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Chapter 18
The Maya Blue Pigment
Manuel Sa´nchez del Rı´o*, Antonio Dome´nech
{
, Marı´a Teresa
Dome´nech-Carbo´
{
, Marı´a Luisa Va´zquez de Agredos Pascual
#
,
Mercedes Sua´rez
}
and Emilia Garcı´a-Romero
k,}
*
European Synchrotron Radiation Facility, BP220, Grenoble Cedex, France
{
Departament de Quı
´
mica Analı
´
tica, Universitat de Vale
`
ncia, Dr. Moliner, 50, Burjassot,
Vale
`
ncia, Spain
{
Departament de Conservacio
´
i Restauracio
´
de Bens Culturals, Institut de Conservacio
´
del
Patrimoni, Universitat Polite
´
cnica de Vale
`
ncia, Camı
´
de Vera 14, Vale
`
ncia, Spain
#
Departament de Histo
`
ria de l’Art, Universitat de Vale
`
ncia, Passeig al Mar, Vale
`
ncia, Spain
}
A
´
rea de Cristalografı
´
a y Mineralogı
´
a, Departamento de Geologı
´
a, Universidad de Salamanca,
37008 Salamanca, Spain
k
Instituto de Geociencias (UCM-CSIC), Ciudad Universitaria, 28003 Madrid, Spain
}
Departamento de Cristalografı
´
a y Mineralogı
´
a, Universidad Complutense de Madrid, Facultad
de Geologı
´
a, 28003 Madrid, Spain
1. HISTORY OF MAYA BLUE
Maya blue (MB) is a pigment that was extensively used in ancient times in
Mesoamerica, cover ing today Mexico, Guatemala, Nicaragua and Belize. This
‘standard’ blue was invented during the first Millennium AD in the Maya
area. The MB use extended to nearly all Mesoamerican cultures. Examples
of its use are in Figures 1–3.
Merwin (1931) described a blue paint found on remains of a Maya wall
painting at the Temple of the Warriors in Chiche
´
n Itza
´
. He recognized the
blue colour as a new pigment (Gettens and Stout, 1942). Early workers
described MB as an unknown material because of its resistance to chemical
and thermal treatments: “... the color is not discharged by boiling nitric acid
nor by heating much below redness. The conclusion seems justified that this is
an inorganic color.” Although Merwin stressed one of its most peculiar char-
acteristics, its resistance to acids, he thought it resembled a clay mineral.
Gettens and Stout (1942), coined the ‘Maya blue’ name.
Gettens (1962) analysed several samples of MB and recognized their sim-
ilarity with the blue described by Merwin, which differs from any blue used
elsewhere. He gave physical and chemical properties and reviewed earlier
studies that “suggested that it might be derived from, or related to, silicates
Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00018-9
#
2011 Elsevier B.V. All rights reserved. 453
of the chlorite group which are colored blue or greenish-blue by ferrous iron”.
However, they did not identify MB as any known material.
X-ray data showed that MB gave a distinct pattern, in agreement with
samples of palygorskite from Attapulgus (Georgia, USA). Thus, MB was con-
firmed as an apparent mixture of palygorskite with other clays, but the mix-
ture did not explain the colouration, as palygorskite is whi te. Experiments
mixing palygorskite with indigo reduced with sodium hydrosulfite produced
a blue pigment that was not resistant to nitric acid. Gettens (1955) described
samples not only from the Maya area but also from other areas in Mexico
and Central Mexico, Veracruz and Oaxaca. Gettens stressed the importance
of MB : (i) tracing Maya trade routes, (ii) historical aspects of preparation
and (iii) that MB may have been continued in native communities until
now. Gettens also remarked the presence of sepiolite in some samples and
suggested “that Maya blue is an artificial blue, and that more than one kind
of clay could be used in its preparation”.
Shepard (1962; Shepard and Gottlieb, 1962) suggested that a blue organic
colourant was present in MB, in addition to palygorskite. The optical proper-
ties and the behaviour of the pigment when heated may indicate an organic
complex. In addition, the intensity of the colour is independent of the grain
FIGURE 1 Fragment of the Mural of the Battle (photo: Christophe Morisset). This fragment
shows a warrior of high rank, with one of them vanquished. The murals in Cacaxtla have details
typical from Teotihuacan, while the figures possess characteristics of the end of the Mayan Classic
period. Although these murals have been dated very early (AD 650–750), the fact that the MB pig-
ment is used indicate that it should come after the ninth or tenth century AD. (Reyes-Valerio, 1993).
Developments in Palygorskite-Sepiolite Research454
size. Grains with low colouration may indicate an incomplete penetration of
the colourant. When MB is heated to 300–400
C, it turns to grey and then
white which may be related to degradation of organic substances. Although
failing to identify the organic complex, Shepard argued: “I fully agree with
Gettens that it cannot be a dye coating the surface of the clay particles. But
there is another relation between the clay and organic matter complex—the
so-called clay organic complex—that should be considered”. Tae Young
Lee, from Gettens team, first identified indigo in MB . See Shepard and
Pollock (1971) and Torres et al. (1976) for additional details.
van Olphen (1966) studied the synthesis of indigo–clay complexes.
He found that to achieve stability to acids, the pigment must be heated at
moderate temperat ures. van Olphen, heating to 75
C for several days or at
105–150
C for shorter periods, produced stable pigments using fibrous clays
(Figure 4) with a tunnel structure (palygorskite and sepiolite) and failed to
obtain stab le pigments using clays neither with plate-like structures (e.g. kao-
linite, nontronite) nor with mordenite (a zeolite with a cage-type structure).
van Olphen noted that “the indigo molecules are undoub tedly too big to enter
the channels of attapulgite or sepiolite, and the relatively small adsorption
capacity of the minerals for indigo suggests that the dye is indeed adsorbed
FIGURE 2 A Warrior figurine from the
Jaina Island (photo: Thomas Aleto). It is
remarkable the good conservation of the
MB pigment as compared to other colours
that are degraded.
Chapter 18 The Maya Blue Pigment 455
on only the external surfaces of the particles.... Yet the precise mechanism of
the stabilization of the complexes by heating is not clear; if one assumes that
the heated indigo–attapulgite complex is indeed the synthetic equivalent of
Maya blue, the solution of this puzzle has created a new one”. This new puz-
zle has not yet been solved after more than 40 years. van Olphen did not argue
that the Mayas prepared the pigment in the same way, just that a pigment with
the same characteristics can be synthesized in a laboratory.
Kleber et al. (1967) studied archaeological samples and considering fixating
indigo onto palygorskite. They created a resistant pigment by mixin g
palygorskite with a solution of leucoindigo and then heating at 150
C for
20 h. They also prepared MB by mixing indigo powder (0.5–50 wt%) with paly-
gorskite subsequent heating (120 or/and 150
C for 5, 20 or 25 h) and removing
the excess of indigo by sublimation at 130 and 190
C at 1 Torr. The pigments
heated at 190
C were resistant to boiling nitric acid but not for those heated
at 120
C, in partial contradiction to results of van Olphen. Kebler et al. unam-
biguously detected indigo only in synthetic samples with > 15% indigo. With
infrared absorption spectroscopy, indigo was detected where samples were
prepared with > 3% indigo. The indigo-characteristic lines at 1485, 1460,
1380 and 1305þ1290 cm
1
were detected in seven archaeological samples.
Kebler et al. suggested that the irreversible fixation of indigo is related to loss
of zeolitic H
2
O at 150–200
C (hygroscopic H
2
O is lost near 100
C and struc-
tural OH
2
at 375–425
C). They suggested that based on steric considerations,
the penetration even of indigo molecules in the tunnels is possible.
FIGURE 3 Colonial wall decoration at
the Oaxtepec Convent (Morelos, Mexico,
sixteenth century) in black and blue
(MB) tonalities (photo: Manuel Sa
´
nchez
del Rı
´
o).
Developments in Palygorskite-Sepiolite Research456
Arnold and Bohor (1975) published a seminal work on materials used by
the Mayas to fabricate MB. More recently, Littmann (1980) disagreed with
earlier theories, suggesting the importance of montmorillonite. Littman stud-
ied experimentally indigo–palygorskite complexes, some using indigo from
Indigofera tinctoria, and some samples were resistant to nitric acid. He sug-
gested the importance of the heat treatment to stabilize the pigments and
suggested complex mechanisms that the Maya may have used in preparation
of MB. Torres (1988; Torres et al., 1976) has provided overviews of the geo-
graphic and chronologic distributions of MB, as well as possible fabrication
techniques.
In 1990, Tagle et al. (1990) reported the presence of MB in Cuban colonial
wall paintings (1750–1860). In 1993, Reyes-Valerio (1993) suggested a fabri-
cation technique that may have been used by the Mayas to produce the
pigment. Jose-Yacam an et al. (1996) published an scanning electron micro-
scopy (SEM) study that suggested the presence of superlattices and nanopar-
ticles, which were later rejected. They are discussed below.
2. EXPERIMENTAL TECHNIQUES
2.1. Diffraction Studies
Diffraction studies provide direct information on the crystal structure of min-
eral phase in MB. High resolution diffraction may be sensitive to clay’s
changes in the hydration state and lattice parameters due to the interaction
with indigo. Solid state indigo is crystallized (Susse et al., 1988) but losses
its crystallinity when combined with palygorskite in MB. Residual peaks of
indigo can be found in MB (Arnold, 1971; Kleber et al., 1967; Sa
´
nchez del
Rı
´
o et al., 2006a, 2009a) but may only indicate an excess of indigo. The paly-
gorskite crystal structure was refined using synchrotron powder diffraction
(Chiari et al., 2003) and neutron diffraction (Giustetto et al., 2006). Chiari
et al. (2003) and Giustetto et al. (2006) examined synthetic MB and concluded
that the indigo molecules enter in the tunnels of the orthorombic or mono-
clinic palygorskite. They suggested that both indigo and zeolitic H
2
O coexist
in the tunnels. Indigo resides in the middle of the tunnel with the centre of the
molecule in the centre of a triple cell. The disposition of the zeolitic H
2
Ois
more disordered in MB than in palygorskite. The affinity of indigo is greater
for orthorhombic palygorskite (30% occupancy) than for monoclinic (19% ).
Sa
´
nchez del Rı
´
o et al. (2006a) studied the effects of acids on MB. The
results (see Figure 5) looked to pigment decolouration and destruction of
the clay structure, as revealed b y X-ray diffraction. They showed that paly-
gorskite- and sepiolite-based pigment s do not decolour when immersed in
concentrated nitric or hydrochloric acids at room temperature for 5–15 min.
However, sepiolite pigments are more degraded than palygorskite pigments,
with the former destroye d with long treatments (several hours to a few days)
Chapter 18 The Maya Blue Pigment 457
in concentrated aci ds at room temperature. No changes are observed by paly-
gorskite pigments after long (greater than 1 week) acid submers ion at room
temperature.
2.2. Infrared Spectroscopies
Fourier transform infrared (FTIR) spectroscopy was used for identifying the
organic component in MB. Kleber et al. (1967) identified a few indigo bands
in archaeological samples spectra. Indigo and palygorskite modifications
under thermal treatment were used to try to unveil the nature of the clay–
colourant interactions (Giustetto et al., 2005; Leona et al., 2004; Sa
´
nchez
del Rı
´
o et al., 2009a; Figure 6).
The mid-infrared (MIR) spectrum of MB, like in palygorskite, can be
analysed in three parts:
(1) A zone with wave numbers to 1200 cm
1
, which corresponds to the
vibrations of the tetrahedral (SiOSi in 950–1250 cm
1
) and octahe-
dral (AlAlOH at 913 cm
1
,AlFeOH at 865 cm
1
and
MgMgOH at 650 cm
1
) sheets.
(2) The 1200–2000 cm
1
range, indigo bands minimally overlap with paly-
gorskite bands. Bands in the range of 1290–1485 cm
1
zone were used
50 nm
A
B
50 nm
FIGURE 4 TEM images for palygorskite
crystals. (A) Pristine palygorskite from
Sacalum (Yucata
´
n, Mexico); (B) MB sam-
ple from the archaeological site of Maya-
pa
´
n (Yucata
´
n, Mexico), post-classic
period. TEM examination reveals fibrous
crystals of palygorskite. In MB samples
the fibres exhibit a corrugate surface,
associated to the evacuation of water dur-
ing the preparation of the pigment (Dome
´
-
nech et al., 2006, 2007a,b)
Developments in Palygorskite-Sepiolite Research458
by Kleber et al. (1967) to identify MB. A broadband around 1652 cm
1
is related to the zeolitic H
2
O (also producing two broadbands in the
3200–3400 cm
1
) and is always present in MB, indicating that fully
H
2
O dehydration never occurs. In Figure 6, the ATR–FTIR spectra of
indigo, MB and indigo–montmorillonite (a non-stable clay–indigo mix -
ture) are compared between 1400 and 1600 cm
1
. The absorption bands
are essentially the same for indigo and the montmorillonite mixture. In
contrast, the number of indigo bands increases in MB with palygorskite.
The complex, multiple-band spectra of MB suggest the presence of dif-
ferent topological isomers of indigo and, possibly, of other indigoid
molecules attached to different sites in the palygorskite matrix. The
1650–1750 cm
1
bands are attributed to the superposition of the
d(H
2
O) mode of structural OH
2
associated with indigo and n(C ¼¼O) anti-
symmetric stretching mode of palygorskite-associated indigo and dehy-
droindigo molecules (Dome
´
nech et al., 2006, 2 007b,c).
(3) The zone with wave numbers higher than 3500 cm
1
includes the vibra-
tions of the hydroxyl groups of palygorskite linked to cations:
AlMgOH at 3540 cm
1
,AlFeOH at 3580 cm
1
and
10
0
20
40
60
80
Intensity (counts/s)
100
120
Untreated palygorskite (scaled by 1/6)
HNO
3
90 °C 5 h
HNO
3
90 °C 30 h
HCl 90 °C 5 h
HCl boiling 30 h
140
20 30
2q (deg)
40
FIGURE 5 XRD spectra of palygorskite pigments, where it can be appreciated the destruction of
the palygorskite structure for several strong acid attacks: (from top to bottom) untreated palygors-
kite, 1% indigo–palygorskite pigment treated with 7N HNO
3
for 1.5 h at 90
C, the same for 6N
HCl, 3% indigo–palygorskite pigment treated with 6N HNO
3
for 30 h at 90
C and 3% indigo–
palygorskite pigment treated with 6N HCl for 30 h in ebullition. The diffractograms are shifted
vertically for clarity. (Sa
´
nchez del Rı
´
o et al., 2009b).
Chapter 18 The Maya Blue Pigment 459
AlAlOH at 3613 cm
1
(Sua
´
rez and Garcı
´
a-Romero, 2006). The latter
is very sharp and was proposed as an indicator for palygorskite in archaeo-
logical MB (Sa
´
nchez del Rı
´
o et al., 2008). The 3613 cm
1
band of paly-
gorskite (AlAlOH) shifts to higher wave numbers (10 cm
1
) with
increasing temperature in palygorskite and palygorskite–indigo com-
pounds (Sa
´
nchez del Rı
´
o et al., 2009a). This shift is explained by the dehy-
dration process, although Manciu et al. (2007) and Polette-Niewold et al.
(2007) assigned these changes to interaction of the indigo with Al.
Near-infrared (NIR) spectroscopy has been used to investigate palygorskite-
containing clays (Chryssikos et al., 2009; Gionis et al., 2006, 2007) because the
anharmonic coefficients of the stretching modes for the structural OH groups are
typically smaller than those of H
2
O. Thus, NIR offers a good separation of the
OH and H
2
O modes compared to MIR. This separation is enhanced using second
derivative analysis that eliminates broad features and background effects. When
a palygorskite–indigo mixture is heated to produc e MB, the surface silanols
dehydrate and then fully rehydrate when returning to room temperature. This
effect is analogous to the case of palygorskite, thus manifesting that indigo does
not obstruct the clay surface to rehydrate (Sa
´
nchez del Rı
´
o et al., 2009a).
Indigo
Indigo +
montmorillonite
Indigo + palygorskite
1500
cm
-1
1600
90
100
100
%T
60
80
100
90
1400
FIGURE 6 ATR–FTIR spec-
tra in the 1400–1600 cm
1
region of indigo, indigo–
montmorillonite and indigo–
palygorskite (MB). The richness
of structures in the MB spectra
is a proof of the complexity of
the indigo–palygorskite interac-
tions. (Dome
´
nech et al., 2009c).
Developments in Palygorskite-Sepiolite Research460
Therefore, MB is not a surface compound, contrary to the proposal by Chiari
et al. (2008), Fuentes et al. (2008), Manciu et al. (2007) and Polett e-Niewold
et al. (2007). Moreover, the NIR technique is sufficiently sensitive to detect
minimum penetration of H
2
O in the MB tunnels.
2.3. Raman
Raman spectroscopy can be used to correlate bands in archaeological pig-
ments with those of well-characterized references (Vandenabeele et al.,
2005) to identify MB. In addition, Raman spectroscopy is used to study the
chemical interaction occurring during heat treatment to obtain acid resistant
pigments (MB). An advantage of Raman over FTIR is that palygorskite is
not seen in Raman unless NIR frequencies are used (McKe own et al., 2002).
Raman spectra of MB were first published, using 442 and 632.8 nm excita-
tions (Grimaldi, 2000) and 1064 nm (Andreev et al., 2001). Witke et al. (2003)
compared the spectrum (excited with 514.5 nm) of indigo to a spectru m from a
Maya clay bead and reported, in addition to the bands of indigo, additional bands
at 1017, 1128 and 1380 cm
1
. Also, the intensity increase for bands at 1253,
1593 and 1633 cm
1
, which occur because of activation of Raman-inactive
modes at 1299, 1592 and 1627 cm
1
. This activation may be a consequence
of perturbations in the planarity of the indigo molecule resulting by the interac-
tion with the clay. Differences between the Raman spectra of indigo and MB
using a 785 nm laser (Leona et al., 2004) suggested (i) modifications affecting
the charge distribution of the indigo during fixation on palygorskite, (ii) disap-
pearance of possible vibrational coupling owing to inter-molecular indigo–
indigo interactions and (iii) different hydrogen bonding conditions between
C¼¼O and NH in pure indigo and in the indigo–palygorskite complex. Gius-
tetto et al. (2005) measured synthetic and archaeological MB with 325 nm exci-
tation. They found the Raman spectra of MB coalesc enced indigo bands at 1576
and 1586 cm
1
into a single peak, the disappearance of indigo bands at 1189
and 1370 cm
1
and the shift of indigo bands at 1315 and 1703 to 1325 and
1688 cm
1
. Calculations (Giustetto et al., 2005) predicted the shift in vibra-
tional frequencies when going from isolated indigo to indigo interacting with
two formula units of the clay, displaying a moderate shift (13 cm
1
) in the
C¼¼O stretch at 1692 (coupled with C¼¼C stretch) and important shifts
(26 cm
1
) in some vibr ations including the NH group.
Sa
´
nchez del Rı
´
o et al. (2006c) found that most features present in the spec-
trum of MB, which are not present in indigo, are not exclusive to MB and they
cannot relate to resistance to chemicals (Figure 7). Montmorillonite and kaolin-
ite produce similar spectral features in Raman spectra to palygorskite– and sepi-
olite–indigo spectra. Ab initio vibrational calculations that allow the assignment
of bands of the indigo spectrum also indicate that a single loss of planarity (a few
degrees rotation of half molecule around the C¼¼C axis) is not sufficient to
explain the acid resistance observed in indigo–clay mixtures.
Chapter 18 The Maya Blue Pigment 461
Recently, Manciu et al. (2007) described the peaks at 425 and 1395 cm
1
,
accompanied by an intensity increase of a peak at 606 cm
1
and concomitant
decrease in intensity of peaks at 635 and 1701 cm
1
with an increase in heat-
ing time for synthetic MB specimens prepared with 6–16% indigo and paly-
gorskite. The peaks at 425 and 606 cm
1
were assigned to band owing to
AlN and AlO bonding, respectively. These attributions should be taken
with caution. The peak at 606 cm
1
has been also observed in the indigo
spectrum, whereas the 425 cm
1
peak may correspond to a Raman-inactive
vibration mode of symmetry A
u
(Tomkinson et al., 2009) activated by planar-
ity perturbation.
Sa
´
nchez del Rı
´
o et al. (2009a) studied changes in the Raman spectra (exci-
tation at 785 nm) of a indigo (1%) palygorskite adduct during thermal treat-
ment to produce MB. Raman spectra showed changes in the temperature
interval of 70–130
C owing to vibrations involving the double bonds C¼¼C
and C¼¼O. In the same temperature interval, XRD showed a reduction of
the a cell parameter by 0.16 A
˚
owing to loss of zeolitic H
2
O. This suggests
that the modification of the indigo chromophore correlates with a reduction
in the tunnel width in palygorskite.
400
0
100
200
Raman intensity/Arbitr.units
300
(5)
1680
1583
1490
1380
1128
1017
1633
(4)
(3)
(2)
(1)
400
600 800 1000
Wavenumber
(
cm
–1
)
1200 1400 1600 1800
1253
FIGURE 7 Experimental Raman spectra (excitation laser at 532 nm, green light) of indigo (1%)
mixed with different clays (99%). From bottom to top: (1) synthetic indigo, (2) palygorskite and
indigo, (3) sepiolite and indigo, (4) montmorillonite and indigo and (5) kaolinite and indigo. The
Raman spectrum of the indigo changes when indigo interacts with clays. Vertical dash-dotted
lines mark the position of the changes from Witke et al. (2003), (Sa
´
nchez del Rı
´
o et al., 2006c).
Developments in Palygorskite-Sepiolite Research462
Raman spectra support the suggestion by Dome
´
nech et al. (2006) that
dehydroindigo, an oxidized form of indi go, occurs with dye in MB. The
enhancement of the Rama n peak at ca. 1635 cm
1
observed in MB by Sa
´
n-
chez del Rı
´
o et al. (2006c) and Witke et al. (2003) can be attributed to the
activation of the Raman-active vibration mode at 1627 cm
1
as a conse-
quence of perturbations in the planarity of the molecule (Witke et al.,
2003), or to the presence of imino units (C¼¼N) whose stret ching mode
appears typically at 1639 cm
1
. Recently, Tomkinson et al. (2009) analysed
the vibrational spectrum of indigo and found that the mid-frequency bands
corresponding to the g(NH) and g(CH) modes are better indicators for
the dye because they are sensitive to local bonding environment. Because
solid indigo possesses intramolecular hydrogen bond between carbonyl and
NH groups, attachment of indigo to the clay matrix in MB should break
the intramolecular hydrogen bond system of indigo. Accordingly, the Raman
band at 750 cm
1
will disappear as a result of the replacement of indigo by
dehydroindigo (Tomkinson et al., (2009); Dome
´
nech et al., 2011a), in agree-
ment with the observations reported by several authors (Giustetto et al., 2005;
Manciu et al., 2007; Sa
´
nchez del Rı
´
o et al., 2006c).
2.4. Optical Spectroscopies
Reinen et al. (2004) studied by UV–vis spectroscopy complexes of palygorskite–
indigo and palygorskite plus other organic colourants and considered changes in
colour after heating. They observed an irreversible shift of the absorption bands
when heating the palygorskite–indigo mixture, with colour shifts to green–blue
hues. These results indicate an interaction between palygorskite and indigo dur-
ing the heating process. They also showed that MB-like stable pigments with dif-
ferent colours can be synthesized using indigoderivates. Leona et al. (2004)
compared the UV–vis spectra of indigo and MB, and other studies (Fuentes
et al., 2008; Tilocca and Fois, 2009) compared quantum chemistry calculations
and optical spectroscopy. The differences in the spectral features of MB as com-
pared with reference compounds (indigo and dehydroindigo) are attributed to: (i)
additional absorbing compounds (Rondao et al., 2010), (ii) different palygors-
kite–dye associations (i.e. different topological dye isomers) and (iii) the appear-
ance of dye–dye associations (Dome
´
nech et al., 2009a).
2.5. Voltammetry
The voltammetry of microparticles considers the voltammetric response of a
micro- or nano-sol id non-conducting sample on a surface of an inert electrode
(usually graphite) immersed in a suitable electrolyte (Grygar et al., 2003;
Scholz and Meyer, 1998; Scholz et al., 2005). Dome
´
nech et al. (2006,
2007b,c, 2009a,b) obtained the electrochemical response of solid indigo and
MB samples attached to graphite electrodes in contact with aqueous electro-
lytes. Figure 8 compares the voltammetric response of two archaeological
Chapter 18 The Maya Blue Pigment 463
MB samples in contact with aqueous sodium acetat e buffer. Here, two indigo-
characteristic peaks at potentials of þ450 and 300 mV vs. AgCl/Ag are
recorded. These peaks are assigned, respectively, to the oxidation of indigo
in dehydroindigo, and the reduction of indigo to leucoindigo via two-proton,
two-electron processes (Figure 9). The variation of peak potentials and peak
current ratios for the above peaks on the timescale of the electrochemical
experiment (potential scan rate, square wave frequency) differs significantly
from indigo to MB. Additionally, MB specimens frequently show peak
splitting. The analysis of these voltammetric parameters indicates that indigo
and dehydrodingo are strongly attached to the palygorskite whe reas tempera-
ture-variable experiments indicate that enthalpy and entropy values associated
in the process of dye attachment to the clay matrix vary (Dome
´
nech et al.,
2006). Chemometric analysis of MB samples from Campeche and Yucata
´
n
archaeological sites permits a classification of such samples into different
‘electrochemical types’ and sugges ts that various preparation procedures were
used by the ancient Maya people (Dome
´
nech et al., 2007a, 2009d).
2.6. Nuclear Magnetic Resonance
Hubbard et al. (2003) used
29
Si CP/MAS to determine that indigo is not located
in tunnels of sepiolite in contrast to what happens to smaller organic molecules,
such as acetone or pyridine. This is in contrast to recent works (Dejoie, 2009;
Ovarlez et al., 2009; Giustetto, 2010). A dramatic change is observed in the
13
C CP/MAS spectra for pure indigo compared to indigo crushed with sepiolite,
suggesting the alteration of indigo. The altered indigo coexists with unaffected
A
B
1.0
Potential
(
V vs. A
g
Cl/A
g)
1 µA
I
I
II
II
0.6 0.2 –0.2 –0.6
FIGURE 8 Square wave voltammograms for
MB samples from (A) Kuluba
´
(Yucata
´
n, Me
´
x-
ico), (B) Dzibilnocac (Campeche, Me
´
xico),
both dated in the Classical Period, attached
to paraffin-impregnated graphite electrodes
immersed into 0.50 M aqueous sodium acetate
buffer at pH 4.75. Potential scan initiated at
0.75 V in the positive direction. Potential
step increment 4 mV; square wave amplitude
25 mV; frequency 5 Hz. Peak I corresponds
to the oxidation of indigo to dehydroindigo,
while peak II corresponds to the reduction of
indigo to leucoindigo.
Developments in Palygorskite-Sepiolite Research464
pure indigo in the unheated mixture but is the only molecule present in heated
sepiolite with indigo (20 wt%) after rinsing with nitric acid. They suggested a
model where the carbonyl and amino groups of the indigo are anchored by
hydrogen bond interactions with edge silanol groups of the clay (not with the
internal surfaces of the tunnels), thus blocking tunnel entrances.
1
HMAS
NMR data on indigo, palygorskite and newly synthesized MB were published
by Giustetto et al. (2005). New features in the NMR spectra (at 13.0 and
17.8 ppm) of MB and not present in the spectra of the reactants suggested the
presence of hydrogen bonds (NHO and C¼¼OHO, respectively).
The
1
H
13
C CP MAS,
27
Al,
29
Si NMR spectra for palygorskite and indigo
plus palygorskite synthetic specimens were studied by Dome
´
nech et al. (2009a).
The
27
Al MAS NMR spectra have an intense peak close to 4 ppm, accompanied
by a weak peak near 60 ppm. The 4 ppm peak is assigned to sixfold coordinated
aluminium to oxygen atoms, and the 60 ppm is attributed to a small amount of
fourfold coordinated aluminium. The close similarity between the spectra of
palygorskite and MB suggests that there are no significant modifications in
the environment of the Al
3þ
centres upon attachment of indigo to the palygors-
kite matrix. These results suggest that aluminium is structurally stable and that
direct interaction with the dye does not exist. In contrast,
29
Si NMR spectra for
palygorskite and MB are significantly different. The
29
Si NMR signals are
shifted to stronger fields and became broader as a consequence of dye incorpor a-
tion in palygorskite. The observed features suggest an increase in the electronic
density close to (SiO
4
)
4
tetrahedra.
N
N
H
H
O
O
N
N
O
O
H
H
N
N
OH
OH
Indigo
+2H
+
, +2e
–
Oxidation
Reduction
–2H
+
, –2e
–
Deh
y
droindi
g
o Leucoindi
g
o
FIGURE 9 Scheme for the electrochemical oxidation of indigo to dehydroindigo and the electro-
chemical reduction of indigo to leucoindigo.
Chapter 18 The Maya Blue Pigment 465
2.7. Computer Modelling
Chiari et al. (2003) calculated energies and forces as well as structure opti-
mization concluding (i) the adsorption of indigo on dehydrated (hydrated)
palygorskite is exothermic (endothermic), (ii) the displacem ent of water if
slightly favoured for the orthorhombic polymorph as compared to the mono-
clinic, and the absorption energy for indigo is similar and (iii) the C¼¼Oof
indigo form hydrogen bond with structural OH
2
. They concluded that indigo
is stabilized by irreversible encapsulation into the tunnels by high tempera-
ture diffusion, after the zeolitic H
2
O is lost. Fois et al. (2003) modelling also
suggested that indigo molecules are trapped into the tunnels by a strong
hydrogen bonds, stressing the role of the tight fit of the sorbate in the tun-
nels because only related guest–host interactions are not sufficient to explain
pigment stability. Giustetto et al. (2005) calculations, also including vibra-
tional frequencies, also supports this model, suggesting that C¼¼OH
bonds, the dye and the clay exist, but not NHO bonds. Tilocca and Fois
(2009) calculated electronic excitation spectra of the carbonyl group in
indigo and dehydroindigo interacting with several clay sites, and concluded
that the direct interaction indigo–cation is more important than the H bond
indigo–OH
2
, being the Al interaction responsible for the change in coloura-
tion. Indigo and dehydroindigo may coexist, but dehydr oindigo better
matches the experiment.
Chianelli et al. (2005) and Polette-Niewold et al. (2007) regarded that MB
is a ‘surface compound’ compatible with their DFT calculations and ab initio
model of the visible MB spectrum. In this model, the indigo bonds to the alu-
minium surface defects in the palygorskite, which holds metallic impurity
ions at a silicon site. Fuentes et al. (2008) used the same structural model with
Al replacing Si in palygorskite for reproducing the features of the MB visible
spectrum and explain ing the unusual stability of MB.
The impact of quantum chemistry computations in the search for a reason-
able structural and interaction model in MB is at present limited by the com-
plexity of palygorskite. The models used for palygorskite are quite simplified:
for example, the use of a fully magnesian (Fois et al., 2003) or magnesium-
less (Ferna
´
ndez et al., 1999) palygorskites. The suggested interaction of
indigo with Al
3þ
rather than with Mg
2þ
(Tilocca and Fois, 2009) is opposite
to the idea that the external octahedral sites are occupied by Mg
2þ
(Chryssi-
kos et al., 2009; Sua
´
rez and Garcı
´
a-Romero, 2006). Also, the fundamental
role assigned to tetrahedral Al
3þ
(Fuentes et al., 2008; Polette-Niewold
et al., 2007) does not seem consistent if one considers that almost 100% of
Al is octahedrally coordinated. Moreover, the simulation of palygorskite with
tunnels empty from zeolitic H
2
O contradicts the experimental evid ence that
only a mild heating (less than 100
C for 20–30 min; Reyes-Valerio, 1993),
unable to evacuate the whole zeolitic H
2
O (as revealed by FTIR), is sufficient
to stabiliz e the complex.
Developments in Palygorskite-Sepiolite Research466
3. THE SYNTHESES, PROPERTIES AND NATURE OF MB
3.1. The Synthesis of MB
We distinguish between two kinds of synthesis procedures of MB: methods
used by the Mayas and other Mesoamerican people with natural indigo, and
the syntheses performed with synthetic indigo, under controlled conditions.
Laboratory synthesis (van Olphen, 1966) used three techniques: (i) soak-
ing palygorskite with indoxylacetate; (ii) vat d yeing, where leucoindigo is
soaked with palygorskite in a water suspension with prolonged stirring and
(iii) crushing indigo with powdered palygorskite. Mixtur es are then heat trea-
ted from a few minutes (Reyes-Valerio, 199 3) to hours or days (van Olphen,
1966) for different temperature ranges [90–100
C (Reyes-Valerio, 1993),
190
C (Kleber et al., 1967) or even 250–300
C (van Olphen, 1966)]. Sp eci-
mens prepared from indigo and sepiolite (Hubbard et al., 2003; Ovarlez et al.,
2009; Sa
´
nchez del Rı
´
o et al., 2006a; van Olphen, 1966) and in colloidal sus-
pensions (Yasarawan and van Duijneveldt, 2008) were reported. Variations of
these modern recipes wer e proposed in Littmann (1982).
The properties of MB are influenced by parameters during the synthesis, such
as pH, temperature, duration of the heating process, indigo concentration and
morphology of the clay (after a more or less intensive crushing). Post-treatments
are applied to remove the excess of reactant: washing in acetone, acid washing,
Soxhlet extraction, etc. Ageing or light resistance test has been employed.
Although palygorskite is not available in synthetic grade, MB-like pigments using
sol–gel or crystallization of the mineral phase with indigo are being studied.
3.2. The Chemical Resistance of the Pigment
MB is remarkably stable to chemical attacks, as noted by Merwin and Gettens
(Gettens, 1962): many workers used this stability as ‘quality control’ for well-
prepared MB. ‘Gettens’ tests’ (Gettens, 1962; Littmann, 1980) consist of using
concentrated reactants (nitric, hydrochloric, sulfuric acids, aqua regia,sodium
hydroxide) at room temperature during 18 h, and then heating to test the chem-
ical stability. With few exceptions, the tests are neither quantified by concentra-
tion, temperature or duration of the attack, nor are they compared to the indigo
and clays contained in MB. Results are derived from a visual inspection of the
loss of pigment colour, not correlated to the molecular or crystallographic struc-
tures changes in the indigo and clay (chemical change is not always accompa-
nied by a change in colour). Sa
´
nchez del Rı
´
o et al. (2006a) studied
palygorskite–indigo and sepiolite–indigo pigments by applying acid attacks of
different intensity (varying concentration, duration and temperature). They
showed the relationship of colour change and crystallographic structure of the
synthetic MB pigment. Total destruction is possible at room temperature for
sepiolite pigments but requires intense heating (e.g. boiling in hydrochloric acid
for 30 h) for palygorskite pigments.
Chapter 18 The Maya Blue Pigment 467
Chemical stability between palygorskite–indigo and sepiolite–indigo pig-
ments together with the common use of palygorskite in archaeological MB
suggest that the name ‘Maya blue’ should only be applied to the palygors-
kite–indigo pigment (Arnold et al., 2008; Sa
´
nchez del Rı
´
o et al., 2006a).
Noneless, sepiolite–indigo interactions are sufficiently close to those of paly-
gorskite–indigo to suggest that indigo molecules attach to the clay matrix in a
similar way for palygorskite or sepiolite when forming MB-like materials
(Dome
´
nech et al., 2009a). However, the fact that indigo links to sepiolite only
at one side of its molecule also contribute to the weakness of sepiolite-based
pigments with respect to MB (Giustetto, 2010).
3.3. The Hue of MB
MB has a characteristic hue, a pale blue, with tonalities that may change from
greenish to turquoise. The colour is bright and thus attracts considerable attention
by archaeologists and researchers. The first hypothesis attributed the colour to an
unknown blue mineral of the palygorskite group. Then, the presence of indigo in
the MB was demonstrated, although Jose-Yacaman et al. (1996) reported iron
metal and iron oxide nanoparticles in MB. They suggested (see also Ferna
´
ndez
et al., 1999; Polette et al., 2002) that Mie-type light dispersion in nanoparticles
should produce the characteristic hue of MB. This hypothesis was later discarded
(e.g. Chiari et al., 2003; Fois et al., 2003; Giustetto et al., 2005; Hubbard et al.,
2003; Ovarlez et al., 2006; Reinen et al., 2004; Sa
´
nchez del Rı
´
oetal.,2004,
2005) and it is now agreed that the MB colouration is caused by bathochromic
shift of the indigo absorption bands resulting from the dye having an inorganic
support. Colour may also be related to the presence of secondary indigoid pro-
ducts, namely leucoindigo (Dome
´
nech et al., 2007b; Vandenabeele et al., 2005),
and indirubin in several MB samples (Dome
´
nech et al., 2007b). Recent results
from Dome
´
nech et al. (2011b) suggest that the ancient Mayas could have prepared
yellow pigments with dye plus palygorskite association similar to MB containing
isatin, dehydroindigo and/or ochre and possibly other minor organic compounds.
In the research of new MB-inspired pigments, Giustetto et al (2011) made a stable
red pigment encapsulating methyl red in palygorskite, and Dejoie et al. (2010)
demonstrated that indigo stabilizes when it is encapsulated in Silicalite.
The peculiar hue of MB and its variability are understood comparing the
visible spectra of MB specimens with indigo and dehydroindigo. In Figure 10,
the visible spectrum of MB from Mayapa
´
n (Yucata
´
n, Mexico) shows two
maxima at 425 and 570 nm. Indigo in ethanolic solution yields an absorption
band in the visible region at l
max
¼605 nm, whereas dehydroindigo produces
a band at l
max
¼440 nm. The spectrum of MB contains the superposition of
indigo and dehydroindigo absorption, both affected by a bathochromic shift
attributable to the interaction of the dyes with palygorskite. The variability
in the colour of MB specimens probably resu lts from the different dehydroin-
digo/indigo ratio in the palygorskite, which is controlled by the heating
Developments in Palygorskite-Sepiolite Research468
temperature (Dome
´
nech et al., 2006, 2007a). The formation of dehydroindigo
by aerobic oxidation of indigo (Dome
´
nech et al., 2006, 2007b) is favoured
with increasing temperature.
The hue of archaeological MB varies geographically and along the history.
Th
e blue in the Aztec zone is darker than in the Maya zone. Moreover, the green
pigment used in the Maya zone, like in Bonampak pain ts, contains palygorskite.
3.4. Structural Aspects: The Attachment of Indigo to the Clay
van Olphen (1966) suggested a structural mechanism of complex formation, by
indigo molecules attaching to surface channels of the clay. Kleber et al. (1967)
suggested that indigo penetrates (partially or deeper) into the tunnels in the clay
structure. Papers often support Kleber et al. (e.g. Chiari et al., 2003; Fois et al.,
2003; Giustetto et al., 2005, 2006; Tilocca and Fois, 2009) or van Olphen (e.g.
Chianelli et al., 2005; Chiari et al., 2008; Fuentes et al., 2008; Manciu et al.,
2007; Polette-Ni ewold et al., 2007). Hubbard et al. (2003) proposed that indigo
attaches to the entrance of the tunnels blocking the entrance. Vibrational and
diffraction data (Sa
´
nchez del Rı
´
o et al., 2009a) support this model.
The three possibilities, penetration of the molecule, covering the nanotun-
nels and attaching to the surface, may coexist depending on preparation. How-
ever, what is the cause of MB chemical stability?
Palygorskite and sepiolite may differ in the attachment of indigo. Pigments
made with these two clays present different resistance to acids (Sa
´
nchez del
Rı
´
o et al., 2006a), and they have different tunnel sizes. The dimensions of
the sepiolite tunnel, which are larger than in palygorskite, may allow for
greater diffusion. Palygorskite has tunnels of nearly the same width as the
0
10
20
30
40
50
60
70
450 550 650 750
Wavelen
g
th
(
nm
)
Abs. u.a.
350
FIGURE 10 Visible spectra for
indigo (diamonds), dehydroin-
digo (squares) and for a Maya
blue sample (triangles) from
Acanceh (Yucata
´
n, Mexico), late
classic period. (Dome
´
nech et al.,
2009d).
Chapter 18 The Maya Blue Pigment 469
indigo molecule. Steric arguments are compatible with the idea that indigo
penetrates in sepiolite (Dejoie, 2009; Giustetto and Wahyudi (2011); Ovarlez
et al., 2009) but blocks the entrances of the tunnels in palygorskite
(Dejoie, 2009; Hubbard et al., 2003; Sa
´
nchez del Rı
´
o et al., 2009a).
3.5. The Nature of the Palygorskite–Indigo Association
The proposed models for the interaction that anchors indigo to palygorskite
include (i) formation of hydrogen bonds between the C¼¼OandNHgroups
with edge silanol units of the clay (Hubbard et al., 2003), (ii) formation of hydro-
gen bonds between indigo molecules and structural OH
2
groups (Chiari et al.,
2003; Giustetto et al., 2005), (iii) hydrogen bond formation between indigo carbo-
nyls and structural OH
2
(Fois et al., 2003), (iv) direct bonding between the clay
octahedral cations and the dye molecules without H
2
OnorOH
2
(Chiari et al.,
2008; Tilocca and Fois, 2009), (v) specific bonding to Al substituted Si sites in tet-
rahedral centres (Chianelli et al., 2005; Fuentes et al., 2008; Polette-Niewold
et al., 2007). The possibility of significant Van der Waals interactions was intro-
duced by Fois et al. (2003) and further considered by Dome
´
nech et al. (2007c)
There are serious arguments against a direct dye–Al
3þ
interaction in MB.
The first is the possibility of formi ng stable aluminium-free MB-type pigments
with sepiolite. Additionally,
27
Al MAS NMR data sugges t that the coord ination
of Al
3þ
in palygorskite is not affected by indigo (Dome
´
nech et al., 2009a). Con-
sistent with NMR data, NIR data on AlAlOH stretching band at ca. 3620 cm
1
in MB probes do not find evidence of Al–indigo interaction (Sa
´
nchez del Rı
´
o
et al., 2009a) . In contrast, the
29
Si NMR spectrum of MB differs sign ificantly
from that of palygors kite. The
29
Si NMR spectrum suggests that electronic den-
sity increases near to (SiO
4
)
4
tetrahedra, thus indicating adsorption of indigo
near silicon atoms. This dye–silicon interaction may occur through surface
SiOH groups of clay and p bonds of dye. This particular bonding does not
seem crucial for inducing chemical stability to MB, and it may also be present
in indigo attached to laminar clays, explaining some similar features in their
Raman spectra compared to MB (Sa
´
nchez del Rı
´
o et al., 2006c).
MB can be regarded as a hybrid organic–inorganic material with polyfunc-
tional characterist ics where differ ent topological isomers of indigoid mole-
cules coexist attached to palygorskite (Dome
´
nech et al., 2009a,c).
4. MB RESEARCH IN RELATION WITH THE
ARCHAEOLOGICAL AND HISTORICAL CONTEXTS
4.1. Historic Relevance of Indigo
Indigo is a natural blue dye formed by indigotin (3H-indol-3-one, 2-(1,3-dihydro-
3-oxo-2H-indol-2-ylidene)-1,2-dihydro), a quasi-planar molecule of approximate
dimensions 4.812 A
˚
, containing a slightly elongated central CCbondand
two elongated CObonds.
Developments in Palygorskite-Sepiolite Research470
The technique of dyeing with indigo was discovered and extensively
applied independently by several cultur es using plants from the Indigofera
family: Indigofera tinctoria in India, Indigofera indigotica or tein-cheing in
China, Polygonum tinctorium in the Far East. In Europe, woad (Isatis tinc-
toria) was cultured from the twelfth to the seventeenth century, having an
important economic, political and cultural role (Harry, 1930). In the late sev-
enteenth century, the woad industry in Europe initiated the decline because
woad plant produces less indigo than the Indigofera plants, which were avail-
able at lower price from India and America. The synt hesis of indigo from isa-
tin was developed by Baeyer in 1870 (Baeyer and Emmerling, 1870) but a
more economical method was devised by Heumann in 1890 (Heumann,
1890a,b). Commercialization of synthetic indigo displaced indigo production
from plants in the late 1890s (McKee and Zanger, 1991).
The recipes used by the ancient Mesoamericans for dyeing with plants
(Herna
´
ndez, 1959) are related to water solutions. The Mesoamericans
obtained indigo from a maceration of leaves of plants (mainly Indigofera
suffruticosa) generically termed an
˜
il (in Spanish) or xiuquitlitl (in Nahuatl,
the Aztec language), followed by a prolonged aeration/stirring process termed
batido (Dome
´
nech et al., 2007c, 2009c). Indigo was traded in Mesoamerica
during the conquest, and great quantities of indigo were sent to Spain (Sarabia
Viejo, 1994). Indigo was prepared in pre-Colum bian times for being used as a
dye, but there are no references of its trade for the fabrication of MB pigment.
Possibly, a similar preparation with the addition of clays was used by the
ancient Mayas for the production of MB (Reyes-Valerio, 1993).
Electrochemical monitoring of indigo during preparation by traditional
procedures suggests that the hydrolysis of the indigo precursors may involve
two competing reac tion pathways, via intermediate formation of leucoindigo
or isatin (Dome
´
nech et al., 2007b). For Indigofera leaves and a suspension
of palygorskite, leucoindigo should be favoured with the use of an alkaline
quick-lime suspension, because this product, contrary to indigo, is slightly sol-
uble in alkaline media (Dome
´
nech et al., 2009d).
4.2. Palygorskite in Contemporary and Ancient Mesoamerica
The use of MB disappeared after the Spanish conquest, and there is no written
record describing its fabrication and use. It has been argued that MB
continued to be used in contemporary communities. Moreover, some aspects
of the use and significance of MB may have been transmitted orally. The link
between palygorskite and Maya culture was first established by the ethno-
graphic work of Arnold in Yucata
´
n (Arnold, 1967, 1971). Palygorskite was
mined, used and traded among the contemporary Yucatec Maya in Ticul
and Sacalum (Arnold, 1967, 1971; Arnold and Bohor, 1975; Folan, 1969)
for pottery temper and for medicinal purposes. Two proba ble ancient mines
for the mineral were suggested, one at the cenote in Sacalum (Arnold, 1967;
Chapter 18 The Maya Blue Pigment 471
Arnold and Bohor, 1975; Folan, 1969) and a second at Yo’ Sah Kab near
Ticul (Arnold, 2005). Palygorskite occurs elsewhere in the Yucata
´
n peninsula
(Arnold et al., 2007; Sa
´
nchez del Rı
´
o et al., 2009b ), but not beyond, except in
samples from Pete
´
n (Guatemala; Arnold et al., 2007). Arnold et al. (2007)
described two hypothesis for palygorskite provenance in MB: (i) The
Shepard/Arnold/Bohor hypothesis, where MB was widely traded from the
Yucata
´
n Peninsula because of its widespread use on pottery intended for
household rituals. Sacalum and Yo’ Sah Kab could be the sources of palygors-
kite. (ii) The Littmann hypothesis: MB was derived from local palygorskite
deposits and the synthesis technique moved rather than the pigment itself.
Sa
´
nchez del Rı
´
o et al. (2009b) found palygorskite of great purity in several
sites 40 km around Uxmal, suggesting that palygorskite is a frequent mineral
in the area as discussed in Arnold et al. (2007) and Littmann (1982). Palygors-
kites within this area have similar crystallographic characteristics, but varia-
tion in trace element concentrations may be used to obtain chemical
fingerprints of the different sources (Arnold et al., 2007). Krekeler and Kearns
(2009) found palygorskite ca. 200 km from Uxmal in the south-eastern
Yucata
´
n Peninsula. Arnold collected samples with the cultural characteristics
of palygorskite, suggesting a source of paly gorskite in Guatemala. Cecil
(2010) determined that pigment from Ixlu (El Peten, Guatemala) has the
MB structure, but suggesting a local manufacture from clays in central Peten.
Although pigments found in Guatemala are MB, no XRD studies on the pris-
tine clays have been done. Thus, the occurrence of palygorskite in Guatemala
has not been fully demonstrated yet.
4.3. Sepiolite and MB
Sepiolite has occasionally been reported in Yucata
´
n(IsphordingandWilson,
1974; Stinnesbeck et al., 2004), but not in surface outcrops. Recent studies using
Yucatecan palygorskites (Chiari et al., 2003; Pablo-Gala
´
n, 1996; Sa
´
nchez del Rı
´
o
et al., 2009b) did not identify sepiolite. However, sepiolite may occur with
palygorskite in archaeological MB (Gettens, 1962; Shepard and Gottlieb, 1962).
Shepard (1962) found a related mineral in Aztec MB, probably sepiolite. Ortega
Avile
´
s (2003) found palygorskite together with sepiolite in a sample from The
Great Temple in Tenochtitlan.
Sepiolite is only found in archaeological samples from the Mexico-
Tenochtitlan area, corresponding to the Aztec Empire, in agreement with
Shepard (Shepard and Gottlieb, 1962): “It is noteworthy that sepiolite has
not yet been found in any Yucatecan or Mayan sample.”
4.4. The Production and Use of MB in Ancient Times
Indigo may have been used as an intermediate ingredient from Indigofera
leaves, then mixed wi th palygorskite to obtain MB. Alternatively, MB may
Developments in Palygorskite-Sepiolite Research472
have been processed directly with paly gorskite and Indigofera leaves (Reyes-
Valerio, 1993), a recipe proposed from interpretations of historical docu-
ments. Reyes-Valerio demonstrated that a pigment with the characteristics
of MB can be obtained by macerating Indigofera leaves in a clay–water solu-
tion, and this method may have been used by the Maya.
Cabrera Garrido (1969) suggested other possible preparations: (i) direct
dyeing of palygorskite using the colourant from the Indigofera plant, (ii) heat-
ing a mixture of palygorskite and indigo, (iii) heating indigo with water
vapour for fixation on palygorskite, (iv) using a vat-dyeing technique, (v)
burning Indigofera leaves and mixing the ashes with other ingredients, (vi)
mixing indigo with palygorskite and heating to adulterate indigo and (vii) rit-
ual ceremonies by burning ingredients like Copal. Copal is a resin widely
used in Prehispanic times. Cabrera detected copal in a sample from Tlatelolco
and determined that fabrication involved heating, which may have been
related with rituals like burning Copal. This idea was recently developed by
Arnold et al. (2008).
Dome
´
nech et al. (2007a) compared the electrochemical response, compo-
sition (from SEM/EDX) and UV–vis spectral properties of samples from 12
archaeological sites of Yucata
´
n and Campeche. The electrochemical
responses provide dehydroindigo/indigo ratios, and mineralogy and textural
properties of the samples can be corr elated with spectral and compositional
signatures. Samples are categorized into ‘electrochemica l types’ that suggest
(i) different procedures for the preparation of MB were used; (ii) these proce-
dures evolved from not using therm al treatment, to moderate and more inten-
sive heat ing; (iii) ochre and other pigments were possibly used during the
crushing/thermal treatment process; (iv) the variation of MB types along time
corresponds to an evolution of fabrication technique, where new processes
were incorporated progressively showing a ramified scheme as a function
of time.
The fabrication of MB and use of colo urs, as well as all the arts, and
nearly all social life in Mesoamerican civilization were under strict religious
control (Reyes-V alerio, 1993). The artist that created codices, sculptures, tem-
ples and pyramids was educated in the different institutional centres. The pos-
sibility that MB production was done in a ritual context was developed by
Cabrera Garrido (1969) and Arnold et al. (2008). It is unclear how artists man-
aged to obtain the different tonalities and hues.
Optical microscopy examination of cross sections of paint layers shows
each MB layer as an apparently homogeneous blue or greenish-blue layer.
According to Dome
´
nech et al. (2006, 2007a, 2009c), the modulation of the
hue of the pigment from dark blue to more or less greenish-blue could be
obtained by varying the dehydroindigo/indigo ratio upon varying the temper-
ature treatment during the preparation of the pigment. In sever al samples, yel-
low aggregates were added to MB pigment to obtain a more greenish colour
(Dome
´
nech et al., 2007a, 2009d). The Mg concentration in a mural sample
Chapter 18 The Maya Blue Pigment 473
from Cacaxtla with two blue hues indicates the proba ble mixing of the pig-
ment with other products (lime water, i.e. calcite; Sa
´
nchez del Rı
´
o et al.,
2004). Another control of the tone is the depth of the pigment layer (one or
several layers). Layer s of different colours are often visible in mural paints
(Reyes-Valerio, 1993). However, multilayers of the same pigment cannot be
excluded, but they are more difficult to detect because there are no composi-
tional differences.
4.5. Trade and Distribution and of MB in Ancient Times
Gettens (1962) gave several reasons to the study of MB: (i) “Maya blue might
have been an important item of trade among the preconquest Maya”, (ii) “the
source, preparation and use of Maya blue are of importance to the history of
technology”, (iii) “it is not yet certain that the Maya blue disappeared after the
conquest”. In addition, if MB is to be used as evidence that artefacts are
true antiquities, then the history of MB must be known. If MB was traded
(Littmann, 1980), then MB “is archaeologically significant not only in demon-
strating trade or cultural contacts, but also in determining whether these contacts
consisted of trade in a material substance or the transmission of a technique. The
difference represents the distinction between discovery and invention”.
There is crystallographic evidence that MB from Central Mexico used a
palygorskite that differs from the Yucatecan palygorskite (Sa
´
nchez del Rı
´
o
et al., 2009b). This is based on the fact that Yucatecan palygorskite is near-
equal mixture of monoclinic and orthorhombic palygorskite (Chiari et al.,
2003; Sa
´
nchez del Rı
´
o et al., 2009b). In contrast, diffractograms from archae-
ological MB from Central Mexico indicate mostly orthorhombic palygorskite
(Sa
´
nchez del Rı
´
o et al., 2 009b). In addition, MB from Central Mexico may
contain sepiolite. Interestingly, Zaachila blue, a variety of MB found in a
tomb explored in 1962 by the archaeologist R. Gallegos in Oaxaca, produces
a diffractogram (Christ et al., 1969) in good agreement of palygors kite used in
Aztec MB (Sa
´
nchez del Rı
´
o et al., 2008). From these data, it appears that the
Aztecs exploited a palygorskite mine different from those of Yucata
´
n (Sa
´
nchez
del Rı
´
o et al., 2009b).
4.6. Chronology and Distribution of MB
The mural paintings of Bonampak (Classic period, eighth century) are consid-
ered the oldest accurately dated artworks containing MB (Reyes-Valerio,
1993). MB is abundant in archaeological finds from the beginning of Maya
Late Classic (ca. sixth century) in the Usumancinta, Puuc and Peten regions,
as well as in Guatemala (Cecil, 2010). Outside the Maya area, MB has been
found in Mexico in El Tajin (Veracruz, Totonac culture), Cacaxtla (Tlaxcala,
Olmec-Xicalanca culture, see Figure 1), Tamuin (San Luis Potosı
´
, Huastec cul-
ture), Zaachila (Oaxaca) and is very abundant in the Aztec area. However, MB
Developments in Palygorskite-Sepiolite Research474
has not been found in important sites like Teotihuacan, Monte Alban or Xochi-
calco, indicating the late arrival in the Mexican plateau (Reyes -Valerio, 1993).
The age of MB has been revised recently, with a possible earlier use in Yaxchi-
lan (Torres et al., 1976) or possibly earlier, in different locations in the archaeo-
logical site of Calakmul, belonging to the Late Preclas sic (Dome
´
nech et al.,
2006; Va
´
zquez de Agredos et al., 2011) and Early Classic (Garcı
´
a Moreno
et al., 2008).
MB was used in Colonial times for decorating convents and churches
(Reyes-Valerio, 1993), at least until the end of the sixteenth century (see Fig-
ure 3). Noteworthy, occurrences include Actopan (Hidalgo), Tecamachalco
(Puebla), Epazoyucan (Hidalgo), San Pedro Tezontepec (Hidalgo) and Jiute-
pec (Morelos). Comparative studies of Precolumbian MB and Colonial MB
(Sa
´
nchez del Rı
´
o et al., 2006b) may be of importance to determine how pig-
ment technology was transferred during the Conquest. Reyes-Valerio showed
that Spanish monks used Indian artists for the decoration of chapels. Juan
Gerso
´
n, an Indian artist, made numerous paintings in Tecamachalco (Puebla;
Reyes-Valerio, 2000) in 1562. Because MB pigment was used frequently
during the first century of the Conquest in religious decorations of the three
important Monastical orders (Augustinians, Dominicans and Franciscans),
the monks proba bly could know how to fabricate MB. Possibly, the technol-
ogy was controlled by a few Indian artists, but the number of convents and
the total painted surface (> 400 m
2
) suggested that the technology had been
acquired by the religious hierarchy.
A variety of MB, known as La Havana blue, decorated civil buildings in
Colonial Cuba (Tagle et al., 1990) during the seventeenth and eighteenth cen-
turies. MB was last used in Mexican colonial mural paints around 1580. It is
unclear if the Cuban MB came from Mexico, or if it was fabricated in situ.
MB probably came from Mexico, because of the small distance that separates
Yucata
´
n from Cuba, and because palygorskite is unknown in Cuba. X-ray data
of Cuban blue (Tagle et al., 1990) are identical to Yucatecan palygorskite
(Sa
´
nchez del Rı
´
o et al., 2009b). The time of first import to Cuba is unknown,
and how MB evolved from religious to civil decorations is uncertain. Because
MB must have coexisted in Mexico and Cuba suggests that either it was used
in Mexico after the end of the sixteenth century, or it was imported to Cuba
earlier. It is probable that Cuba obtained the recipe in colonial times, where
it is known that slave markets and trade exist between Cuba and Yucata
´
n.
4.7. Symbology of MB
Colours were essential in the Prehispa nic world. However, the colour symbol-
ogy is different for the Maya than for the cultures in the valley of Mexico
(Reyes-Valerio, 1993). The Maya associated the blue to several meanings,
among them, the sacrifice. Based on Landa (1566), this is noted by Thomson
(Gan and Thompson, 1931) “Before being sacrificed the victim was stripped
Chapter 18 The Maya Blue Pigment 475
of his clothing and painted with a blue unguent, blue being the sacrificial
color”, and lately by Gettens (1962). Blue is associated also with water and
rain, and therefore related to several deities, for example, god of rain for both
Mayas (Chaac) and Aztecs (Tlaloc). Colour s are also related to the Cardinal
Points by the peoples of Central Mexico, and blue is sometimes related to
West (Reyes-Valerio, 1993).
ACKNOWLEDGEMENTS
MSR dedicates this work to the memory of Constantino Reyes-Valerio (1922–
2006), historian, chemist, photographer, who made great contributions to the
study and popularization of MB. His guidance and dedication were essential
for the accomplishment of several of the works cited here. S. Guggenheim
is acknowledged for a pertinent review and useful editing. Financial support
by CICYT (project CGL2009-10764 ) is acknowledged.
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Chapter 18 The Maya Blue Pigment 481
