CHSOS Application note #1: Testing GorgiasUV on Pigments Checker

From 2021 the Reflectance Spectrometer GorgiasUV is joining our tools kit for scientific art and archaeology examination.
It is based on the standard Gorgias system but it is optimized to cover a wider range into the UV, up until 200 nm.
GorgiasUV explores with much more reliability the extreme part of the UV spectrum, providing helpful information for pigments identification.

Used from the late 80′ [1] Reflectance Spectroscopy (RS) is a powerful tool for the identification of pigments and dyes [2]. A reflectance spectrum shows for each wavelength the ratio between the intensity of the reflected and incident radiation. This ratio is called reflectance and is given in percentage (%). Pigments are identified using a spectral database and comparing the spectral features of the investigated unknown spectrum with the spectra available in the database.
The advantages of this method with respect to the other spectroscopic methods are: affordable equipment, small dimensions, and portability.
Along with the usual halogen lamp,

GorgiasUV features a Deuterium radiation source and a dedicated solarization-resistant fiber probe to effectively work within the UV range, figure [1].
We discuss the advantages of GorgiasUV over the standard Gorgias by testing the 2 instruments on the Pigments Checker, our collection of historical pigments, ranging from antiquity to the early 1950′. The colors are laid with an acrylic binder on a cardboard support. We collected the spectra of the pigments and that of the binder alone on the cardboard. All these spectra are available online on the Pigments Checker webpage.

Most pigments show the same spectra

For most pigments, there is no difference in using GorgiasUV or the standard Gorgias. This is because often the spectral information is in the visible and infrared regions.
This is the case for chrome oxide green. Figure [2] displays the spectra of this pigment acquired with Gorgias and GorgiasUV. We understand there is a maximum centered at 410 nm, in the visible region. This maximum is still clear and there is no uncertainty. The standard Gorgias covers the near UV region where the left shoulder of the this maximum is found. The spectrum acquired with GorgiasUV does not add any other information. The same can be said for most of the other pigments.
Though, there are a few pigments where GorgiasUV provides extra information that make their identification more reliable.

Better maxima and absorption bands in the UV

GorgiasUV provides better spectra of maxima and absorption bands in the UV region. This is the case for viridian which has a maximum across the UV-VIS edge.
As chrome oxide green, this pigment is a chrome oxide but it also contains a water molecule. Figure [3] displays the spectrum of this pigment acquired with Gorgias and GorgiasUV. Gorgias shows that there is a maximum centered at 370 nm but we miss its shoulder in the UV region. On the other hand, the spectrum taken with GorgiasUV thoroughly captures this maximum. In the case of viridian, using GorgiasUV provides new information and a more satisfactory characterization of the pigment.
Other examples are carmine lake and alizarine which have a maximum across the UV-VIS region which becomes apparent in the GorgiasUV spectra, figure [4, 5].

Discovering new bands in the UV

Some pigments have characterizing bands in the far UV which only GorgiasUV can discover. This is the case for some pigments, such as lead white, cobalt yellow, and ultramarine blue.
As a first example, we focus on the 6 white pigments in pigments checker: chalk, gypsum, lead white, zinc white, lithopone, and titanium white. Figure [6] and [7] show the spectra acquired, respectively, with the standard Gorgias and GorgiasUV. The standard Gorgias is already able to identify without ambiguity 3 white pigments: titanium white, zinc white and, lithopone. The other ones, chalk, gypsum, and lead white have

similar flat and featureless spectra. We are not able to distinguish them using standard Gorgias. On the other hand, GorgiasUV reveals a characteristic absorption for lead white in the UV region. Chalk and gypsum have just flat spectra in the UV as well as in the visible and infrared region.
Figure [8] shows the spectra of just lead white acquired with the standard Gorgias and GorgiasUV. We can see the characterizing absorption band in the UV. The figure displays also the spectrum of the cardboard just painted with the same acrylic binder, so we are sure that the absorption band belongs just to lead white and not to the binder or cardboard.
Figure [9] illustrates another example where GorgiasUV can make a difference for the identification of pigments. Cobalt yellow exhibits 2 farther absorption bands in the UV region that we can’t record with the standard Gorgias.
The spectra of Ultramarine blue natural and artificial are represented in figure [10]. GorgiasUV reveals a characterizing curve in the UV region. There is also a study showing that is possible to distinguish, in some cases, natural ultramarine from the artificial one [3]. Testing the natural and artificial ultramarine in our Pigments Checker, we got similar results. The maximum appears shifted toward the UV for the artificial ultramarine.

Conclusions

Gorgias is surprisingly portable and a low-cost reflectance spectroscopy system. GorgiasUV provides better results in the UV range thanks to its Deuterium lamp, but because this special lamp is a bit bulkier. Also, it is more costly since it uses special fiber optics that must resist the UV radiation from the Deuterium lamp. We suggest the standard Gorgias for most applications and in particular for those that require mobility and travel with the equipment. GorgiasUV provides better results in the UV and we consider it a complementary tool to the standard Gorgias.

References
[1] BACCI, M., CAPPELLINI, V., CARLA’, R. (1987). Diffuse reflectance spectroscopy: An application to the analysis of art works, Journal of Photochemistry and Photobiology B: Biology, 1, Issue 1, 132.
[2] FONSECA, B., SCHMIDT PATTERSON, C., GANIO, M., MACLENNAN, D., & TRENTELMAN, K. (2019). Seeing red: towards an improved protocol for the identification of madder- and cochineal-based pigments by fiber optics reflectance spectroscopy (FORS). Heritage Science.
[3] ACETO, M., AGOSTINO, A., FENOGLIO, G., & PICOLLO, M. (2013). Non-invasive differentiation between natural and synthetic ultramarine blue pigments by means of 250–900 nm FORS analysis. Analytical Methods. 5, 4184.