Doménech, A. et al. Study blue maya. 2006

8/3/2019 Doménech, A. et al. Study blue maya. 2006

http://slidepdf.com/reader/full/domenech-a-et-al-study-blue-maya-2006 1/10

Chemometric Study of Maya Blue from theVoltammetry of Microparticles Approach

Antonio Domenech,*,† Marıa Teresa Domenech-Carbo,‡ andMarıa Luisa Vazquez de Agredos Pascual‡,§

Departament de Quı mica Analı tica, Universitat de Vale `ncia, Dr. Moliner, 50, 46100 Burjassot, Vale `ncia, Spain, Institut de Restauracio ´ del Patrimoni/Departament de Conservacio ´ i Restauracio ´ de Bens Culturals, Universitat Polite `cnica de Vale `ncia,Camı de Vera 14, 46022, Vale `ncia, Spain, and Departament de Histo `ria de l’Art, Universitat de Vale `ncia, Passeig al Mar,Vale `ncia, Spain

The use of the voltammetry of microparticles at paraffin-

impregnated graphite electrodes allows for the character-

ization of different types of Maya Blue (MB) used in wall

paintings from different archaeological sites of Campeche

and Yucata ´ n (Mexico). Using voltammetric signals for

electron-transfer processes involving palygorskite-associ-

ated indigo and quinone functionalities generated by scratching the graphite surface, voltammograms provide

information on the composition and texture of MB samples.

Application of hierarchical cluster analysis and other

chemometric methods allows us to characterize samples

from different archaeological sites and to distinguish

between samples proceeding from different chronological

periods. Comparison between microscopic, spectro-

scopic, and electrochemical examination of genuine MB

samples and synthetic specimens indicated that the

preparation procedure of the pigment evolved in time via

successive steps anticipating modern synthetic proce-

dures, namely, hybrid organic-inorganic synthesis, tem-

perature control of chemical reactivity, and template-like

synthesis.

Maya Blue (MB), a pigment produced by the ancient Mayas,

has claimed considerable attention because of its enormous

stability and peculiar hue, ranging from a bright turquoise to a

dark greenish blue. The use of MB in Central America, mostly in

Mexico, is well documented in archaeological sites between the

8th and the 16th centuries,1 but its use is prolonged even to recent

times.2

MB can be considered as a hybrid organic-inorganic material3

resulting from the attachment of indigo, a blue dye extracted from

Indigofera suffruticosaand other species, and a local clay, palygor-

skite.4,5 A Yucatan site, Sak lu’um, served as the main and almost

unique source of palygorskite in Mesoamerica. Natural indigo is

formed by indigotin (3 H -indol-3-one, 2-(1,3-dihydro-3-oxo-2 H -indol-

2-ylidene)-1,2-dihydro), a quasi-planar molecule of approximate

dimensions 4.8 × 12 Å.6 The inorganic component of MB,

palygorskite, is a fibrous phyllosilicate of ideal composition (Mg,-

Al)4Si8(O,OH,H2O)24‚nH2O. Palygorskite appears usually as a

mixture of two different polytypes, one monoclinic and one

orthorhombic, whose structure can be described in terms of continuous, two-dimensional tetrahedral and octahedral sheets.7,8

Both polytytpes show discontinuous 2:1 tetrahedral/octahedral

layers with one 2:1 unit joined to the next by inversion of the

SiO4 tetrahedra along Si-O-Si bonds. The tetrahedral and

octahedral mesh gives rise to a series of rectangular tunnels of

dimensions 6.4 × 3.7 Å. Such clays are, therefore, crossed by

zeolite-like channels and permeated by weakly bound, nonstruc-

tural (zeolitic) water. Magnesium and aluminum cations complete

their coordination with tightly bound water molecules (structural

water).

Although the procedure of extraction of indigo from plants has

remained in use until current times, the procedure for preparing

MB is unknown.1,4,5 It is believed that the Mayas prepared the

pigment by crushing indigo and palygorskite with a moderate

thermal treatment, but the details of the process and the way in

which the ancient Mayas modulated the hue of the pigment have

not been elucidated.

The nature of the indigo-palygorskite association has claimed

considerable attention in the last years.9-22 Thus, Jose-Yacaman

et al.9,10 discovered that most palygorskite particles exhibit a

superstructure along the a-axis about 14 Å, which roughly

* Corresponding author. E-mail: [email protected].† Departament de Quımica Analıtica, Universitat de Valencia.‡ Universit at Politecnica d e Valencia.§ Departament de Historia de l’Art, Universitat de Valencia.

(1) Magaloni Kerpel, D. Materiales y Tecnicas de la Pintura Maya. Facultad de

Filosofıa y Letras, Universidad Nacional Autonoma de Mexico, 1996.

(2) Tagle, A.; Paschinger, H.; Richard, H.; Infante, G. Stud. Conserv. 1990, 35 ,

156-159.

(3) Romero, P.; Sanchez, C. New J. Chem. 2005, 29 , 57-58.

(4) Torres, L. M. Mater. Res. Soc. Symp. Proc. 1988, 123.

(5) Reyes-Valerio, C. De Bonampak al Templo Mayor: el azul Maya en

Mesoamerica; Siglo XXI, Madrid, 1993.

(6) Gordon, P. F.; Gregory, P. Indigoid Dyes in Organic Chemistry in Colour ;

Springer-Verlag: Berlin 1983; pp. 208-211.

(7) Chisholm, J. E. Can. Miner. 1990, 28, 329-339.(8) Chisholm, J. E. Can. Miner. 1992, 30 , 61-73.

(9) Jose-Yacaman, M.; Rendon, L.; Arenas, J. ; Serra Puche, M. C. Science1996,

273, 223-225.

(10) Polette, L. A.; Meitzner, G.; Jose-Yacaman, M.; Chianelli, R. R. Microchem.

J. 2002, 71, 167-174.

(11) Hubbard, B.; Kuang, W.; Moser, A.; Facey, G. A.; Detellier, C. Clays Miner.

2003, 51, 318-326.

(12) Chiari, G.; Giustetto, R.; Riccihiardi, G. Eur. J. Mineral. 2003, 15 , 21-33.

(13) Fois, E.; Gamba, A.; Tilocca, A. Microporous Mesoporous Mater. 2003, 57 ,

263-272.

(14) Witke, K.; Brzezinka, K.-W.; Lamprecht, I. J. Mol. Struct. 2003, 661-662 ,

235-238.

(15) Reinen, D.; Kohl, P.; Muller, C. Z. Anorg. Allg. Chem. 2004, 630 , 97-103.

Anal. Chem. 2007, 79, 2812-2821

2812 Analytical Chemistry, Vol. 79, No. 7, April 1, 2007 10.1021/ac0623686 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 03/08/2007



corresponds to three times the original lattice constant and might

well be explained by the presence of indigo in the channels. Such

authors reported the presence of iron nanoparticles outside the

lattice of the crystallites of palygorskite as well as inside the

channels accompanying iron oxide and an amorphous phase of

FeO(OH) in MB samples10 and suggested that light dispersion

on metal nanoparticles contributes significantly to the optical

properties of the pigment. Sanchez del Rıo et al.,14,16,19 however,

did not find neither iron in metallic form nor goethite in

archaeological MB while Chiari et al.,12

Reinen et al.,15

andGiustetto et al.,17,18 suggested that the MB hue is mainly attribut-

able to bathochromic shift of the indigo spectrum originated when

single indigo molecules become attached to the palygorskite

matrix. Molecular modeling and spectral data indicate that

hydrogen bonding between indigo and the hydroxy groups in the

palygorskite channels12,13,22 or edge silanol groups11 occurs.

In previous reports, we have studied the solid-state electro-

chemistry of genuine MB samples in contact with aqueous23 and

nonaqueous24 electrolytes using the voltammetry of microparticles

approach. This methodology, developed by Scholz et al.,25,26

provides information on the composition of solid micro- and

nanosamples of solid materials. Application of solid-state electro-

chemistry combined with electron microscopy, atomic forcemicroscopy, FT-IR, and visible spectroscopies and nuclear mag-

netic resonance data allowed us to conclude that not only indigo

but also its oxidized form, dehydroindigo, is attached to palygor-

skite in MB, contributing significantly to modulate the hue of the

pigment.23 The voltammetry of microparticles allowed detection

of the presence of MB in wall paintings of the substructure IIC in

the archaeological site of Calakmul, dated in the Early Classical

period (440-450 B.C.), thus providing evidence on the use of this

pigment 700-750 years prior to the date currently accepted.23

In order to explore the capabilities of the voltammetry of

microparticles for acquiring analytical information, a chemometric

study was performed on a series of 32 MB samples from 12

archaeological sites in Yucatan (Chacmultun, D’zula, Ek Balam,

Acanceh, Kuluba, and Mulchic, all of the Late Classical period,

Chichen Itza, corresponding to the Terminal Classical period, and

Mayapan, dated at the Postclassical period) and 3 sites in

Campeche (Dzibilnocac and El Tabasqueno, Late Classical period,

and the substructure A-6 of the site of Calakmul, corresponding

to the Early Classical period, and the substructure II-C of the same

archaeological site, dated in the Late Preclassical period). The

response in contact with aqueous acetate buffer of genuine MB

samples was compared with that of synthetic specimens prepared

by crushing palygorskite and indigo under different thermal

treatments.

Multivariate chemometric methods have been applied to

voltammetric data for MB samples and synthetic specimens in

order to identify possible regularities in the electrochemicalresponse of such material and obtain analytical information

concerning two unsolved questions: (i) the method of preparation

of the pigment by the ancient Mayas and (ii) the existence of a

possible geographical or chronological variation in the Maya Blue

technology. Multivariate statistical techniques, including pattern

recognition and hierarchical cluster analysis have claimed con-

siderable attention for analytical purposes. Such methods have

been recently applied to different electroanalytical problems.27-29

EXPERIMENTAL SECTION

Microsamples ( ∼1 µg) and eventually nanosamples ( ∼20-50

ng) of MB were taken from blue or greenish-blue regions of wall

paintings in the listed archaeological sites, with the help of a microscalpel, during their routine examination and restoration.

Figure 1 shows a typical Maya wall painting. A light microscope

Leica DMR ( ×25-×400) was used for selecting the samples to

be analyzed and for morphological examination of them.

Palygorskite was collected from the Sak lu’um classical

deposits in Yucatan. Synthetic indigo (Fluka) was used as a

reference material. To mimic possible preparation procedures of

MB employed by the ancient Mayas, a series of synthetic

specimens (S-1 to S-6) were prepared by crushing palygorskite

with synthetic indigo (3% w/w) plus calcite (10% w/w) and heating

at temperatures of 25, 70, 160, 260, 320. and 400 °C for periods of

12 h. With except sample S-6, which acquired a brown hue, all

Maya Blue synthetic specimens became greenish-blue, the green-

ish hue increasing on increasing temperature along the S-1 to S-5

series. After thermal treatment, a portion of the specimens was

modified by adding ochre from Sak lu’um (10% w/w), (series SC-1

to SC-6). A third series of synthetic specimens (ST-1 to ST-6) were

heated at 200 °C during 1, 2, 4, 6, 12, and 24 h. A fourth series

(SO-1 to SO-6) was prepared by adding ochre and then heating

at the above listed temperatures.

Pictorial samples were examined with a Jeol JSM 6300

scanning electron microscope operating with a Link-Oxford-Isis

X-ray microanalysis system. The analytical conditions were as

follows: accelerating voltage 20 kV, beam current 2 × 10-9 A,

and working distance 15 mm. Samples were carbon coated to

eliminate charging effects. Quantitative microanalysis was carried

out using the ZAF method for correcting interelemental effects.

The counting time was 100 s for major and minor elements.

Voltammetry of microparticles experiments were performed

using paraffin-impregnated graphite electrodes (PIGEs) immersed

into acetic acid/sodium acetate buffer (total acetate concentration

(16) Sanchez del Rıo, M. S.; Martinetto, P.; Somogyi, A.; Reyes-Valerio, C.;

Dooryhee, E.; Peltier, N.; Alianelli, L.; Moignard, B.; Pichon, L.; Calligaro,

T.; Dran, J.-C. Spectrochim. Acta, B 2004, 59 , 1619-1625.

(17) Vandenabeele, P.; Bode, S.; Alonso, A.; Moens, L. Spectrochim. Acta, A 2005,

61, 2349-2356.

(18) Chianelli, R. R.; Perez de la Rosa, M.; Meitzner, G.; Siadati, M.; Berhault,

G.; Mehta, A.; Pople, J.; Fuentes, S.; Alonzo-Nunez, G.; Polette, L. A. J.

Synchrotron Radiat. 2005, 12 , 129-134.

(19) Sanchez del Rio, M. S.; Sodo, A.; Eeckhout, S. G.; Neisius, T.; Martinetto,

P.; Dooryhee, E.; Reyes-Valerio, C. Nucl. Instrum., Methods Phys. Res., Sect. B 2005, 238, 50-54.

(20) Giustetto, R.; Llabres i Xamena, F. X.; Ricchiardi, G.; Bordiga, S.; Damin,

A.; Gobetto, R.; Chierotti, M. R. J. Phys. Chem. B 2005, 109 , 19360-19368.

(21) Sanchez del Rıo, M.; Martinett o, P.; Reyes-Valerio, C.; Dooryhee, E.; Suarez,

M. Archaeometry 2006, 48, 115-130.

(22) Giustetto, R.; Levy, D.; Chiari, G. Eur. J. Mineral. 2006, 18, 629-640.

(23) Domenech, A.; Domenech, M. T. ; Vazquez, M. L. J. Phys. Chem. B 2006,

110 , 6027-6039.

(24) Domenech, A.; Domenech, M. T.; Vazquez, M. L. J. Phys. Chem. C , submitted.

(25) Scholz, F.; Meyer, B. In Electroanalytical Chemistry , A Series of Advances;

Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998; Vol. 20,

pp 1-87.

(26) Grygar, T.; Marken, F.; Schroder, U.; Scholz, F. Collect. Czech. Chem.

Commun. 2002, 67 , 163-208.

(27) Scampicchio, M.; Mannino, S.; Zima, J.; Wang, J. Electroanalysis 2005, 17 ,

1215-1221.

(28) Gutes, A.; Ibanez, A. B.; Cespedes, F.; Alegret, S.; del Valle, M. Anal. Bioanal.

Chem. 2005, 382 , 471-476.

(29) Richards, E.; Bessant, C.; Saini, S. Electroanalysis 2002, 14, 1533-1542.

Analytical Chemistry, Vol. 79, No. 7, April 1, 2007 2813



0.50 M, pH 4.85). PIGEs consisted of cylindric rods of 5-mm

diameter prepared as described by Scholz et al.25,26 Measurements

were performed in a thermostated three-electrode cell under argon

atmosphere using a AgCl (3 M NaCl)/Ag reference electrode anda platinum wire auxiliary electrode. Square wave voltammograms

(SQWVs) were obtained with CH I420 equipment. Statistical

analyses were performed with the Minitab 14 software package.

For modified electrode preparation, the samples were ac-

curately powdered in an agate mortar and pestle and extended,

forming a spot of finely distributed material. The lower end of

the graphite electrode was pressed over that spot of sample to

obtain a sample-modified surface.

Transmission electron microscopy (TEM) was carried out with

a Philips CM10 with Keen view camera. Soft imaging system was

used operating at 100 kV. MB samples were prepared by grinding

a few micrograms of the samples in an agate mortar and then

dispersing them by the help of an ultrasonic bath in CH 2Cl2. A

drop of the dispersion was poured on TEM grids pretreated with

a polymer film layer with holes in order to improve the quality of

the images.

FT-IR-ATR spectra of samples and reference materials were

obtained with a Vertex 70 Fourier transform infrared spectrometer

with a fast recovery deuterated triglycine sulfate temperature-

stabilized coated detector. Number of coadded scans, 32; resolu-

tion, 4 cm-1.

Visible spectra were obtained with a Minolta CM-503i spec-

trophotometer using a Xe arc lamp and a Si photodiode detector.

The instrument was calibrated with a standard white (chrom

coordinates Y ) 95.8, x ) 0.3167, y ) 0.3344).

RESULTS AND DISCUSSION Voltammetry of Maya Blue Samples. In Figure 2 is com-

pared the SQWV response of PIGEs modified with (a, d) indigo

microparticles and (b, c) Maya Blue sample from the substruc-

ture II-C of Calakmul. This last corresponds to the oldest Maya

Blue sample, dated in the Late Preclassical period. In both cases,

two indigo-localized electrochemical processes, characterized by

sharp peaks at +475 (I) and -285 mV (II), occur. These processes

correspond to the almost-reversible two-electron, two-proton

oxidation of indigo to dehydroindigo and the two-electron, two-

proton reduction of indigo to leucoindigo,30-32 depicted in Scheme

1. These processes can be represented as

where { } represents palygorskite-anchored species. Notice that

charge conservation is ensured in both cases by the expulsion/

insertion of two protons and two electrons from/to the solid.

(30) Bond, A. M.; Marken, F.; Hill, E.; Compton, R. G.; Hugel, H. J. Chem. Soc.,

Perkin Trans. 2 1997, 1735-1742.

(31) Grygar, T.; Kuckova, S.; Hradil, D.; Hradilova, D. J. Solid State Electrochem.

2003, 7 , 706-713.

Figure 1. Image of Maya wall paintings in the substructure I of Calakmul (Campeche, Mexico), Early Classical Maya period.

{H2IN}f

{IN} + 2H+

(aq) + 2e-

(1)

{H2IN} + 2H+ (aq) + 2e-f {H4In} (2)

2814 Analytical Chemistry, Vol. 79, No. 7, April 1, 2007

http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/ac0623686&iName=master.img-000.jpg&w=475&h=356



Current data confirmed previous results,23 showing that the

relative intensity of peaks I and II differs substantially from that

determined for indigo microparticles, a feature attributable to the

presence of a significant amount of dehydroindigo (5-15%)

accompanying indigo in MB. Coupled electrochemical/AFM

experiments indicate that extensive electrooxidation of the play-

gorskite-indigo complex produces a significant alteration in the

clay structure.24

Apart from indigo-centered peaks I and II, voltammograms of

MB exhibit two peaks at +200 (III) and -15 mV (IV). These last

two peaks, although with different peak profile, were also recorded

in blank experiments performed with PIGEs modified with pristine

palygorskite.

As can be seen in Figure 3, the profile of voltammetric peaksIII and IV varied significantly from one sample to another.

Examination of SQWVs for the studied samples allowed us to

distinguish four main morphological types, labeled a -d. Chemo-

metric analysis (vide infra) allowed us to distinguish five subtypes

(A1- A5) within the type A. Representative voltamograms are

presented in Figure 2 for MB samples from (a) Chacmultun (type

A3), (b) El Tabasqueno (type B), (c) D’zula (type C), and (d)

Mayapan (type D). Type A4, corresponding to the oldest MB

sample, is shown in Figure 1, while Figure 4 shows SQWVs of

samples from (a) Acanceh (type A1), (b) El Tabasqueno (type

A2), and (c) Dzibilnocac (type A5). These voltammograms were

taken as “holotypes” of the groups of MB samples. Diagnostic

criteria for grouping voltammetric profiles are provided as Sup-porting Information.

In agreement with literature data,33-37 peaks III and IV can be

attributed to quinone functionalities produced in the graphite

surface. Such processes are promoted by scratching of the

graphite surface, thus yielding relatively intense voltammetric

peaks. The observed differences in the shape of peaks III and IV

is attributable to the differences in the mineralogy and textural

properties of the sample, so that the variability in their profile

should be representative of differences, not only in mineralogy,

but also in the mechanical properties of the sample. Since these

differences may result from differences in the preparation proce-

dure, the analysis of voltammetric profiles can provide information

on the procedure of preparation of MB.Chemometrics. Chemometric analysis was performed by

defining eight quantitative parameters representative of the shape

of the voltamograms. On first examination, peak potentials, peak

currents, and peak areas were used. The variables concerning

(32) Domenech, A.; Domenech, M. T. J. Solid State Electrochem. 2006, 10 , 459-

468.

(33) Evans, J. F.; Kuwana, T. Anal. Chem. 1977, 49 , 1632-1635.

(34) Gunasingham, H.; Fleet, B. Analyst 1982, 107 , 896-902.

(35) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56 , 136-141.

(36) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. Electrochim. Acta 2000,

46 , 509-517.

(37) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. J. Electroanal. Chem. 2000,

488, 92-98.

Figure 2. SQWVs of (a, c) indigo microparticles and (b, d) Maya

Blue sample from Calakmul, substructure II-C, immersed into 0.50M acetate buffer, pH 4.85. (a, b) Potential scan initiated at +0.85 V

in the negative direction; (c, d) potential scan initiated at -750 mV inthe positive direction. Potential step increment 4 mV; square wave

amplitude 25 mV; frequency 5 Hz.

Scheme 1. Electrochemistry of Indigo (H2IN)

Oxidation to Dehydroindigo (IN) and Reduction

to Leucoindigo (H4IH)


http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/ac0623686&iName=master.img-002.png&w=226&h=143




peak current and peak area were found to be strongly coupled, a

common situation in chromatography, voltammetry, and electro-

phoresis,27 so that in subsequent data handling only peak currents

were used. Since absolute peak currents varied slightly from one

sample to another, peak current ratios were used. Pertinent data

for the holotypes are summarized in Table 1.

Different two-variable diagrams were constructed for several

pairs of electrochemical parameters. This can be seen in Figure

5, in which the peak III/peak current IV ratio, I (III)/ I (IV ), is

plotted as a function of the separation between the peak potential

of peaks III and IV, E p(III)- E p(IV). As can be seen in this figure,

data points for the different main morphological types (A -D)

cover different regions of the diagram but subtypes A1- A5

become overlapped. The principal component method provided

similar results, thus reinforcing the idea that the different

electrochemical types correspond to differently prepared MB

samples. Distribution of samples into the different electrochemical

types suggests that there is a chronological variation in the

properties of MB, as can be seen in Table 2 (see also Supporting

Information).

In a second step, hierarchical cluster analysis was carried out

to reveal hidden relationships between voltammograms and the

32 MB samples. The used parameters were as follows: (i-iv)

peak potentials of peaks I-IV, (v) peak III/peak current IV ratio

( I (III)/ I (IV), (vi) the quotient between the current at the shoulder

between peaks III and IV (located ∼ +100 mV) and the peak

current of peak IV ( I (100)/ I (IV)), (vii) the peak current I/peak

current II ratio ( I (I)/ I (II)), and (viii) the quotient between the

sum of peak currents for peaks I and II and the sum of the peak

Figure 3. SQWVs of PIGEs modified with Maya Blue samples from(a) Chacmultun (type A3), (b) El Tabasqueno (type B), (c) D’zula (type

C), and (d) Mayapan (type D), immersed into 0.50 M acetate buffer,pH 4.85. Potential scan initiated at -750 mV in the positive direction.

Potential step increment 4 mV; square wave amplitude 25 mV;frequency 5 Hz.

Figure 4. SQWVs of PIGEs modified with Maya Blue samples from(a) Acanceh (type A1), (b) El Tabasqueno (type A2), and (c)

Dzibilnocac (type A5), immersed into 0.50 M acetate buffer, pH 4.85.Potential scan initiated at -850 mV in the positive direction. Potential

step increment 4 mV; square wave amplitude 25 mV; frequency 5Hz.






currents for III and IV ( I (I + II)/ I (III + IV)). This last parameter

can be taken as a measure of the “indigo/clay” ratio, i.e., of the

concentration of indigo in the palygorskite. Confirming prior

results,23 the values of the I (I)/ I (II) ratio in MB samples,

representative of the existing relation dehydroindigo/indigo, were

clearly larger than that determined for indigo microparticles under

the same conditions. A matrix was constructed with rows

representing 32 MB samples and columns corresponding to the

above parameters. Pertinent data are included as Supporting

Information.

Dendrogram building using this analysis is shown in Figure

6a. First, two branches, corresponding to D and A + B + C types

are separated. In this second branch, C first separates from the

A + B group, further divided into A and B branches. The former

subsequently displays a separation in A1- A5 types. This separa-

tion scheme is reproduced by building the dendrogram from the

eight electrochemical holotypes alone, as shown in Figure 6b. In

Table 2, the sample distribution along chronologically ordered

archaeological sites is depicted. Here, electrochemical types are

distributed following the similarity sequence derived from cluster

analysis in Figure 6. MB sample grouping in Table 2 suggests

that there is a progressive replacement of some electrochemical

types by other ones along time following a ramified scheme.

Electrochemical Types and MB Preparation. The existence

of net linkages between the different “electrochemical” types acts

in support of the idea that voltammetric profiles are representative

of mineralogical and textural differences and, ultimately, of the

existence of different preparation procedures of MB samples. In

order to verify this idea, electrochemical data were crossed by microscopic and spectroscopic data on genuine MB samples. First,

MB contains calcite grains, as confirmed by ATR/FT-IR data.23,24

Calcite appears as a result of the preparation of indigo from

macerated leaves of I, suffruticosa in quick lime. As shown in

Figure 7a, spectra of MB samples show a prominent band a 1408

cm-1, attributable to the stretching vibration of the carbonate

group. This peak is entirely absent in the spectra of pristine

palygorskite and MB specimens prepared by crushing the clay

with synthetic indigo. MB overlapped bands in the 1600-1750-

cm-1 region correspond to the carbonyl frequency of indigo

(theoretical 1629 cm-1 ),38 dehydroindigo (theoretical 1736 cm-1 ),38

and zeolitic water.20,22 Such bands can be clearly seen of removing

calcite by treating the sample with diluted HCl, as can be seen inFigure 7b.

Second, MB often appears in paint layers accompanied by

ochre grains, as indicated by SEM/EDX data and microscopical

examination of cross-section stratigraphies (see Supporting In-

formation). This ochre can, in principle, be attributed to a

deliberated addition, “in situ” of ochre for modulating the hue of

the pictorial layer. The presence of ochre in these samples is

clearly reflected in their visible spectrum, as can be seen in Figure

(38) Klessinger, M.; Luttke, W. Tetrahedron Lett. 1963, 19 (Suppl. 2), 315-

335.

Table 1. Voltametric Data for MB Holotypes Defined in This Studya

sample E p(I)b E p(III)b Ep(IV)b Ep(II)b I (III)/ I (IV) I (I+II)/ I (III+IV) I (I)/ I (II) I (100)/ I (IV) ∆ E p2(IV)b type

Acanceh-2 +460 +210 0 -285 0.938 3.250 1.524 0.438 130 A1El Tabasqueno-2A +455 +190 0 -265 0.320 0.485 1.909 0.330 130 A2Kuluba-9 +485 +195 -30 -265 0.536 0.556 0.326 0.446 163 A3Calkmul IIC +515 +180 -45 -250 0.320 0.242 0.389 0.329 167 A4Dzibilnocac-9A +475 +175 0 -265 0.132 0.225 3.667 0.150 113 A5Mayapan 2A +470 +180 +40/0 -285 0.172 0.034 1.143 0.146 135 BD’zula-2 +450 +185 +10 -285 1.400 0.354 2.200 0.900 216 CDzibilnocac-9 +450 +190 +40 -275 1.539 0.188 1.308 1.129 288 D

a From SQWVs recorded at sample-modified PIGEs immersed into 0.50 M acetate buffer, pH 4.85. Potential step increment 4 mV; square waveamplitude 25 mV; frequency 5 Hz. bPotentials in mV.

Figure 5. Two-dimensional I (III)/ I (IV) vs E p(III)-E p(IV) (in mV)diagram for MB samples grouped into electrochemical types A (solid

rhombs), B (rhombs), C (squares), and D (triangles). Froms SQWVsof sample-modified electrodes; conditions such as described in

precedent figures.

Table 2. Chronological Distribution of Samples into the

Different Archaeological Sitesa

site A2 A1 A4 A3 A5 B C D

postclass. Mayapan • •• •

terminal class. Chichen Itza •• ••

D’zula • ••

late Ek Balam •

Dzibilnocac • •

classical Kuluba •

Chacmultun •

Mulchic • ••• •El Tabasqueno •• • •• •

Acanceh ••

early class. Calakmul A6 • •

late preclass. Calakmul IIC •

a Each point corresponds to one sample.





8. Diffuse reflectance spectra of many MB samples (curve A in

Figure 8) exhibit typically two bands at ∼420 and ∼540 nm, close

to those described for indigo and dehydroindigo in solution

phase.38 In contrast, samples of paint layers containing addi-

tions of ochre (curve B in Figure 8) exhibit a diffuse reflec-tance spectrum close to that of goethite,39 shown in curve C of

Figure 8.

It should be noted that palygorskite contains iron ions.

Although such ions do not influence the hue of the pristine clay

(white in all samples from Sak lu’um), their participation in

chemical processes (acting, for instance, as catalytic redox sites

or modulating water/indigo coordination) involved in the prepara-

tion of MB cannot be entirely discarded.

Remarkably, TEM examination of MB samples revealed

significant differences between pristine palygorskite crystals from

the classical site of Sak lu’um and palygorskite in MB samples,

as well as between the different types of MB. As can be seen in

Figure 9A, pristine palygorskite consists of aggregates of elon-gated crystals 0.5-1- µm sized having fine fiber structures with

thicknesses from 300 to 600 Å, as shown in Figure 9A. In contrast,

palygorskite from MB samples showed narrow fibbers with a

corrugated structure. Remarkably, a clear correlation between

“electrochemical” types and TEM images was obtained. Thus,

A-type MB samples presented irregular-shaped palygorskite

crystals, whereas B-type samples exhibited numerous 10-20-nm-

sized pores irregularly distributed on the surface of the crystals

(see Figure 9B). In contrast, MB samples of the C and D types

showed a dense array of fine (2-5-nm-sized) pores covering the

entire surface of palygorskite crystals (see Figure 9C). See details

in Supporting Information.

Such textural differences can be attributed to differences in

the preparation of MB via crushing indigo and palygorskite with

application of a given thermal treatment and, eventually, with

addition of other components. Similarly to other minerals,40 pores

observed in palygorskite crystals can be attributed to segregation

and evacuation of loosely bound (physisorbed) water at and,(39) Elias, M.; Chartier, C.; Prevot, G.; Garay, H.; Vignaud, C. Mater. Sci. Eng.

B 2006, 127 , 70-80.

Figure 6. Hierarchical cluster analysis with euclidean distance ofthe autoscaled variables applied to voltammetric parameters recorded

for MB samples. Data matrix using the electrochemical parametersdescribed in the text for (a) 32 MB samples and (b) the holotypes

listed in Table 1.

Figure 7. ATR/FT-IR spectra of MB sample from Mulchic (a) before

and (b) after treatment with diluted HCl for removing calciumcarbonate.

Figure 8. Diffuse reflectance spectra of (A) MB sample from El

Tabasqueno, (B) MB sample from Kuluba, and (C) goethite (fromref 40).







mainly, zeolitic or structural water. Thermal data 11,17,18 indicate

that such processes occur at 110-120 and 220-230 °C, respec-

tively, temperatures able to be handled by the ancient Mayas. The

upper limit of usable temperatures must be situated at 360 °C,

where thermal decomposition of indigo occurs.11 Release of the

gross structural water at 460-480 °C and dehydroxylation and

phase transformation to clinoestatite at 700 °C complete the

thermal evolution of palygorskite.17,18 Our data on MB samples

and synthetic specimens (vide infra) suggest that some types of

MB were prepared with no or smooth thermal treatment, in

agreement with Hubbard et al.11 This implies that indigo adsorp-tion in the palygorskite system does not necessarily follow the

release of zeolitic water.

In order to study the influence of the thermal treatment on

the preparation of MB, different synthetic specimens were

prepared from synthetic indigo combining different thermal

treatments with the addition of calcite and, in some cases, ochre.

Specimens were prepared by crushing palygorskite from the

classical site of Sak lu’um, synthetic indigo (3% w/w) plus calcite

(10% w/w), and, eventually, ochre (10% w/w) and heating at

temperatures between room temperature (25 °C) and 400 °C

during times between 6 and 48 h. Ochre was selected as a possible

additive in view of its use as a pigmenting agent by the ancient

Mayas and its availability as a common mineral in the region. In

a second series of samples, ochre was added after the thermal

treatment acting merely as a pigmenting agent.

Experimental data indicated that the response of the specimens

varied mainly with the duration of the crushing process and the

temperature of the thermal treatment, while the duration of the

thermal treatment produced minor variations in the voltammetric

response. Increasing the duration of the crushing process pro-

duced a decrease in the indigo-localized peaks I and II with respect

to peaks III and IV. This suggests that palygorskite channels are

partially empty in MB specimens prepared with no or minimal(40) Ruan, H. D.; Frost, R. L.; Kloprogge, J. T. Spectrochim. Acta, A 2001, 57 ,

2575-2586.

Figure 9. TEM images of (A) pristine palygorskite crystals from Saklu’um, (B) MB sample from Chichen Itza (electrochemical type B),

and (C) MB sample from Mayapan (electrochemical type D).

Figure 10. Variation of I (100)/ I (IV) ()W ) with I (III)/ I (IV) for MB

samples (solid figures, type D, triangles; type C, squares; types A3,A4, and B, rhombs; A1 and A2, circles) and synthetic specimens

prepared from synthetic indigo and palygorskite submitted at differ-ent thermal treatments (squares, temperatures in the figure) during

24 h.



http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/ac0623686&iName=master.img-009.jpg&w=223&h=528



thermal treatment and are progressively filled on increasing

temperature, so that indigo molecules become increasingly at-

tached to the inorganic host. Additionally, the I (I)/ I (II) ratio

increases on increasing temperature, denoting the concomitant

increase in the relative amount of dehydroindigo, a feature

consistent with prior thermochemical data.23,24

Using the above preparation procedures, voltammetric profiles

of types A3 and A4 were reproduced satisfactorily by specimens

S1-S3, prepared by crushing indigo and palygorskite with no or

smooth thermal treatment. A5-type voltammograms were obtained

for heated synthetic specimens S4 and S5. Types A1 and A2,however, were approached by nonheated or moderately heated

synthetic specimens after adding ochre (SO1-SO3). The elec-

trochemical type C was approached by specimens SC3-SC5,

which were heated after adding ochre. However, the electro-

chemical types B and D were not completely attained. Comparing

data for selected pairs of electrochemical parameters for genuine

MB samples and synthetic specimens indicated that the temper-

ature of the thermal treatment probably increased along the

sequence A -B-C-D. This can be seen in Figure 10, where the

variation of I (100)/ I (IV) with I (III)/ I (IV) is given for genuine MB

samples (solid figures) and a series of synthetic specimens

prepared without addition of ochre at different temperatures. In

this figure, the different electrochemical types are sequentially

grouped along an almost linear representation. Data points for

nonheated synthetic specimens fall within the region of A3- A4

types and, remarkably, on increasing the temperature of the

thermal treatment, synthetic specimens approach, successively,

the regions of A5, B and C-D genuine MB samples. Interestingly,

synthetic specimens prepared with ochre without thermal treat-

ment approached the region of A1- A2 types.

Implications. Crossing microscopical, spectral, and voltam-

metric data for MB samples and synthetic specimens suggested

the following: (i) electrochemical types A3 and A4 correspond

to MB prepared by crushing indigo and palygorskite with no (or

smooth) thermal treatment and no significant addition of ochre;

(ii) types A1 and A2 were prepared similarly but incorporating a

smooth thermal treatment and, frequently, a postheating addition

of ochre-type minerals; (iii) preparation of type A5 MB required

the use of a thermal treatment between 160 and 200 °C; (iv)

preparation of types B-D required the use of more strong thermal

treatments, probably with temperatures above 200 °C and addi-

tives, ochre being one of the possibly used materials.

Crossing the linkages derived from the chemometric analysis

of voltammetric data with chronological data for the studiedarchaeological sites allows presentation of a scheme in which the

preparation of MB evolved in time. Accordingly, a seminal

preparation procedure, dated, at least, from the Late Preclassical

period, consisted of crushing palygorskite with indigo (and,

eventually, adding ochre in situ for modulating the hue of the

paint layer). From the actual perspective, this step can be viewed

as a first “technological revolution”, where the modern synthesis

of hybrid organic-inorganic materials was precluded. This

preparation procedure, corresponding to A3 and A4 electrochemi-

cal types, was maintained until the Postclassical Maya period with

local variations (A1, A2 types) attributable to the use of additives

such as ochre.

During the Late Classical period, a second technological

revolution consisting of the performance of vigorous thermal

treatments was introduced, yielding MB samples of types A5, B,

C, and D. Differences between such types are attributable to the

use of different temperatures and the introduction of ochre or

other unknown additives prior to the thermal treatment for types

B-D. Iron ions and other species existing in the parent palygor-

skite can eventually influence MB preparation. This scheme

should explain the conflict between the observations concerning

the absence9,11,14 and the presence4,5 of iron oxide materials in

some MB samples. The use of additives during the crushing/

Scheme 2. Possible Chronological Scheme for the Evolution of MB in the Late

Preclassical-Postclassical Maya Periodsa

a Relationships between electrochemical types are crossed with the accepted chronology of archaeological sites. Three possibletechnological revolutions are localized.



Doménech, A. et al. Study blue maya. 2006

Documents

Transcript of Doménech, A. et al. Study blue maya. 2006