# Revisiting Maya Blue and Designing Hybrid Pigments by Archaeomimetism

**Introduction** : The search for resistant dyes led past civilizations to the conception of several artificial pigments. One of the first ever organic-inorganic hybrids is Maya Blue (MB), manufactured in pre-Columbian Mesoamerica (ca. 800 B.C.). Despite 50 years of sustained interest, its microscopic structure and its relation to durability remain an open question [1-2]. Here we address the problem by comparison with modern analogues of Maya blue based on dyes in zeolites.

**Objectifs** : Knowledge of the chemical composition of materials constituting an artistic object Maya Blue and of new pigments ease of use by
* Quantitizing visual appearance, physical and chemical stability
* Understanding the mechanism of degradation in response to changes in the environment (temperature, pressure, pH,...)

<figure><img src="image_1.png" /><figcaption>S. Blanc<sup>a)</sup></figcaption></figure>

<caption>P. Bordat<sup>a)</sup></caption>

<caption>R. Brown<sup>a)</sup></caption>

<caption>C. Dejoie<sup>b)</sup></caption>

<caption>P. Martinetto<sup>b)</sup></caption>

<caption>M. Anne<sup>b)</sup></caption>

<caption>E. Dooryhée<sup>b)</sup></caption>

<caption>P. Strobel<sup>b)</sup></caption>

<caption>F. Porcher<sup>c)</sup></caption>

## Synthesis

### “ Maya Blue”

* ancient synthetic hybrid material
indigo from *Indigofera* leaves
a clay (palygorskite, sepiolite)
heating

* exceptional stability :
acid-resistant (Gettens test [3])
no biodegradation
stable up to ~300°C

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>indigo</figcaption></figure>

### Archaeoinspired material

* co-grinding to mimic the archeological processes
indigo (1 to 10% wt)
zeolite MFI (silicalite, channel section 5.5×5.6 Å²)

* heating up
100-300°C
from 2h to 15h

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

## UV Visible Diffuse Réflectance

### reconstituted MB : Indigo (2%wt) -palygorskite

<figure><img src="image_7.png" /><figcaption>Spectra are normalised at 665 nm</figcaption></figure>

□ unheated mixtures;

○ heated hybrids (at 190°C during 5h);

△ heated hybrids after the nitric acid test

### Oxidation test

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

### indigo(2%wt)-silicalite

<figure><img src="image_11.png" /><figcaption>Spectra are normalised at 610 nm</figcaption></figure>

□ unheated mixtures;

○ heated hybrids (at 300°C during 5h);

△ heated hybrids after the nitric acid test

The absorption band responsible for the blue colour persists after the nitric acid test

## Fluorescence

### Fresco fragment from Cacaxtla (Mexico Coll. Louvres)

<figure><img src="image_12.png" /></figure>

(○) yellow part;

(□) red;

(△) blue (Maya blue, MB)

λ<sub>ex</sub> = 488 nm

Strong emission from all parts at ca. 550 nm, might be due to the substrate
The blue part shows an additional weak fluorescence band at ca. 750 nm.

<figure><img src="image_13.png" /><figcaption>Diluted Indigo in CHCl<sub>3</sub></figcaption></figure>

### Indigo (2%wt.) silicalite

<figure><img src="image_14.png" /></figure>

sample after 5 minutes laser irradiation
λ<sub>ex</sub> = 488 nm,
P0 corresponds to a power of ca. 3kWcm<sup>-2</sup>

The 650nm band, internal monomer indigo, is not notably affected

The 730nm band, surface indigo, disappears with the increase of the irradiation power

### M. Sanchez del Rio

IR

<figure><img src="image_15.png" /></figure>

d)

### E. Van Eslande<sup>e)</sup>

### P. Walter<sup>e)</sup>

sylvie.blanc@univ-pau.fr

Water content unchanged in palygorskite
Water band disappears on heating in silicalite

DRX

### palygorskite + Indigo

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

Indigo molecules stabilize
the palygorskite structure without modification

### silicalite + Indigo

<figure><img src="image_18.png" /></figure>

after heating

before heating

Indigo molecules enter
into silicalite channels

## TGA/DSC

<figure><img src="image_19.png" /></figure>

Indigo molecules are very stable once
incorporated inside the silicalite channels
(decomposition in the 550-700°C range)

The first TGA mass loss is attributed to the
decomposition of indigo powder
aggregated on the silicalite surface

## References

[1] A. Doménech, M.T Doménech-Carbó, M.L.V. De Agredos Pascual, J. Phys. Chem. C, 2007, 111, 4585-4595.

[2] G. Chiari, R. Giustetto, J. Druzik, E. Doehne, G. Ricchiardi, Appl. Phys. A, 2008, 90(1), 3-7.

[3] R. J. Gettens, American Antiquity, 1962, 7(4): 557-64.

[4] A. Romani, C. Zuccaccia, C. Clementi, Dyes Pigm., 2006, 71(3), 218-223.

[5] C. Dejoie, P. Martinetto, E. Dooryhée, P. Strobel, S. Blanc, P. Bordat, R. Brown, F. Porcher, M. Sánchez del Río, M. Anne, accepted Applied Materials & Interfaces

[6] C. Dejoie, P. Martinetto, E. Dooryhée, J. Kreisel, E. Van Elslande, S. Blanc, P. Bordat, R. Brown, F. Porcher, M. Anne, submitted Applied spectroscopy

## Molecular dynamics optimizations

### Palygorskyte : adsorption site of indigo

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

indigo first populates the very stable site

-at the fibre tip (1) partially obstructing the channels
(E1=-22 kcal/mol)

- within the bulk channels (2)
(E2=-7.7 kcal/mol)

- in the the groove sites (3)
(E3=-3.1 kcal/mol- and

partial orthographic and perspective views down,
with water withdrawn for clarity

'Doorkeeper' molecules in site (1) allow escape of cavity water but oppose population of the internal channels (2) until at higher temperatures they are activated further into the channel entrances

<figure><img src="image_22.png" /><figcaption>Fresco from Cacaxtla (Mexico)</figcaption></figure>

## Microscopy

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /><figcaption>Transmission</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>Fluorescence</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Overlay</figcaption></figure>

Preliminary fluorescence microscopy on an indigo-doped MFI zeolite,
showing an accumulation of the dye along the twin planar faults

## Conclusion

The present example inspired by MB leads to a non-toxic, acid-proof, low-cost and stable hybrid pigment.

Archaeomimetism undoubtedly opens a wide range of future studies concerning materials and properties with counterparts in our cultural heritage.