Pre-hispanic goldwork technology. The Quimbaya Treasure, Colombia

Abstract

One of the most important pre-Columbian gold assemblages made up of more than one hundred objects from two tombs in the Cauca river valley, Colombia, was studied to obtain archaeometric information. Although several attempts have been made to arrange gold production in time and space from the stylistic point of view, no firm conclusions were possible due to the lack of archaeological contexts. This paper presents first results of a new, fully instrumental approach, in which different analytical techniques, including OM, SEM-EDS, XRF, PIXE, RBS, AMS and X-ray imaging, were applied in order to determine a technological, metallurgical and chronological frame of the so-called Quimbaya Treasure.

Full text

Pre-hispanic goldwork technology. The Quimbaya Treasure, Colombia
A. Perea
a
,
*
, P.C. Gutiérrez-Neira
a
, A. Climent-Font
b
,
c
, P. Fernández-Esquivel
d
, S. Rovira-Llorens
e
,
J.L. Ruvalcaba-Sil
f
, A. Verde
g
, A. Zucchiatti
b
a
Centro de Ciencias Humanas y Sociales, CSIC, Albasanz 26-28, 28037 Madrid, Spain
b
Centro de Microanálisis de Materiales, UAM, Faraday 3, 28049 Madrid, Spain
c
Dpto. Física Aplicada C-12, UAM, 28049 Madrid, Spain
d
Fundación Museos Banco Central de Costa Rica, San José, Costa Rica
e
Museo Arqueológico Nacional, Serrano 13, 28006 Madrid, Spain
f
Instituto de Física, UNAM, Mexico DF, Mexico
g
Museo de América, Avda. de los Reyes Católicos 6, 28040 Madrid, Spain
article info
Article history:
Received 13 July 2012
Received in revised form
19 December 2012
Accepted 28 December 2012
Keywords:
Gold
Archaeometry
Colombia
Archaeometallurgy
Technology
abstract
One of the most important pre-Columbian gold assemblages made up of more than one hundred objects
from two tombs in the Cauca river valley, Colombia, was studied to obtain archaeometric information.
Although several attempts have been made to arrange gold production in time and space from the
stylistic point of view, no firm conclusions were possible due to the lack of archaeological contexts.
This paper presents first results of a new, fully instrumental approach, in which different analytical
techniques, including OM, SEM-EDS, XRF, PIXE, RBS, AMS and X-ray imaging, were applied in order to
determine a technological, metallurgical and chronological frame of the so-called Quimbaya Treasure.
Ó2013 Elsevier Ltd. All rights reserved.
The Quimbaya Treasure is the most important pre-Columbian
gold assemblage kept since 1941 at the Museo de América in
Madrid. We have accomplished an interdisciplinary study of these
135 (museum inventory numbers) gold objects with a twofold
purpose, firstly the technological and metallurgical characterization
of the assemblage, and furthermore an attempt for dating it. Due to
the restricted extension of this paper we present an overview. A
detailed archaeometric study will be published in the near future.
This well-known treasure has embodied sensitive social con-
cerns, appropriating ideological meanings that were never at its
origin. For this reason we prefer to introduce our work within the
frame of current Colombian archaeology.
1. Background
We can distinguish three main issues that have conditioned
Colombian archaeology, the first one stems from the construction
of the Colombian state after the independence from Spanish rule in
1819, and it is related to the search for a national identity. The need
to integrate the different social groups that made up the new
society was soon made evident. These groups, created by Spanish
colonial segregation, included the indigenous people. The identi-
tarian discourse arose from anthropology with the aim of solving
a dichotomy: on the one hand to redeem native communities and
convert them to civilization, and on the other hand to rationalize
pre-hispanic society as the cornerstone of national identity. As
Gnecco (2008) states archaeology was built upon anthropological
premises and contributed only to perpetuate internal colonialism.
The second issue concerning research in past societies refers to
the theoretical premises at its base. During the first half of the XX
century foreign researchers undertook major archaeological and
anthropological enterprises, not only in Colombia but all over Latin
America. Meanwhile, Colombian archaeologists went abroad to be
prepared as scientists. For example the National Ethnological Insti-
tute was founded by the French anthropologist Paul Rivet in 1941;
the Spaniard José Pérez de Barradas and the Austrian Gerardo
Reichel-Dolmatoff laid out the bases for pre-Columbian goldworking
chronology and interpretation. In this case, while anthropology
overcame the old fashioned nationalistic discourse, vindicating the
indigenous legacy, archaeology “kept strengthening nationalism by
incorporating native societies into a common history”(Gnecco,
2008: 1108). In the opinion of G. G. Politis (2003) the culture-
historical paradigm for the reconstruction of the past remains
*Corresponding author. Tel.: þ34 91 602 24 88.
E-mail address: alicia.perea@cchs.csic.es (A. Perea).
Contents lists available at SciVerse ScienceDirect
Journal of Archaeological Science
journal homepage: http://www.elsevier.com/locate/jas
0305-4403/$ esee front matter Ó2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jas.2012.12.033
Journal of Archaeological Science 40 (2013) 2326e2334

strong in Colombia. Present-day researchers have adopted modern
scientific techniques and discourse, but in general, ethnographic
analogy and current extrapolation are long standing traditions used
to explain the archaeological record. One of the reasons of this state
of affairs is the primaryconcern for accumulating descriptive data of
the archaeologicalrecord due to lack of excavations and the existence
of poorly known vast regions (Politis, 2003: 130).
The systematic looting of archaeological sites in search for val-
uable objects is a common activity known as guaquerismo (or
huaquerismo) and widespread throughout Latin America. Gua-
querism is not only a normal activity, but a way of earning a living
(Gamboa Hinestrosa, 2002). The result is large archaeological col-
lections, mainly of gold but also pottery and textiles, without
a context and even without a place of origin. This practice already
occurred during the 16th century, when Royal documents attest
that the Spanish conquistadors plundered many sites searching for
gold in the Cauca river valley. The second large plundering wave in
this same region occurred during the second half of the 19th cen-
tury when this activity became one of the factors for capital accu-
mulation (Valencia Llano, 1989). Since 1826 the mining activity of
the Colombian Mining Society was another factor behind the in-
direct spoiling of archaeological sites. The Museo del Oro in Bogotá,
founded in 1939, has played an important role in recovering pre-
Columbian gold objects from the Antiquaries market, collecting
some 34,000 gold items at present.
The absence of archaeological contexts has prevented the
construction of a safe chronological frame where to place the
archaeological record. As a result archaeology turned firstly to
stylistic analysis, and secondly to ethnology in search for symbolic
and functional explanations.
As far as analytical data is concerned, there is only a small
amount of studies about pre-Columbian gold-work except for some
generalities regarding the CueAueAg alloy known as tumbaga
(Ruvalcaba Sil and Demortier, 1997).
In the last decade field archaeology programmes have been
developed, building up regional chronologies with an emphasis on
social change and organization. For example the Valle de la Plata
project (Drennan, 2000, 2008) or more recently the Proyecto
Arqueológico Tierradentro, funded by the Instituto Colombiano de
Antropología e Historia ICANH (Langebaeck and Dever, 2009). Still,
the wheels of field-archaeology/pottery based chronologies and
stylistic/gold periodizations are not definitely assembled.
2. Materials & methods
2.1. The Quimbaya Treasure
The so-called Quimbaya Treasure was looted in 1890 from two
tombs in the site of La Soledad, near the Municipality of Filandia
(Quindío Department, Colombia), amidst the Central Cauca Valley.
They say it was made up of more than 20 0 gold objects, but only 123
objects were acquired in 1891 by the President of the Republic,
Carlos Holguín, with three purposes in mind. Firstly, to present the
treasure at the 1892 exhibition in Madrid commemorating the 4th
Centennial of the discovery of America. Secondly, to display the
treasure at the International Exhibition of Chicago dedicated to
Columbus. And finally to give it as a present to the Regent Queen of
Spain, Doña María Cristina de Absburgo Lorena, in appreciation for
her mediation in a frontier conflict with Venezuela. The treasure was
kept at the National Archaeological Museum in Madrid, until the
opening of the Museo de América in 1965 where it continues to be in
permanent exhibition (Plazas, 1978;Cuesta Domingo and Rovira
Llorens, 1982;Rovira Llorens, 1992;Gamboa Hinestrosa, 2002).
Ernesto Restrepo (1892a,1892b,1929) was the first to publish this
assemblage associating it with the Quimbaya ethnic group that the
Spanish chronicles mentionedwhen describing the regionin the 16th
century. From then on all the goldfindings in the area were attributed
to the historic Quimbaya group. Not until the middle of the 20th
century was a more elaborate classification for pre-Columbian gold
production. José Pérez de Barradas (de Carrera Hontana and Martín
Flores, 2008), had worked in Colombia between 1936 and 1938 in
the archaeological area of San Agustín and Tierradentro. In 1946 he
was charged by Luís Ángel Arango, manager of the Banco de la
República, with the classification of the gold collection at the Museo
del Oro which had 7000 gold objects at the moment. His method was
based on the concept of style as defined by Meyer Schapiro who put
an emphasis in its communicative function. Without archaeological
contexts he warned about the real implications of this classification,
stating its use only as a spatial manifestation of recurrent icono-
graphic features. He was very conscious of the feeble connection
between the names of the historical people describedin the Spanish
chronicles and the archaeological people who really produced that
goldwork. With all these drawbacks in mind he defined eight stylistic
groups: Calima, Quimbaya, Darién, Sinú, Tairona, Muisca, Tolima and
Invasionist (Aceituno, 2008). His classification lacked chronological
references and archaeological connections, resulting in long periods
which spanned for over a millennium.
The groups stated by Pérez de Barradas are still in use, although
their importance and meaning have slightly changed in favour of
a more general division for metal production in two metallurgical
provinces (Plazas and Falchetti, 1986), the North province and the
Southwest.
In the meantime Gerardo Reichel-Dolmatoff carried out his
researchwith the aim of establishing an evolutionarysequence of the
human occupation in the country. In his book Colombia (Reichel-
Dolmatoff, 1965) description was left out in favour of social change
processes from a diffusionist point of view, developing important
concepts like the Intermediate Area that covers from Centroamérica
to the central Andean region or the idea of “tradition”or “horizon”.
He played an important rolein the academic life of the country, and
he was the first professor in charge of a Department of Anthropology
at the Universidad de los Andes in 1963. He began to be more
interested in ideology and ethnography due to his reading of Lévy-
Strauss withan idea in his mind: the past could be explainedthrough
contemporary native communities (Langebaeck, 2005). One of his
most influential works refers to the interpretation of pre-hispanic
goldwork in terms of shamanism (Reichel-Dolmatoff, 1988).
Research in pre-Columbian gold at the inception of the 21st
century entails a double task. On the one hand one must accom-
plish the enormous classification of the archaeological record, fill-
ing gaps in connection with the ever increasing data from scientific
field archaeology (McEwan, 2000). On the other, it is necessary to
pose questions that call for new methods under an autochthonous
and independent discourse, negotiating the relation between
research, the academy and the active social and political move-
ments (Rodriguez, 2002).
2.2. Experimental
Today the Quimbaya gold production includes the old Quimbaya
and Invasionist groups from Pérez de Barradas. According to M.A.
Uribe (1991, 2004) there are two periods, Classic/Early Quimbaya
between 500 BC and 600 AD, associated to the so called marrón inciso
pottery, and Late Quimbaya that extends until the Spanish conquest
and should be better named Sonsoide considering its association to
this particular archaeological group. The connections between both
periods are not at all explained, but from the typological and tech-
nological points of view both productions are very different
(Rodriguez, 2002). The Quimbaya Treasure belongs to the first of
these periods, which it helped to define. After Pérez de Barradas
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e2334 2327

there has been a limited attempt to study this important assemblage
(Cuesta Domingo and Rovira Llorens, 1982;Rovira Llorens, 1994).
Aware of the limitations and risks of dealing with politically
sensitive and, ideologically meaningful, archaeological materials
we undertook its study persuaded that only an archaeometrically-
oriented analysis could help to shed some light on this long shad-
owed assemblage.
To cope with the complex technology of these archaeological
materials it is necessary to combine different analytical procedures
to allow the physical characterization of the samples (Contreras
et al., 2007;Ruvalcaba Sil et al., 2004). After a first character-
ization by optical microscopy (OM), all the objects were analysed
using XRF with portable equipment. The use of Scanning Electron
Microscopy (SEM) was ruled out due to the size of the objects
(Perea and García-Vuelta, 2012), and only the gold micro-beads
were examined. The high cost of moving the assemblage obli-
gated the selection of 63 objects for the elemental characterization
using PIXE-RBS. In the third phase the inside of the funerary urns
were explored for any organic remains which could be dated using
AMS. Finally, X-ray pictures were taken of the more complex ob-
jects, helping us to understand the technical features.
2.3. Samples
We had unrestricted access to all types of objects from the
treasure. These comprise anthropomorphic and phytomorphic
vessels; headed pins for mixing coca leaves with lime; helmets;
a crown; a whistle and possibly a second wind instrument; nose
rings, earrings and rattle earrings; bells; bracelets; quite a variety of
beads, micro-beads and zoomorphic and anthropomorphic pen-
dants. All are described as made up of tumbaga alloy with an
enriched surface in the literature. Traditionally the characteristic of
these objects that attracted more attention was the very detailed
representations of men and women in ecstatic attitudes although
we lack an iconographic study of this assemblage.
3. Results and discussion
3.1. OM
Optical microscopy is a simple, efficient method for identifying
tool marks, technical processes, usage, wear damage and deterio-
ration in burial/museum conditions (Armbruster et al., 2003;Perea,
2010). Quimbaya technological processes as documented in the
Madrid assemblage include lost wax castings in four variants:
- open cast,
- with one interior core,
- with two interior cores, as in the rattle earrings (Fig. 1),
- casting on for unions and repairs (Fig. 2).
Apart from the latter we could identify post-casting retouching
with punches and chasing tools, cutting, inlaying of materials other
than gold (Fig. 3), plastic deformation by hammering and burn-
ishing, coloured surface finishings by depletion gilding, multi-
coloured surfaces ered, yellow, white eand mechanical twisting
interlockings for added elements (Fig. 4). A useful index for the
complexity and excellence of these processes is the size span of the
castings that varies from 53.0 cm long of the biggest object to
0.16 cm diameter of the smallest one. We observed some casting
defaults like voids or porosity but in general the quality standard is
very high. Wear or usage marks were not visible, this circumstance
together with the fact that some of the inner cores were not
removed after casting, lead us to think that at least some of the
objects were manufactured just before their interment.
Smooth shiny surfaces (Fig. 5) show colour stains due to burial
conditions but also to repeated cleaning with abrasive products
during modern times to the point of eliminating the enriched
surface almost completely in some objects.
3.2. SEM-EDS
The Madrid assemblage includes 18 gold anthropomorphic and
phytomorphic vessels known as poporos and totumas. They were
used to keep the lime necessary to mix with coca leaves for its
consumption, but also they served as funerary urns to keep a por-
tion of the ashes after the cremation of the corpse, and in some
cases they introduced a handful of gold micro-beads at the same
time. The variable pressure Hitachi 3400N Scanning Electron Mi-
croscope, equipped with a Bruker Quantax 4010 energy dispersive
X-ray spectrometer, was used for the topographic and analytical
study of these remains at the MicroLab of the CCHS-CSIC.
Nearly one hundred of tiny gold beads of different shapes were
recovered. Some of them show very fresh cutting traces from a lost
wax hollow rod of about 1.6 mm in diameter (Fig. 6). Others were
made shaping a small piece of gold sheet. Elemental analysis shows
Fig. 1. Rattle earring lost wax casted with two interior cores, one for the ring body and
the second for the inner metal sphere (photo: Archivo Au, F. Cuesta).
Fig. 2. Casting on repair on of a void in the abdomen of a female anthropomorphic
vessel. The same repair is noticeable in the X-ray image of Fig. 13 (photo: Archivo Au,
CCHS-CSIC).
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e23342328

a variety of Cu contents (from not detected to 17%) on the contrary
Ag remains around 15%, the mean value.
The ashes were identified as burned bone and sent for radio-
carbon dating (see below, 3.5).
3.3. XRF
Quantitative elemental analysis using XRF were done in situ
with a portable Innov X tube-based Alpha Series analyzer. The
primary photon beam was produced with a silver anode X-ray tube.
The source is equipped with a 2 mm Al filter in order to remove
interfering components of the tube radiation and to improve the
signal-to-noise ratio. The X-ray source provides a maximum power
of 8 W and it was operated in normal conditions of voltage (35 kV)
and current (2
m
A). The characteristic X-rays emitted by the sam-
ples were measured by a Peltier-cooled Si-PiN diode detector
(230 eV FWHM). The circular beam port cross section of 15 mm
2
permitted global analyses on each analysed area. Data results are
automatically processed by the instrument with the Data Analysis
Software in Alloy Analytical Mode, which utilizes a fundamental
parameters algorithm to determine the elemental chemistry. Sta-
tistical errors associated to the measure are below 5% for Cu,1% for
Ag and 1% for Au.
In total, 287 XRF spectra were recollected from all Quimbaya
items. Various spectra in different areas from the same object
were taken in order to assess the homogeneity of the alloy, as it
Fig. 4. Helmet number 17428 decorated with two female figures whose heads are
removable. Museo de América, Madrid (photo: Archivo Au, CCHS, CSIC).
Fig. 5. Anthropomorphic vessel, representing a seated man, in front of the detector at
the CMAM accelerator while taking measures. Museo de América, Madrid (photo:
Archivo Au, CCHS, CSIC).
Fig. 6. Micrography of one of the micro-beads showing fresh cutting traces. Museo de
América, Madrid (photo: Archivo Au_MicroLab, CCHS, CSIC).
Fig. 3. Anthropomorphic vessel in the form of a man’s head with openwork to inlay
other material than gold. Museo de América, Madrid (photo: Archivo Au, CCHS, CSIC).
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e2334 2329

was the case, although the beam size of our instrument restricts
a proper study. Nevertheless we observed noticeable differences
in composition between reverse and obverse on circular
pendants.
Major elements found were Au, Ag and Cu (Fig. 7a), together
with low levels of iron, normally less than 1%, though in some areas
of a sample it reached 4%. Samples are clearly positioned in the
zone of the ternary diagram that comprises copper contents lower
than 50% and silver levels between 12 and 25%. A small group of
objects (beads, anthropomorphic vessels, helmets, ornamented pin
heads, a Darien pendant, earrings and a nose ring) stand out for
their high gold content (no copper was detected, LOQ for Cu: 3%).
Absence of other elements confirms that there were no modern
materials added to the samples, this is an important fact consid-
ering the difficult biography of the treasure.
3.4. PIXE-RBS
PIXE is widely used in the analysis of gold pieces as a totally non-
destructive elemental analysis technique (Demortier and
Ruvalcaba-Sil, 2005;Guerra and Calligaro, 2004). RBS technique is
employed to determine thickness of thin layers and depth con-
centration profile. In particular the use of an external micro-beam
allows the study of samples of different size and shape. RBS and
PIXE were applied simultaneously on 63 Quimbaya objects in order
to obtain detailed information about the elemental composition
and its distribution in depth.
PIXE-RBS measurements were carried out in the external micro-
beam line at the CMAM facility with a proton beam of 3 MeV. Also,
a proton beam of 5 MeV was used in order to increase the analysed
depth. The proton beam crosses an exit Si
3
N
4
window (100 nm
thick) and 3 mm in atmosphere of helium (to minimize absorption
of the incident beam and emitted X-rays) until it reaches the
sample.
Emitted X-rays were collected by two Si(Li) detectors located
at 43 and 50

with respect to the sample normal. The first one,
witharesolutionof130eVandanactiveareaof10mm
2
was
used for the detection of low energy X-rays, while the second
detector with 150 eV FWHM, active area of 80 mm
2
and filtered
by a 1000 or 500
m
m Mylar absorber (utilized to suppress the
K alpha lines of copper) was employed for high energy X-ray
detection. In general, several spots on each sample were chosen
for analysis.
Quantification of PIXE spectra was done with the GUPIX soft-
ware (Maxwell et al., 1995) assuming thick samples, i.e. constant
concentration in depth and thickness larger than the particle range.
Reproducibility of PIXE measurements has been checked by
repeatedly analysing a 17 carats gold ring whose results were
always consistent one to another within the experimental errors.
Backscattered ions were collected using a solid state Si diode
with an active area of 50 mm
2
placed at backward angle of 140

under an atmosphere of helium. Data analysis of RBS spectra was
performed with the computer code for simulation of elastic scat-
tering spectra SIMNRA (Mayer, 1997).
3.4.1. PIXE results
The range of 3 MeV and 5 MeV protons in Au is 26.78 and 58.19
microns respectively (SRIM code, Ziegler et al., 2008). Taking into
account that in XRF 95% of the Ag X-ray from the tube cathode is
converted in 24 microns of Au and 95% of the 35 keV X-rays
(maximum energy delivered by the tube) is converted in 70 mi-
crons, we may say that the thickness explored by the probe is
comparable in the two cases. Furthermore we have proven
(Zucchiatti, 2012;Ahlberg, 1977) that, when we use for the calcu-
lation of elemental concentrations the X-ray lines of maximum
energy (L-lines for Au, K-lines for Cu and Ag), which suffer the
lowest attenuation by the sample itself in their path to the detector,
the PIXE and XRF compositions are very similar and refer to the
sample bulk. The great advantage of PIXE is the spatial resolution:
in the external beam of the CMAM was of the order of 50e
80 microns, which allowed the examination of particular struc-
tures in the objects surface.
For all the 63 Quimbaya objects analysed at CMAM were taken
242 data points. PIXE spectra showed the presence of only a few
elements: major elements Au, Ag and Cu (Fig. 7b) and in some of
cases minor or trace elements Fe and Zn, probably due to inclusions
of dust not uniformly distributed over the porous surface or to
modern cleaning products. In the majority of cases multiple PIXE
measurements on the same sample show that the bulk composition
is essentially homogeneous (Fig. 8a) as it was known from the XRF
measurements. In other cases it is possible to appreciate, thanks to
the high spatial resolution of PIXE, composition differences that
might or not have a trivial explanation.
In Fig. 8b we plot the results of different measurements on six
objects identified by their catalogue number. For example, the
object with catalogue number 17400 is a circular pendant in which
one of the points (351 obverse) appeared less guilded than the
other two (349-reverse, 350-obverse). Indeed the Cu content in
point 351 is higher. The same occurs with objet 17401, another
circular pendant, where measurements on the front (352,353) and
back (354,355) are different; the back being richer in Cu. Objet
17413 is a bracelet where one of the measurements (261) was taken
close to the joint and the other (262) in a dark spot. In the phyto-
morphic vessel 17440 (442e446), the lid is particularly rich in gold.
Fig. 7. Ternary diagrams showing AueCueAg compositions for all measures, where three main groups of objects are identified: a. XRF results (287 data points from all 135
Quimbaya items). b. PIXE results (242 data points from 63 Quimbaya items).
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e23342330

As we said above, considering that PIXE measurements give
substantially the bulk composition it is possible to distinguish at
least three groups of objects: one is characterised by a low Ag
content (<6%); the second one by a high Ag content (>27%)
(Fig. 7b); and the third and largest group features a broadly dis-
tributed composition with an intermediate roughly constant Ag
content, as it is the case for the XRF measurements (Fig. 7a). As
a matter of fact only three objects (17426, 17429, 17432) are fully
separated from the main group and two (17408, 17440) have only
one point showing an outlier composition. For object 17408 this is
a measurement taken on a recess and for object 17440 it is
a measurement taken on a yellowish area.
The multi-technique examination of the objects has indicated
on one side that depletion gilding was applied to surfaces but also
that heavy surface cleaning, performed at some stage of the objects
conservation may have removed most of the enriched surface. PIXE
quantification of layered objects with the program GUPIX is not
possible when Au, Ag and Cu are present, as it is our case, in various
layers at different concentrations. However PIXE can give a qual-
itative indication of the presence of an enriched micrometric layer
by looking at the intensity ratio between the K
a
and K
b
lines of Cu.
The branching ratio of these two X-ray lines gives K
a
/K
b
¼7.1. This
will be the yield ratio observed measuring an ultra-thin copper
sample that does not absorb either line. For any other sample the
absorption of the K
a
(at 8.048 keV) will be higher than that of the
K
b
(at 8.906) and the yield ratio will be lower than 7.1 depending on
the matrix, the sample finite thickness and the angle of detection of
the PIXE spectrum. We have calculated the yield ratio in an infinite
sample (that in which protons loose all their energy) for a normal
proton incidence and the detection at 45

(as in our PIXE set up) for
various binary bulk compositions going from 100% Au to 100% Cu.
To note that, as regards the Cu X-rays absorption Au and Ag are
practically equivalent because their mass absorption coefficients
for the K
a
line (Au 48.80
m
g/cm
2
, Ag 48.35
m
g/cm
2
) and the K
b
line
(Au 63.66
m
g/cm
2
, Ag 63.67
m
g/cm
2
) are practically identical.
Therefore a binary AueCu sample is equivalent to a ternary Aue
AgeCu ternary sample. Then we have calculated the variation of
the ratio due to a surface Au layer with thickness going from 0 to 5
microns. The results are shown in Fig. 9.
The calculated yield ratio, obtained at 45

for a bulk AueCu
alloy, goes from 5.8 to 6.5 and then drops down with the
increase of the absorbing surface gold layer. The distribution of
experimental values has extremes at 6.5 and 4.0 and two maxima
at 5.80 and 5.26. The majority of the measurements (peak at 5.80)
is compatible with no surface layer and a Cu rich bulk (40e50%)
but the lowest experimental values can only be explained start-
ing from a gold rich bulk (60e80%) followed by a 1e4 microns of
gold on the surface. Guilding is therefore observed but only
qualitatively being a quantification possible only through a specific
technique like the RBS.
3.4.2. RBS results
A general inspection of the spectra allowed to distinguish be-
tween layered objects and those with homogeneous composition in
depth although only for a few we could extract layer thicknesses.
Elemental composition obtained with PIXE was used to provide
starting values in the fitting of gold enriched samples and they
represent the composition on the bulk alloy.
Only some anthropomorphic vessels, helmets, the crown, the
whistle and one phytomorphic vessel exhibit a clear gold enriched
surface; other samples may have lost the enriched surface as
a consequence of abrasive cleaning. Different analysed spots of
these same items show irregular gold surfaces layers regarding
thickness and composition.
RBS fitted spectrum from the obverse of an anthropomorphic
vessel (Fig. 10) show firstly a pure gold layer of about 0.6
m
m
Fig. 8. a. Elemental composition of five objects showing internal homogeneity and inter object variability. b. Elemental composition of six objects taken in different points, showing
internal variability.
Fig. 9. Variation of the Cu K
a
/K
b
ratio due to the alloy bulk composition and a surface
Au layer with thickness going from 0 to 5 microns.
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e2334 2331

followed by an intermediate layer of 0.15
m
m thickness with 90%
gold and 10% silver, and finally the bulk alloy of 81% Au, 17% Ag and
2%Cu.
3.5. AMS dating
Although we use the term “assemblage”to name this group of
objects found in the two looted tombs, we have no idea of the time
span the necropolis was in use, or how many interment episodes
occurred. To tackle the chronological problem we had the fortune
that two of the anthropomorphic vessels still contained uncon-
taminated ashes which were used for radiocarbon dating. In
addition, more samples were obtained from the inner cores (sedi-
ment) of an ear spool and a whistle (Table 1):
Some years ago the Museo del Oro (Bogotá, Colombia) under-
took a dating programme for pre-hispanic metallurgy. There are six
dates directly associated with Quimbaya goldwork all from the
inner cores of gold objects except one that dates the charcoal
(without specifying its origin) of a tomb. One of these dates
(MA17424) corresponds to a pendant of the Madrid Quimbaya
assemblage. The only data available was published by Plazas (1998)
and Uribe (2005) as in Table 2.
The chronology of Classic Quimbaya period is nowadays based
on these recent radiocarbon dates although it is surprising the date
of 400 BC for a complex lost wax casting poporo according to Uribe
(2005: 66). We must also take into account that the typology of the
other dated items, zoomorphic pendants and a nose ring, are not
diagnostic of the Quimbaya production but common to a very wide
area of the river Cauca valley and in use for a long period (Plazas,
1998).
The time span of our dates, obtained from ashes and inner cores
(sediment), 410e590 AD, is a reasonable period for a necropolis to
be in use and explains variability in terms of technological and
analytical features of gold production.
3.6. Radiography
X-ray imaging techniques were applied at the Museo de
América using a Yxlon International portable equipment. The
Table 1
Radiocarbon dates (Beta Analytic laboratory) from burned bone and inner cores
contained in some objects of the Quimbaya Treasure (Museo de América, Madrid).
Lab. N

Mus. N

Material BP 2
s
Cal.
date
Object
B-320629 MA17436 Sediment 1650 30 340e430 AD 410 AD Whistle
B-315972 MA17456 Burned
bone
1620 30 390e540 AD 420 AD Poporo
B-315970 MA17422-5 Sediment 1600 30 400e540 AD 430 AD Ear
spool
B-290934 MA17447 Burned
bone
1480 30 540e640 AD 590 AD Poporo
B-: Beta Analytic laboratory.
Table 2
Radiocarbon dates as published by Plazas (1998) and Uribe (2005) obtained from
Quimbaya gold objects in the Museo del Oro, Bogotá.
Lab. N

Mus. N

Material BP (Plazas)
a
Cal. date
(Uribe)
Object
B-144489 MO00382 Sediment e400 40 BC Poporo
B-97373 MO02023 Sediment 2190 40 BP 240 40 BC Pendant
B-107961 MO00275 Sediment 1900 50 BP 50 50 AD Nose ring
B-108843 MO06039 Sediment 1760 40 BP 190 40 AD Pendant
B-190947 Charcoal e250 50 AD Tomb
B-175663 MA17424 Sediment e260 40 AD Pendant
a
Data available as published by Plazas (1998): conventional radiocarbon age is
missing in some of the samples.
Fig. 11. X-ray image of a phytomorphic vessel showing the perfect modelling of the
wax, and fine and homogeneous porosity (darker spots). Museo de América, Madrid
(photo: Archivo Gabaldón-Antelo, IPCE).
Fig. 10. RBS spectrum of an anthropomorphic vessel. Experimental data are shown in
red line (top) and the simulated spectrum in blue. Contribution of Au, Ag and Cu el-
ements, and He and air coming from the environmental are marked on the graphic.
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e23342332

X-ray beam, generated with a constant potential of 160 kV, crossed
an exit window before reaching the object. Radiographic images
were achieved in order to confirm technological features of the
bulk metal which are not evident with an OM examination and to
check actual state of conservation. We used this technique in
technological complex items, like vessels, and only in those sam-
ples we could ascertain they did not contain more organic
residues.
All the images show skilled modelling of the inner core of the
wax casting and the control of very homogeneous and, in some
cases extremely thin, wax layers (Fig. 11). The pegs for holding
the inner cores in place leave perfectly circular holes filled in by
burnishing gold rivets that are difficult to see with nude eyes,
furthermore, they are situated in strict symmetry (Fig. 12). Some
of these rivets have disappeared. In one case there were two
gold micro-beads still left stuck to the inner wall of a vessel
(Fig. 13).
The main problem the artisan had to cope with during the
casting process was the evacuation of gases produced by the filling
of the mould and subsequent solidification of the alloy. This pro-
duced microporosity and small voids in many of the items. Only in
one case have we observed a big metal gap due to low temperature
or excessive density of the alloy that could not fill the mould
completely before cooling. All these small flaws were repaired by
casting on.
Two of the items presented small fissures, probably developed
from porous areas that ended in fatigue failure and finally
fracture.
4. Conclusions
According to analytical results the Quimbaya assemblageshows
variable bulk elemental compositions, comprising three main
groups according to silver content. The largest group corresponds
to silver levels of around 15%. A small number of objects stands out
for being manufactured in a very high gold content alloy. Relating to
objects themselves, they generally exhibit high bulk homogeneity.
Finally, the existence of an enriched surface layer was proved by
RBS, in those cases that this gilded surface was preserved from
modern abrasive cleaning.
The Quimbaya Treasure is important not only for the quality and
large number of items but for its common archaeological funerary
context. It is in this sense that we have used the term assemblage.
Considering the big items only, such as vessels, helmets, crowns
and pins, we can ascertain a common craftmanship origin or
technological tradition that could have developed only within
a restricted period of time. It is very improbable that these tech-
nological traditions and processes could persist over a millennium.
Even if we take tradition and continuity in pre-Columbian com-
munities as the main feature in gold production, it is hard to accept
Fig. 13. X-ray image of an anthropomorphic vessel representing a seated woman.
Some repaired voids can be seen on the abdomen (brighter spots) and a gold
microbead stuck to the inner part of the left arm (square shaped and brighter). Museo
de América, Madrid (photo: Archivo Gabaldón-Antelo, IPCE).
Fig. 12. X-ray image of a phytomorphic vessel showing porosity areas (darker) and
the symmetrical position of the perfectly circular holes (brighter) of the inner core
sustaining pegs. Museo de América, Madrid (photo: Archivo Gabaldón-Antelo,
IPCE).
A. Perea et al. / Journal of Archaeological Science 40 (2013) 2326e2334 2333

the persistence of particular technical behaviours and iconographic
characteristics without any change all throughout the Classical
Quimbaya period 500 BCe600 AD. We suggest that this assemblage
should be considered as characterising a specific phase in the long
goldwork production period of the area. The new radiocarbon
dates, 410e590 AD, would mark the time span of this phase.
Acknowledgements
This paper is part of two Research Projects HUM2009-09298
and HAR2011-12809-E funded by the Ministerio de Economía y
Competitividad, Spain. The authors thank the Museo de América for
allowing us access to the archaeological material and for providing
all kind of facilities. We are also indebted to the Instituto de Patri-
monio Cultural de España, IPCE, for taking charge of the Radio-
graphic work and to the Laboratorio de Arqueología del Paisaje y
Teledetección (LabTel, CCHS-CSIC) for their help with the graphical
abstract. We are specially indebted to the anonimous reviewers for
their valuable comments.
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