Studying pigments on painted plaster in Minoan, Roman and Early Byzantine Crete. A multi-analytical technique approach

Abstract

Wall paintings spanning two millennia of Cretan painting history and technology were analysed in an effort to determine similarities and evolutions of painting materials and technology. A multi-technique approach was employed that combined the use of a) laser-induced breakdown spectroscopy (LIBS) and Raman microspectroscopy, based on mobile instrumentation, appropriate for rapid, routine-level object characterization, and b) non-destructive x-ray diffractometry (XRD), performed directly on the wall painting fragment, that provides detailed information on the minerals constituting the paint. Elemental analysis data obtained through LIBS were compared to molecular and crystal structure information featured by Raman spectroscopy and XRD. Cross-sections from selected samples were also investigated by means of optical microscopy and scanning electron microscopy coupled to microprobe analysis and x-ray mapping that enabled identification of several mineral components of the paint confirming the results of the XRD analysis. In parallel, replica wall paintings, created with known pigments and binding media, for reference purposes; were examined with optical microscopy and stain tested for organic materials. The overall study shows that the LIBS and Raman techniques offer key advantages, such as instrument mobility and speed of data collection and interpretation that are particularly important when dealing with on site investigations. Thus they are capable of providing important compositional information in an effective manner that enables quick surveying of wall paintings and permit targeted sample selection for further analysis by advanced laboratory techniques. The present multi-analytical technique study is part of a broader project that aims to explore the wall painting pigments used on the island of Crete from the Bronze Age to the Roman and Byzantine periods. A fairly large body of work exists on the analysis of pigments used in Bronze Age wall paintings from Crete [9,10,11] in relation to issues concerning on the one hand painting materials and technology and on the other painting style and thematic content. In contrast, information regarding wall paintings from later periods is scarce. The current project begins to redress the balance, and to augment our understanding of the whole palette of Cretan wall painting pigments. Keywords: archaeological pigments, wall paintings, analysis, LIBS, Raman, XRD, SEM-EDXThe principal materials and methods of constructing a wall painting remained almost unchanged onCrete from the Bronze Age until (and in many cases including) the 20 th century. Levelling layers of plaster, usually a combination of hydrated lime (calcium hydroxide) with various inorganic and organic filler materials such as calcitic or quartzitic sand, crushed ceramics, or straw, were applied to the wall, prior to the application of a thin, fine layer of almost pure slaked lime. In most cases, pigments were ground to powder, mixed with water, and applied to the still wet uppermost plaster

A multi-technique approach, based on mobile/portable laser instruments, for the in situ pigment characterization of stone sculptures on the island of Crete dating from Venetian and Ottoman period

“…Considering that paints can be significantly heterogeneous, careful aiming of the laser beam is critical for obtaining reliable results. However, given that Raman signals in some cases turn out to be quite weak, or masked by fluorescence emission, arising from pigments, binders or impurities, it is not uncommon that Raman analysis may not be fully conclusive concerning the identity of the pigment or pigments used in a painted object [26] particularly when analysis is performed in situ.…”

“…Particularly, in the context of wall painting analysis, research studies by Bruder et al [25] illustrate the complementarity of LIBS and Raman microscopy for the in situ analysis of fresco replicas while Westlake et al [26] have demonstrated the combined use of the techniques in the analysis of mural painting fragments, still performing analyses on small plaster fragments in the analytical laboratory. At this stage it is noted that hybrid LIBS-Raman systems, in which the two spectroscopic techniques utilize the same pulsed laser source, spectrometer and detector, have been proposed that perform combined measurements on heritage samples.…”

Towards the understanding of the two wavelength laser cleaning in avoiding yellowing on stonework: a micro-Raman and LIBS study

“…The mobile Raman spectrometer (HE 785, Horiba-Jobin Yvon, France) was employed with excitation at 785 nm provided by a cw (continuous wave) diode laser coupled to an optical head ( [21,22]). A light-emitting diode (LED) and a high resolution colour camera (video microscope) are also part of the optical head and offer a very clear view of the area under investigation, necessary for positioning the beam on individual areas.…”

“…The mobile LIBS spectrometer (LMNTII+), developed and constructed at IESL-FORTH, was used for this study [21,22]. The instrument uses a compact passively Q-switched Nd:YAG laser emitting pulses at 1064 nm (10 mJ/ pulse, 10 ns).…”

Multianalytical characterization of Late Roman glasses including nanosecond and femtosecond laser induced breakdown spectroscopy

“…Although portable XRF systems are becoming more available, the above mentioned techniques cannot be applied for non-destructible samples of considerable or relatively big size, from museum collections, or for glass items that must be reintegrated into a work of art, as for instance, stained glass windows. Although LIBS has been used in several archaeometrical studies of different cultural heritage materials, [21][22][23][24][25][26] those devoted to historical glasses are scarce. These studies have focused on finding the optimal LIBS parameters for the analysis of model soda lime silicate 27 and historical lead silicate glasses, 28 on the characterization of chromophores and opacifiers of ancient glasses, and on degradation pathologies.…”