We perform molecular analysis using an Agilent 4300 Handheld FTIR spectrometer. This system enables direct, non-invasive analysis of artworks and archaeological objects on their surfaces, without any damage or sampling. The technique, known as FTIR Diffuse Reflectance Spectroscopy, requires a specialized probe.

This method differs significantly from standard FTIR spectroscopy and can produce very different results. Therefore, researchers must develop new spectral databases for historical pigments and other materials used in art and archaeology.

Standard FTIR spectroscopy has been widely used in this field [1]. Its spectra are relatively easy to interpret since the IR radiation is either absorbed or transmitted. In contrast, FTIR Diffuse Reflectance involves additional scattering, which makes interpretation more complex. Scattering can vary significantly—even between samples of the same material [2]—leading to spectra that differ from those in standard databases.

CHSOS supports this effort by publishing spectra from the Pigments Checker, our reference collection of historical pigments from antiquity to the early 1950s. We applied the pigments with an acrylic binder on cardboard supports and collected spectra of both the pigments and the binder alone. All spectra are freely available on the Pigments Checker webpage.

Pure powder pigments

The FTIR-DR spectrum of a pigment in the Pigments Checker (Figure [1]) reflects contributions not only from the pigment itself, but also from the acrylic binder and the cardboard support (100% cotton paper). As a result, we cannot clearly identify the absorbance bands of each individual pigment.

Figure [2], for example, compares the spectrum of pure azurite powder with that of the azurite swatch in the Pigments Checker. This comparison shows the importance of having spectra of pure powder pigments to correctly interpret the spectra from painted surfaces.

To address this need, we created a spectral database of pure powder pigments. Users can download this database, along with the others, from the Pigments Checker webpage.

 

In order not to contaminate the probe, the spectra of the pure pigments were acquired by depositing some powder on an IR transparent window, which does not affect the resulting spectrum, figure [4].
This window is then loaded on the probe, figure [5].

 

Case study: antiphonary

We tested a large (68 x 47 cm) three-color antiphonary parchment sheet, 16th century, figure[6]. Figure [7] shows the spectra of the parchment, the blue color of a letter, and the spectrum of pure azurite in our database. Availability of the pure spectrum of azurite is mandatory for a correct interpretation.

 

 

Conclusions

Non-invasive FTIR with diffuse reflectance probe is a quite novel field. While standard FTIR spectra are easy to interpret, in the case of diffuse reflectance, the use of databases is mandatory. This application note discussed the spectra taken on Pigments Checker, but also the database taken from powder pigments. This helps to understand the effect that the binder has on the spectrum of the pigment. In upcoming application notes we will discuss the effect of other binders on the same pigments.

References
[1] DERRICK, M. R., STULIK, D., & LANDRY, J. M. (1999). Infrared spectroscopy in conservation science. Los Angeles, Getty Conservation Institute. http://www.getty.edu/conservation/publications/pdf_publications/infrared_spectroscopy.pdf.
[2] MILOSEVIC, M., & BERETS, S. L. (2002). A REVIEW OF FT-IR DIFFUSE REFLECTION SAMPLING CONSIDERATIONS. Applied Spectroscopy Reviews. 37, 347-364.

DOWNLOAD Application note #2: FTIR Diffuse Reflectance. Pigments Checker Database