Behaviour of monazite and evolution of polymetamorphic pelites from the Monte Rosa nappe, Western Central Alps, Italy

Despite the recognised importance of monazite in geochronology, the contextural information is lost when using standard mineral separation techniques. To overcome this problem, new techniques were developped to date monazite in thin section or as grains drilled out from thin sections. Chemical Th-U-Pb dating of monazite on the microprobe is limited to grains older than 200 Ma, due to the high detection limit. Moreover, a range of fundamental analytical and preparational problems remains. For example, for monazite containing 12% Th, the commonly disregarded interference of Th Mz on Pb Ma causes an overestimation of 11% (relative) in Pb. This propagates to an age overestimation of ~50 Ma for a sample of 400 to 500 Ma in age. A judicious choice of X-ray peaks used in quantitative EMP analysis avoids or minimises peak overlap for all elements, including REE. A newly developped XRF-microprobe achieves superior performance, permitting to date small grains (< 100 μm) as young as Miocene in age. As an example, the precision achieved with the XRF-microprobe for a well characterised monazite age standard FC-1 (TIMS age 54.3 Ma; μ-XRF age 55.3 ± 2.6 Ma), doubly polished to 30 μm in thickness, is below 5 % (2 sigma) after 90 minutes integration time (50 kV; 30 mA) at a spatial resolution of 90 μm. Special sample preparation techniques using a PE-polishing disk permit the sequential use of thermobarometric and geochronometric analytical tools and thus the potential to derive time-calibrated P-T paths. The first two papers describe methods for sample preparation, analysis techniques and data interpretation for electron microprobe and XRF-microprobe dating. In the other two papers the techniques are tested on pelitic rocks of the Monte Rosa nappe, western Central Alps, which is believed to have played a key role in the tectonometamorphic evolution of the Central and Western Alps. In these rocks monazite occurs in different textures. Similarities to the better known Adula - Cima Lunga nappes further east hint at a potentially equivalent late emplacement history from depths in excess of 70 km during Alpine orogenesis. Studying the polymetamorphic basement rocks of the Monte Rosa nappe successfully revealed new insight into the complex history of this fragment of continental crust with European affinity. Careful analysis applying the chemical Th-U-Pb dating technique unveiled two distinct phases of monazite growth in these rocks: a Permian one around 280 Ma, believed to be associated with the intrusion VIII of the Monte Rosa granite; and an Alpine one around 35 Ma, attributed to the timing of high pressure metamorphism when maximum depths were reached during continental subduction. The combination of geochronometry and thermobarometry, sequentially applied ‘in-situ’ demonstrates the potential of this new technique. While monazite dating is frequently used for geochronometry, there exists also a relative lack of knowledge of monazite-forming reactions which is commonly fundamental for an age interpretation. Fine-grained symplectites of monazite, apatite and corundum within allanite of the Monte Rosa nappe have been recognized as breackdown product from bearthite. Similar textures have also been observed in the Dora-Maira high pressure terrane. Bearthite Ca2Al[PO4]2(OH), an aluminium phosphate contains up to ~10 wt% of light rare-earth elements (LREE) + Th. The documented symplectitic textures are assumed to be related to rapid decompression, following high to ultra-high pressure conditions during Tertiary subduction. The systematic study of bearthite from the Monte Rosa type locality and the Dora Maira area revealed that bearthite fractionates LREE, Th and U similar as monazite. This indictes a potential use of bearthite for age dating of ultra-high pressure rocks

Traditional and modern pigments in Gino Severini's Swiss murals

Between 1924 and 1947, Gino Severini decorated five churches in the Romand region of Switzerland with monumental religious wall paintings. This mural art played an important role in the Italian artist's career despite being mostly unknown and not easily found. The painting methods and materials of these modern Swiss murals were characterized by combining historical and archival research with onsite visual and scientific examination. Most pigments were identified with noninvasive methods integrating mapping techniques (i.e., technical photography and digital microscopy) with point analysis (x-ray fluorescence and reflection FT-IR spectroscopy). The results of these portable techniques were completed with SEM-EDS, μFT-IR FPA imaging, and μRaman analyses on micro-samples which provided stratigraphic and compositional information to complement the noninvasive results. Overall, the data obtained show that Severini used both traditional pigments (e.g., Sangiovanni white and ochres/earths of various color) and modern ones, such as cadmium-based pigments, with different painting techniques (e.g., zinc white was applied exclusively a secco). Apart from cerulean blue (cobalt stannate) and Naples yellow (lead antimonate) found only in two locations, the same set of pigments was documented in all of Severini's Swiss murals. The characterization of the artist's palette is important to understand the technical painting process followed by Severini, his interest on new painting materials available on the market and, at the same time, loyalty towards traditional painting methods, such as painting a fresco

Sequential SEM imaging of microbial calcite precipitation consolidation treatment

Cultural heritage built from limestone is prone to deterioration by chemical weathering, a natural process, that is enhanced by pollution. There are many historic monuments built from calcareous rocks that suffer from deterioration, and thus there have been a number of approaches over the last few decades to consolidate these types of rocks and surfaces. Using natural biological processes by fostering the activity of calcite-producing bacteria, also referred to as biomineralization, is one strategy that has also been commercialized. The base of proving the effectiveness of any surface treatment is the observation of the surface at sequential stages before and after treatment, as well as after exposure to weathering. Due to the heterogeneity of natural materials and processes, our aim was to observe identical test areas at the micron scale throughout the observation period. In order to achieve this on a tungsten SEM, we employed a beam deceleration accessory that allowed low kV imaging on non-conductive surfaces at a sufficiently high image resolution with a modified sample holder accommodating drill cores of 25 mm diameter and up to 15 mm height. The presented method is capable of producing time-sequenced images on the same test area on natural rock surface samples without manipulation for imaging purposes. This offers interesting perspectives for effective documentation of such processes in various fields

COMBINED C-14 ANALYSIS OF CANVAS AND ORGANIC BINDER FOR DATING A PAINTING

The use of accelerator mass spectrometry (AMS) for age determinations of paintings is growing due to decreasing sample size requirements. However, as only the support material is usually dated, the validity of the results may be questioned. This work describes a novel sampling and preparation technique for dating the natural organic binder using radiocarbon (C-14) AMS. In the particular case of oil paintings, the natural oil used has a high probability of being representative of the time of creation, hereby circumventing the problem of the originality of the support material. A multi-technique approach was developed for a detailed characterization of all paint components to identify the binder type as well as pigments and additives present in the sample. The technique was showcased on a painting of the 20th century. The results by C-14 AMS dating show that both the canvas and binding medium predate the signed date by 4-5 yr. This could be the time span for keeping painting material in the atelier. The method developed provides, especially given the low amounts of material needed for analysis, a superior precision and accuracy in dating and has potential to become a standard method for oil painting dating

Uncovering modern paint forgeries by radiocarbon dating

Art forgeries have existed since antiquity, but with the recent rapidly expanding commercialization of art, the approach to art authentication has demanded increasingly sophisticated detection schemes. So far, the most conclusive criterion in the field of counterfeit detection is the scientific proof of material anachronisms. The establishment of the earliest possible date of realization of a painting, called the terminus post quem, is based on the comparison of materials present in an artwork with information on their earliest date of discovery or production. This approach provides relative age information only and thus may fail in proving a forgery. Radiocarbon (C-14) dating is an attractive alternative, as it delivers absolute ages with a definite time frame for the materials used. The method, however, is invasive and in its early days required sampling tens of grams of material. With the advent of accelerator mass spectrometry (AMS) and further development of gas ion sources (GIS), a reduction of sample size down to microgram amounts of carbon became possible, opening the possibility to date individual paint layers in artworks. Here we discuss two microsamples taken from an artwork carrying the date of 1866: a canvas fiber and a paint chip (<200 mu g), each delivering a different radiocarbon response. This discrepancy uncovers the specific strategy of the forger: Dating of the organic binder delivers clear evidence of a post-1950 creation on reused canvas. This microscale C-14 analysis technique is a powerful method to reveal technically complex forgery cases with hard facts at a minimal sampling impact

Investigation of the foil structure and corrosion mechanisms of modern Zwischgold using advanced analysis techniques

Zwischgold is a two-sided metal foil made by adhering a gold leaf over a silver leaf to present a gold surface while using less gold than gold foils. Corroded Zwischgold surfaces appear dark, accompanied by gloss loss and possible mechanical stability issues. Zwischgold applied artefacts are commonly found in museums and churches across Europe and they currently face an uncertain future as conservators have little knowledge to base conservation treatments on. We present a comprehensive material analysis of Zwischgold models through advanced characterization techniques including focused ion beam coupled with scanning electron microscopy (FIB-SEM), transmission electron microscopy (TEM), scanning transmission X-ray microscopy (STXM), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and Rutherford backscattering spectrometry (RBS). Complementary information on the foil thickness, sharpness of the gold-silver interface, gold purity, and the formation as well as distribution of corrosion products on Zwischgold models have been obtained, representing a starting point for understanding the morphology and the long-term chemistry of Zwischgold artefacts. (C) 2017 The Authors. Published by Elsevier Masson SAS

Surfaces and interfaces in multilayered paint systems – analytical challenges in art conservation sciences

The surface of an artwork is the interface to its environment and its viewer. The choice of a specific behaviour and appearance by the artist is an essential part of the making and – of how we perceive the expression of the artist. Colour hue and intensity, reflectivity, surface structure, surface finishing and protection, etc. are elemental properties that are achieved with an often complex, multilayered and multicomponent system build-up of paintings or painted and varnished artwork. Chemical interaction between these components and layers, which age over time, influenced by environmental conditions, can lead to reactions that alter the visual impression and/or integrity of an artwork. Conservation science is thus confronted with alterations at different interfaces and levels such as for example the pigment–environment (e.g. Cato et al. 2017), pigment-binder (Fig. 1, Ferreira et al. 2015) and the varnish-environment (e.g. Soulier et al. 2012) interface. Contrary to the development of new materials, understanding and reconstructing the past is generally hampered by alterations through time, poor or lost documentation, the limited availability of sample material that is scarce and, in the case of e.g. paintings or musical instruments, is barely accessible due to the high value of the objects. Furthermore, the experimental nature of the artists leads to the situation where the analyst is confronted with non-standardised materials and unconventional working techniques, such as for example the “tempera” painting technique (e.g Zumbühl et al. 2014; Ferreira et al. 2015). The variety of painted substrates (paper, polymer, textile, leather, wood, metal, glass, rock, mortar…) also implies that we have to deal with a wide range of surface properties and interface behaviour. Generalised approaches thus end in casework when it comes to interpretation of analytical data. The lack of historic reference material may lead to experimental re-production according to ancient recipes, in the case of Guignet green unveiling temperature dependant surface behaviour of pigment particles and as such variable end products (Zumbühl et al. 2009). Our current strategy is minimally invasive, combines the complementary techniques of infrared imaging (ATR-FTIR-FPA), Raman spectroscopy and scanning electron microscopy (VP-SEM, SE/BSE/EDS), applied sequentially to micro-samples. This combination not only covers a wide range of materials, but also delivers morphological and 2D stratigraphic information from minimal amounts of sample material. Specific developments such as the methodology to derivatise interfering compounds prior to FTIR measurement have further helped to significantly increase the selectivity of infrared spectroscopy and reach a new level of information in reconstructing historic coatings (Zumbühl et al. 2017). Figure 1. a) ‚Portrait of a young girl’ by Filippo Franzoni (ca. 1888, oil on canvas). b) Close-up view of the surface showing ‚gold’ particles turning green. c-e) crosssection of metal particle showing alteration products from pigment-binder interaction. (Ferreira et al. 2015) Literature References Cato E. et al. (2017) J Raman Spectrosc 1-10. doi.org/10.1002/jrs.5256 Ferreira E. et al. (2015) Herit Sci 3(1): 1-11. doi.org/10.1186/s40494-015-0052-3 Ferreira E. et al. (2016) In: Beltinger K. & Nadolny J. (Eds) KUNSTmaterial 4, Archetype Pub., Zürich, 205-227. Soulier et al. (2012). Z Kunsttechnol Konserv 108-117. Zumbühl S. et al. (2009) Stud Conserv 54, 149-159. doi.org/10.1179/sic.2009.54.3.149 Zumbühl S. et al. (2014) Appl Spectrosc 68(4): 458-465. doi.org/10.1366/13-07280 Zumbühl, S. et al. (2017) Microchem J 134, 317-326. doi.org/10.1016/j.microc.2017.06.013 https://www.hkb.bfh.ch/en/campus/art-technological-laboratory

Some practical aspects of pigment analysis using Raman spectroscopy

An artwork often is a complex, multilayered and multicomponent system build-up. Chemical interaction between these components and layers, which age over time, influenced by environmental conditions, can lead to reactions that alter the visual impression, the integrity and the materials of an artwork. Conservation science is thus confronted with alterations at different interfaces and levels such as for example the pigment–environment (e.g. Cato et al. 2017), pigment-binder (Fig. 1, Ferreira et al. 2015) and the varnish-environment (e.g. Soulier et al. 2012) interface. To learn more about these phenomena, our current strategy is minimally invasive, combines the complementary techniques of infrared imaging (ATR-FTIR-FPA), Raman spectroscopy and scanning electron microscopy (VP-SEM, SE/BSE/EDS), applied sequentially to micro-samples. This combination not only covers a wide range of materials, but also delivers morphological and 2D stratigraphic information from minimal amounts of sample material. With respect to pigment identification on micro samples, Raman has many strong advantages in tracking down pigment traces, both inorganic and synthetic organic pigments. It probably even may be called ‘THE’ technique, when it comes to synthetic organic pigment detection and identification (e.g. Scherrer et al. 2009). Yet, in many cases, Raman cannot rely on simple routine measurements. This presentation gives some insight into non-standard behaviour at the laser-pigment interface and possible approaches to retrieve a useful spectrum. Literature References Cato E. et al. (2017). J Raman Spectrosc 1-10. doi.org/10.1002/jrs.5256 Ferreira E. et al. (2015). Herit Sci 3(1): 1-11. doi.org/10.1186/s40494-015-0052-3 Scherrer et al. (2009). Spectroc. Acta Pt. A-Molec. Biomolec. Spectr. 505-524. doi.org/10.1016/J.Saa.2008.11.029 Soulier et al. (2012). Z Kunsttechnol Konserv 108-117. https://www.hkb.bfh.ch/en/conservation-and-restoration/consulting-services

Laser dependent shifting of Raman bands with phthalocyanine pigments

Phthalocyanine pigments are commonly used in artist’s paints [PG7 (1936), PG36 (1957), PB15 (1928), PB16 (1939)], can easily be detected by Raman spectroscopy and are thus of interest for authentication purposes. Apart from PG36 and PB16, they all give a good response with the 785nm laser, which seems the most versatile wavelength for pigment analysis. Pigment blue 15 exists in different crystal modifications (α, β, γ, δ, ε), whereby the distinction of the alpha (1928) versus the beta (1953) modification is of particular interest. Phthalocyanine pigments exhibit a peculiar behaviour when exposed to variable laser power of standard Raman excitation sources. Several observations were made: a) spectra of the phthalocyanine PG7 acquired with 514, 633 and 785nm excitation wavelengths differ such that they will not be matched in a reference database unless acquired by the same laser; b) increasing laser intensity produces a significant shift (up to 10cm-1) of the main bands of the macrocycle (C-N bond lengths); c) band shifting is reversible, despite visible alteration at the spot of analysis; d) sharp and distinct multiple bands are reduced to broad bands with excessive laser power. Due to the thermal and chemical stability of phthalocyanines, excessive laser power may not be noticed, as a spectrum will be delivered in any case. Explanations for the peak shifting in relation to excessive excitation energy is likely related to the unique physical properties such as reflectivity and dielectric constant as functions of the photon energy