Applications of surface enhanced Raman Spectroscopy to the identification of traditional artists’ red dyestuffs

Since the mid-1980s, Raman spectroscopy has been increasingly used in the museum and archaeological fields to investigate artists’ materials. On the other hand, the potential of Surface Enhanced-Raman Spectroscopy (SERS) has been left largely unexplored until very recent years, despite its advantages of increased sensitivity and significant quenching of fluorescence, a phenomenon often occurring during the investigation of cultural heritage materials.

In this project, we explore SERS’ enormous capabilities for the unambiguous identification of artists’ lake pigments. Lake pigments are obtained by precipitation of deeply colored organic dyestuffs onto inert, finely divided particles of alumina trihydrate or, alternatively, calcium carbonate or gypsum. Red lakes were extremely prized for their transparent, yet intense colors and are often found in paintings and illuminated manuscripts as thin glaze layers, usually characterized by a high proportion of oil medium. The dyestuffs themselves have been used for dyeing textiles since the earliest times of human history, bound to the fibers through formation of a metal complex with metal salts (called mordants, most commonly alum – Al₂(SO₄)·K₂SO₄·24H₂O; iron sulfate – FeSO₄·7H₂O - and tin chloride -SnCl₂ ) A very small amount of dyestuff is necessary to achieve intense deep colors; therefore a characterization method with enhanced sensitivity is needed to unambiguously identify the dyestuff. Unfortunately, red lake pigments and dyes commonly employed in artistic production from antiquity to the mid-nineteenth century are often extremely fluorescent, an effect that dominates the weak optical process of Raman scattering.
SERS is a process whereby the Raman scattering signal is increased when a Raman-active molecule is spatially confined within the electromagnetic fields generated upon excitation of the localized surface plasmon resonance (LSPR) of nanostructured noble metal surfaces (most commonly Ag or Au). The SERS signals of ensemble-averaged molecules demonstrate enhancements up to 8 orders of magnitude over normal Raman signal.

We have developed an innovative sensing method using 6 nm silver island films (AgIFs) fabricated with electron beam (e-beam) deposition on the colorant particles under investigation. Several reference materials including the synthetic dyestuffs alizarin, purpurin, carminic acid, and eosin as well as historic red lake pigments such as madder lake, cochineal, brazilwood, lac lake, and kermes were probed with SERS and highly detailed Raman spectra were obtained.

Our collaborative research has allowed us to make a giant step forward in the identification of these often elusive materials. Further research is ongoing on the use of silver film over nanosphere substrates (AgFONs) and will involve the fine-tuning of sample preparation procedures to allow precise identification of dyes in samples from paintings and textiles.

Project Participants:

At AIC:
Francesca Casadio (A.W. Mellon Conservation Scientist)

At NU:
Richard P. Van Duyne (Charles E. and Emma H. Morrison Professor of Chemistry)
Alyson Whitney (graduate student)

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