In situ diagnostics of artwork

In recent years, non-invasive diagnostic applications for artwork have become one of the most stimulating challenges for scientists involved in the field of cultural heritage. Several spectroscopic techniques have been developed to obtain a significant amount of information specific to the materials used in the very different pieces of artwork realized over the centuries. Each technique records different types of information, representing separate pieces of a puzzle, which have to be combined to completely characterize the substances used in a work of art. It has proven difficult to assemble and optimize compact and portable instruments that are usable in situ and capable of investigating all the spectroscopic properties of a surface in a non-invasive way. However, we have now been able to design and build one such apparatus.

Reflectance ultraviolet-visible spectroscopy is a technique that studies the light (in the spectral regions of the ultraviolet and visible) that is reflected by a solid object, providing information on the fraction of the light absorbed by the solid surface. Such a technique can help understand the chemical nature of the object, and it has been largely used in the diagnostics of materials in works of art for the in situ non-destructive identification of pigments.^{1, 2} In the same spectral range, emission spectroscopy is another powerful technique that provides information on the organic and inorganic materials used in artwork production.³ This method is used both in steady-state and time-resolved modes, so it is performed using a continuous and a pulsed irradiation source, respectively. Each mode provides different but complementary information about the materials, including luminescence (the emission of light in a substance consequently to its irradiation, such as in fluorescence) and decay lifetime (measure of the time duration of a transient phenomenon such as luminescence). By coupling the data coming from the steady-state and the time-resolved techniques, we can distinguish different fluorophores (chemical substances that provide fluorescence) with similar spectral properties.^{4, 5}

With this evidence in mind, we have designed an ‘all in one’ compact and easily manageable instrument that includes the above mentioned techniques. A dedicated fiber optic system collects reflectance, luminescence and decay lifetime from exactly the same point in a selected surface. This instrument allows us to determine the complete photophysical behaviors of materials because we can completely characterize molecules by determining what happens to them after they absorb light. To date, our technique represents the most practicable non-invasive way of obtaining relevant information concerning organic dyes and colorants. The identification of these materials, used in almost all figurative arts, represents one of the biggest challenges in the non-invasive diagnostics in cultural heritage. The data provides selective optical signatures for different chemical compositions, and these can then be used to identify absorbing or fluorescing molecules by comparison with known standards.

The complete instrument is mainly composed of three different excitation sources and three detection systems (see Figure 1). The sources are a deuterium-halogen lamp for reflectance, three interchangeable continuous wavelength diode lasers (375, 450, 630nm) for luminescence, and three interchangeable pulsed diode lasers (375, 458, 635nm) for lifetime measurements. Two CCD spectrophotometers detect reflectance and luminescence. A highly sensitive photocathode takes lifetime measurements using a time-correlated single-photon counting experimental method.

Figure 1. The portable instrument can be used to identify organic dyes and pigments in artwork.

The instrument, which weighs less than 30kg, can be quickly disassembled into several parts and is easily transportable to places where artwork is located. For this reason, we selected the instrument's parts to allow a compromise between the best performance and most compact size and weight. The wavelength selection of both the luminescence excitation sources can be adjusted to excite the most wide range of materials used in artwork. The instrument's ultraviolet source (around 370nm) is suitable for uncolored and yellow materials. Its visible source (around 450nm) is used for orange, red and purple targets, and another visible source (around 600nm) selectively excites green and blue dyes or pigments.

The dedicated fiber-optic cable performs all measurements in the same point without any need for further movement after probe positioning (see Figure 2). Each single optical fiber has a 400μm diameter, leading to an inner probe core dimension of approximately 5mm² that is contained into a 6mm diameter cylinder. These dimensions are suitable for analysis of almost all painted surfaces, including very small manuscript illuminations.

Figure 2. Fiber optics cable scheme used to perform measurements on artwork. CW: Continuous wavelength (used to describe a continuous emitting light source).

Our portable instrument diagnoses artwork in situ by characterizing its dyes and pigments non-invasively. Over the next few months, we will test the device on several raw materials and pictorial mock-ups. We developed these models using ancient and modern recipes in order to achieve an ample database of substances that are suitable for in situ diagnostic purposes. After this preliminary work, the new instrument will be integrated into MOLAB,⁶ the mobile laboratory, which is a set of portable instruments that are capable of in situ measurements on unmovable or low-mobility objects.

The research presented here was funded by the European Union CHARISMA Project (Cultural Heritage Advanced Research Infrastructures: Synergy for a Multidisciplinary Approach to Conservation/Restoration, FP7 Capacities – Specific Programme Integrating Activities for Research Infrastructures, n. 228330).