Variable-temperature Micro-Raman Spectra of Selected Artists’ Pigments. Oral presentation at 96th...

Variable-‐temperature Micro-‐Raman Spectra of Ar6sts’ Pigments:

Cadmium Yellow (CdS) Chrome Yellow (PbCrO4)

and Maya Blue Jacqueline Wagner Riddle

Prof. I.S. Butler Emily Hopkins

96th Canadian Chemistry Conference and ExhibiMon Quebec City, 30 May 2013, 9 am

Spectroscopy in art conservaMon

•  Knowledge of arMfact composiMon

•  StabilizaMon without further damage –  Temperature, humidity and light exposure controlled

•  IdenMficaMon of forgeries

Some recent work

•  Lead chromate (PbCrO4) – High-‐pressure Raman study by Bandiello et al. (2012)

•  Cadmium sulphide (CdS) –  Temperature-‐dependent (-‐258 to 42˚C) Raman study of CdS quantum dots by Neto et al. (2012)

•  Maya blue – Ambient-‐temperature Raman study of Maya blue pigments in different clay structures by Sanchez del Rio et al. (2006)

Instrument SpecificaMons •  inVia Renishaw microscope system •  Long-‐working distance lens (50X/0.75) •  514-‐nm (16 mW) and 785-‐nm lasers

•  Linkam THMS600 temperature stage

My samples

CdS and PbCrO4 Pigment InformaMon Cadmium Yellow (CdS) •  Hexagonal wurtzite polymorph,

occurs naturally as Greenockite •  Used in photoresistors, solar cells •  Commercially available starMng 1840 •  Used by Monet, van Gogh, MaMsse

Chrome Yellow (PbCrO4) •  Occurs naturally as

crocoite (monoclinic prismaMc)

•  The colour associated with school buses

CdS Raman spectrum 514 nm laser, 5% of 16mW

•  301 and 603 cm-‐1 corresponding to the longitudinal phonon mode (1-‐LO) and its overtone (2-‐LO)

•  906 cm-‐1 peak corresponding to second overtone (3-‐LO), now visible due to resonance Raman effect

1-‐LO

2-‐LO

3-‐LO

CdS Temperature dependency of peak posiMon 514 nm laser, 5% of 16mW

Overall shihs for the 1-‐LO, 2-‐LO and 3-‐LO (not

shown here) peaks in the

temperature range are

–8.1, –17.7, and –22.4 cm-‐1.

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Raman shiD (cm-‐1)

CdS Temperature dependency of FWHM

•  The 1-‐LO and 2-‐LO bands broaden with increasing temperature

Raman spectrum of PbCrO4 at 25˚C 785 nm laser, 20% power

•  840 cm-‐1 (s) peak corresponding to v1 symmetric stretching of CrO4

2-‐ •  856 cm-‐1 (overlap)

•  337, 357, 376, 399 cm-‐1 v4 and ν2 bending modes

Temperature dependency of PbCrO4 Raman spectrum

785 nm laser, 20% power

Maya Blue

Mechanochemical Synthesis and Variable-‐temperature Raman study

Maya Blue Pigment History

•  Used by the ancient Mayans on polery, sculptures, murals

•  Associated with sacrifices and offerings

•  Resistant to acids, alkalis, solvents, oxidants, reducing agents, moderate heat and biocorrosion

Mechanochemical Synthesis of Maya blue

•  syntheMc indigo and sepiolite (Mg4Si6O15(OH)2·∙6H2O)

purchased from Sigma-‐Aldrich

•  A mixture of indigo (2%) and sepiolite (98%) was placed in a ball milling jar and milled for 0.5 hours

From the top: structure of indigo; encapsulaMon of indigo molecule inside a sepiolite tunnel (with dashed lines indicate H-‐bonding between the indigo C=O group and the clay OH2) Image from Giuste6o et al., 2010.

Raman spectra of Maya blue at 25 °C 514 nm laser

Raman spectra of indigo and Maya blue at 25 °C

•  AcMvaMon of vibraMonal modes with Bu symmetry due to the deformaMon of indigo in the sepiolite framework

Temperature series Maya blue 514 nm laser

Conclusions •  Cadmium Sulphide (CdS) –  Peaks redshih and broaden with increasing temp.

•  Chrome Yellow (PbCrO4) –  Broadening and amorphizaMon with increasing temp.

•  Maya Blue – No change up to 250˚C

Three very stable pigments!

Acknowledgments Thanks to Prof. I.S. Butler, Emily Hopkins, Yann Desjardins-‐

Langlais, Julia Bayne, and Joël Poisson. Thanks to NSERC for financial support used toward the Raman microscope and materials.

Contact informa6on: Jacqueline Riddle [email protected]

Ian S. Butler [email protected]

References Arnold, D., et al., An<quity, 2008, 82, 151–164. Bandiello, E., et al., Physical Review B, 2012, 85, 024108. Giustelo, R., et al. Periodico di Mineralogia, 2010, Special Issue, 21-‐37. Gonzalez-‐Alcudia, M., et al., Adv. Tech. Mater. & Mater. Proc. J., 2007, 9, 81-‐86.

Kleber, R., et al., Studies in conserva<on, 1967, 12 (2), 41-‐56. Neto, E., et al., Nanotechnology, 2012, 23, 125701-‐125709. Frost, R., J. Raman Spectrosc., 2004, 35 (2), 153-‐158. Leona, M., et al., Journal of the American Ins<tute for Conserva<on, 2004, 43 (1), 39-‐54.

Sanchez del Rio, M., et al., J. Raman Spectrosc. 2006, 37, 1046-‐1053.

Raman spectra of indigo and Maya blue at 25 °C

•  AcMvaMon of vibraMonal modes with Bu symmetry due to the deformaMon of indigo in the sepiolite framework

1633 1380 1380

Raman shih (cm-‐1)

Indigo

Maya Blue

1680

Raman spectrum, 514 nm laser of PbCrO4 •  840 cm-‐1 (s) peak corresponding to v1 symmetric stretching of CrO4

•  856 cm-‐1 (overlap) second v1 band

•  337, 357, 376, 399 cm-‐1 v4 and ν2 bending modes

•  1672 cm-‐1 overtone of v1 symm. stretch

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PbCrO4 variable-‐temperature Raman series, 514 nm laser

•  Peaks redshih and broaden •  Strong v1 peak shihs –6.8 cm-‐1 over the temperature range, v1 overtone shihs –9.1 cm-‐1

•  Quartet ca. 350 cm-‐1 no longer disMnguishable at higher temperatures

Variable-temperature Micro-Raman Spectra of Selected Artists’ Pigments. Oral presentation at 96th...

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