters of these signals to increase the amount of information that they will
yield, but it is clear, even at this preliminary stage, that radical carbon has considerable
utility as a marker of ancient heating or burning.
The signal development in the two series of Nimrud ivories parallels that of the modern
ivory shown in Figure 3, and we noted with the modern ivory that maximum signal
intensity is associated with the colour ascribed to c. 550 “C and seemsto be independent
of heating regimes employed. The weak signal in the cream coloured ivory is indicative
of pyrolysis c. 200 “C which is consistent with the archaeological evidence that very little
of the ivory in the citadel completely escaped burning. The development of the radical
carbon signal in the cream coloured ivory on subsequent pyrolysis in the laboratory

7. STUDY OF NIMRUD IVORIES 391
closely paralleled that in modern ivory and suggested that the growth of the signal on
subsequent heating can be used to establish a measure of the temperature of pyrolysis:
thus, signal growth on heating to 550 “C in the laboratory will indicate a temperature on
the descending curve. In this way it is possible to suggest that palaeotemperatures might
eventually be measured with increasing precision once our study of the signal has pro-
gressed into more quantitative measurements.
Even in the absence of such quantification, however, the detection of the radical carbon
signal with ESR is clearly important in that it can identify ancient heating unambiguously
without recourse to semi-subjective colour and texture evaluation.
As part of the present study we examined Pleistocene mammoth and mastodon ivory
and samples of unheated bone gathered from palaeolithic sites in South Western France.
In no case was the radical carbon signal observed in the unheated sample but was readily
generated by pyrolysis above 200 “C and once generated it followed the same profile of
development shown in Figure 3. Similarly, we examined a sample of ivory from the
associated site of Fort Shalmaneser which had escaped the main conflagration which
had destroyed Nimrud. We found no ESR signal in the original sample but we were able
to generate this on pyrolysis above 200 “C.
Infrared spectroscopy
Infrared spectroscopy has been used routinely in chemical analysis for many years but
its application to archaeology has not been extensive apart from a series of studies of
amber. Baer et al. (197 1) reported a limited investigation of the temperature dependence
of the proteinaceous carbonyl stretching frequency in modern ivory, and we now report
an extension of this study to the amide N-H stretching frequency centred on 3500 cm-l
which appears to be sensitive to pyrolysis. We also observed that the anionic P-O
stretching frequency centred on 1030 cm-l also showed a temperature dependency and
this band in our sample suites was accordingly investigated.
For this study all samples of the powdered ivory were prepared as Nujol mulls and
run between sodium chloride plates in the region 4000-625 cm-l on a Perkin-Elmer
580A/B spectrophotometer. The temperature dependence of the N-H and P-O bands
bands are shown in Figures 4 and 5, respectively, where sample spectra are grouped
according to these bands.
The amide N-H stretching frequency. Proteins, which consist of polypeptide chains,
show amide absorption characteristic of strong hydrogen bonding. In simple primary
amines two such frequencies are observed centred at 3360 and 3180 cm-l, assignable to
asymmetric and symmetric stretching modes (Griffiths et al., 1981), respectively, and
these bands are identifiable in long chain polypeptides although the exact stretching
frequency is a function of crystal packing and chain folding, both features which con-
siderably influence hydrogen bonding.
As shown in Figure 4, the 3360 and 3180 cm-l bands are observed in all samples
pyrolysed below 800 “C indicating that amide residues remained. The relative intensities
of these bands, however, show some marked differences over the range of temperature
studied. Thus, in the modern ivory samples the 3180 cm-’ band is more intense at 300 “C
and above 700 “C whereas at intermediate temperatures the bands have approximately
equal intensities. However, the cream and black Nimrud samples have a greater intensity
in the 3360 cm-l band wheras the grey sample shows a reversal of this intensity with
respect to the 3180 cm-’ band.
This behaviour is paralleled in the Nimrud samples heated in the laboratory, as shown
in Figure 4, and together they highlight the importance of the 500-600 “C region in the

8. 392 G. V. ROBINS ET AL.
Block
b)
Unburnt
(Fort Shalmaneser
c)
4ocil 3500 3000 4000 3500
Frequency (cm-‘)
3003 4000 3500 3CCo
Figure 4. The thermal dependence of the amide N-H infrared stretching
frequency in ancient and modern ivory. Spectral groups as designated in
Figure 5.
deterioration of ivory which was observed in the previous section in the discussion of the
ESR data.
Nevertheless, the information obtainable from the comparison of intensities is limited
and the examination of the higher frequency absorptions is much more fruitful. Most of
the samples pyrolysed above 400 “C show an absorption envelope with a maximum above
3360 cm-’ and the feature observed c. 3530 cm-’ is assignable to non-hydrogen bonded
N-H absorption, which is not normally anticipated in polypeptide structures. The pro-
duction of this band upon pyrolysis is seen as a consequence of thermal deterioration of
the polypeptide matrix: it is present in all three sample suites, being strongest in the
laboratory-heated ancient samples above the pyrolysis temperature of 500 “C and be-
coming the most intense peak in this spectrum at 700 “C. This strong absorption feature
contrasts sharply with the minimal corresponding feature in the other sample suites and
we suggest that its presence in the laboratory heated samples is a consequence of the
pyrolysis of the organic matrix which has already undergone deterioration in the ground :
when thermal deterioration precedes later chemical breakdown, as in the coloured
samples, free N-H is not produced in such quantity, and in the modern ivory the
similar thermal profile uncovered also supports this contention. Such a rationale would
support the suggestion that free N-H would be short-lived in such a context and would
not be expected to be present as an indicator of pyrolysis.
It is noteworthy that in the laboratory-heated ancient samples the thermal develop-
ment of the free N-H feature corresponds with the development of the radical carbon

9. STUDY OF NIMRUD IVORIES 393
1200 1000 E
b)
lxx, 1000 800
Frequency (cm-’ 1
1200 1000 E 0
Figure 5. The thermal dependence of the P-O infrared stretching frequency
in ancient and modern ivory. (a) Nimrud coloured ivory fragments as supplied
[NI series (Table l)]; (b) nimrud ivory fragments heated in the laboratory
(BNI series); (c) modern ivory samples heated in the laboratory (MI series).
profile observed with ESR in terms of signal intensity and we suggest that both para-
meters reflect the final breakdown point of the organic matrix. Another feature that
emerges in this temperature range is the weak but sharp absorption centred at 3680 cm-l
observed in the laboratory-heated ancient samples and the modern ivory: this wavelength
is above those normally associated with free N-H but does correspond with the range
observed for free O-H. Again, it may be noted that the ancient pyrolysed samples do
not show this band in their spectra.
Clearly, this preliminary survey of amide N-H band indicates that it possesses some
diagnostic potential in any investigation of deterioration of ivory, and we suggest that
this be further explored in the examination of other sample suites involving ancient
pyrolysed and non-pyrolysed ivory to see whether it is of utility in determining between
thermal and non-thermal deterioration modes.
The P-O stretching frequency. Inorganic ions, measured as solid powders or Nujol
mulls, often give broad and diffuse IR spectra unless the particle size is sufficiently small.
This is particularly true when either polymeric ions or ionic mixtures are present and from
these observations it is anticipated that the diagnostic potential of the inorganic absorp-
tion band in ivory would be limited. Bellamy (1975) cites studies of orthophosphate
series which all absorbed strongly between 1030 cm-’ and 1000 cm-’ whereas basic
phosphates were observed to absorb at a somewhat higher frequency, although the small

10. 394 G. V. ROBINS ET AL.
differences in absorption maxima, coupled with the diffuseness of the absorption band,
made this distinction of little diagnostic value.
The spectra for the three sample suites in the 100 cm-l region are shown in Figure 5,
and the thermal dependence of the P-O band is strikingly apparent. Below 500 “C all
suites show a broad diffuse band centred at 1030 cm-’ but above that temperature there
is a marked change in the band profile: the band becomes sharp and strong, developing
sharp sidebands at 1120 and 980 cm-’ whilst retaining the now central 1030 cm-’ feature.
This development argues for a change in the inorganic matrix in the 500 “C region in all
three sample series which is of fundamental structural importance. Again, it may be noted
that a compositional change occurs in the 500-600 “C region, and this P-O observation,
together with the previous N-H data and the ESR profiles suggest that the two tech-
niques may be used in conjunction to give considerable information about the previous
history, in terms of both thermal and non-thermal deterioration, of ivory and associated
materials, particularly where deterioration is such that total organic contents amenable
to gross analysis are small.
Conclusions
Our spectroscopic studies of the Nimrud and related ancient and modern ivories have
shown that previous pyrolysis can be identified by the detection of a radical carbon ESR
signal and that laboratory heating of ancient samples to examine the subsequent be-
haviour of this signal can give an indication of the extent of previous heating. Structural
changes in both inorganic and organic matrices of ivory can be identified with IR spectro-
scopy and both techniques point to 500-600 “C as being the critical range for the thermal
deterioration of ivory.
Acknowledgements
We wish to thank the following for the provision of samples: Professor D. Oates and
Lady Mallowan of the Institute of Archaeology, University of London for allowing us
accessto the Nimrud samples, which are the property of the British School in Iraq; the
British Museum (Natural History) for samples of mammoth, mastodon and modern
ivory; Dr P. Armitage of the Department of Urban Archaeology, Museum of London,
for the sample of 16-17th Century ivory; one of us (G.V.R.) would like to acknowledge
the efforts of R. R. Larick, J.-M. Geneste and S. Maury of Direction des Antiquites Prt-
historique, Bordeaux, France for their help in securing the bone fragments from French
sites. We are grateful to C. J. Barker of Metal Box Research and Development Labora-
tories, Wantage, Oxon., who helped us to run infrared spectra of our samples on a
Perkin-Elmer 580/AB spectrophotometer. One of us (C.d.R.) would like to thank the
Samuel H. Kress Foundation, New York, U.S.A. for financial support and another
(G.V.R.) acknowledges the support of SERC and Ealing College of Higher Education,
London, during this work.
Part of this work was carried out by C.d.R. in partial fulfilment of the requirements
of the B.Sc. of London University.
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