Background and degree of novelty

Background and current state of research

Despite being extensively studied over time, Roman mural paintings continue to attract interest and stimulate new research. Until recently Roman wall paintings were regarded exclusively as frescoes (in the a fresco technique the paint is applied without the aid of an organic binder) 1–3. However, latest studies have highlighted the presence of various types of organic binders in the composition of Roman wall paintings 1,4–8. The theory of fresco as the standard painting technique versus the potential use of organic binders (a secco painting technique) in Roman wall paintings is a much-debated question in the current literature 1,6. As already highlighted in previous studies 1,2,5, several factors may limit the ability of the scientific instruments to accurately identify the presence of organic materials in ancient wall paintings: (1) the low concentrations of the binder-to-pigment ratio (as low as one-tenth of 1% of binder); (2) the natural ageing/degradation of the organic binders; (3) the vast and porous wall painting system, highly susceptible to degradation and biological contamination; (4) the complex binder-pigment interactions. Moreover, the difficulty in extracting organic materials from ancient wall paintings can lead to distorted results or to an incomplete characterization of their composition 1,5,7.

Until now only a small number of studies addressed the efficiency of various investigation techniques for organic materials identification in lime-based wall paintings 2,5. This lack of data may cause uncertainty in the degree of reliability of the different results obtained within various studies leading to ambiguity around the painting techniques used by the Romans as well as to possible misguided restoration interventions of the wall paintings. A wide range of invasive and non-invasive techniques have been tested for their ability to provide information on the presence of organic materials in wall paintings, best results so far being obtained via vibrational spectroscopy (FTIR, Raman) and chromatographic techniques (liquid chromatography, gas chromatography with flame-ionization detection, and gas chromatography–mass spectrometry), frequently only after the successive use of extraction methods 2,4,5,7. At this moment the current state of research within the field is focused on the implementation of an analytical protocol that can assess the efficiency of the extraction methods (of the binder components) as well as the influence of biological contamination 1,4,5. 

A recent study 9 carried out by our research group on several 2nd century Roman wall painting fragments coming from two archaeological sites located on the former territory of the Roman province of Dacia (Ulpia Traiana Sarmizegetusa and Alburnus Maior) indicated for some of the investigated sample the presence of an organic binder. These results, obtained via FTIR analysis, suggest that the pigments were applied a secco, and not a fresco as initially thought 9,10. Given the small number of samples investigated as well as the limitations of standard FTIR measurements (no sample pretreatment was carried out), at this moment there is still a lack of understanding regarding the nature and distribution of the organic materials initially employed.

As shown in studies carried by others 2, as well as in our own 9,10, the rich calcium carbonate support (lime-based plaster) can hinder or distort the characteristic bands of organic compounds in those regions of the spectrum where the vibrational bands of calcium carbonate are active. The poor selectivity in complex material systems (heterogeneous mix of compounds) due to signal interferences of different functional groups within similar spectral ranges, is a major drawback of infrared spectroscopy as the minor components can remain frequently undetected or undifferentiated 11. Taking into account the high content of inorganic materials that can easily mask the binders (matrix effect) within the FTIR spectrum registered on wall painting samples, refined procedures or more sensitive techniques need to be employed in order to accurately assess the presence of an organic binder. Extraction of the organic components with polar solvents (such as dichloromethane) has proved to significantly increase the selectivity of the FTIR technique by eliminating signal overlaps 4. In order to achieve an enhancement of the underlying binder compounds, similar extraction procedures with the one used by Cerrato et al. 4 and by Gelzo et al. 7 will also be carried in the current project.

Another important limitation of the FTIR technique is given by its micro-invasive character, sampling being required 12. Further limitations can also be related to the selection of areas/sampling sites that may not be representative of the entire artwork, or to the small sample size (often too thin for repeatable analysis and sufficient signal-to-noise ratio) 2. Moreover, an important aspect when working with historical objects is given by the minimal sampling allowed, non-invasive techniques being preferred 13. In this regard, the use of remote non-invasive analysis such as laser-induced fluorescence (LIF) is a major advantage.

A spectroscopic and imaging technique, LIF allows detection of fluorescent molecular species present in the outermost layer of a surface, due to the short penetration of UV radiation into solids. LIF has been successfully applied to the study of a wide range of materials associated with works of art, such as: stone and minerals, pigments and colorants, organic binding media 14–20. Biological attack as well as trace of former restorations were also investigated by laser-induced fluorescence 21,22. The technique offers numerous advantages: (1) high sensitivity and selectivity (most organic or bio-organic materials as well as a wide range of inorganic materials contain chromophores that upon excitation exhibit characteristic fluorescence emission); (2) can be used in laboratory or in-situ without the need of sampling or surface pretreatment (remote analysis); (3) offers prompt real-time diagnosis and mapping of the distribution of materials. The technique has also been tested on wall paintings 18,23–25, but none of the studies published so far focused on ancient wall painting fragments from the Roman period.

The potential and limitations of the LIF technique for the characterization of organic binders in Roman wall painting fragments were studied so far only in wall painting replicas 2. As shown in previous studies in-situ identification of organic materials used on complex painted surfaces using fluorescence spectroscopy is not always straightforward due to several factors: (1) the intrinsic similarities in emission spectra for some of these materials; (2) auto-absorption and selective absorption phenomena; (3) the complex attenuation of emission by optical absorption from other non-emitting materials 2,26. As a result, the emission spectrum is frequently deformed (shifts of the emission maxima and/or to the formation of new emission bands) leading to the erroneous characterization or identification of the fluorescent materials. Despite these analytical challenges, the technique is well suited for non-invasive analysis of large areas and for discrimination or classification of materials (based on the variation in the fluorescence signal related to the various materials present within the investigated painted surfaces) 17.

Degree of novelty and relevance of the proposed idea in relation to national and international state of the art

The artMAP project proposes development of an investigation methodology that starts with a non-invasive approach (without sampling) via LIF analysis, followed by a minimally invasive FTIR investigation carried on targeted micro-samples. This first step will provide a first level of identification, the LIF analysis being aimed to identify the presence or absence of an organic binder (the presence of an emission band with a clearly defined maximum indicating the presence of an organic binder). The information obtained via LIF measurements will indicate the points on the painted surfaces (areas with strongest fluorescence signal assigned to organic compounds) at which to take micro-samples to be analyzed in the laboratory by FTIR spectroscopy. An enhanced FTIR procedure (extraction with polar solvents) for the analysis of organic binders will be used in order to achieve refined results. FTIR measurements are expected to provide a second level of identification, meaning identification of specific classes of organic binding media. This combined methodology will give a synergic effect as the chemical identification of the organic materials provided by FTIR spectroscopy can be transferred to the whole painted surface thanks to the imaging capabilities of the LIF system, without the need for extensive micro-sampling.

The proposed methodology will include state-of-the-art methods (upgraded LIF system and refined FTIR procedures) never tested before in this combination for the investigation of organic compounds (present in low amounts) in ancient wall paintings. A similar analytical protocol based on LIF and standard FTIR measurements has been tested before only in a limited number of studies: on renaissance fresco paintings in Italy and on Michelangelo’s David sculpture 23,27. Compared to our proposed approach, these previous studies have addressed identification and mapping of organic residue of various types left from previous restorations (surface contamination) only, and have not used refined FTIR procedures (sample pretreatment).

The proposed methodology is aligned with the current research trends and tries to solve a difficult problem of interest at the level of the international scientific community: the identification and mapping of organic binders present in low concentrations in ancient painted surfaces. Preliminary results obtained by our group support the success of this project proposal: innovative development of optoelectronic systems including the first LIF scanning system developed in Romania; numerous patents obtained for innovative approaches in in the investigation of cultural heritage goods; numerous research papers published in top international journals (Q1/Q2 rank) including studies performed on ancient wall paintings.

References

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