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Microscope slide collections represent extremely valuable depositories of research material in a natural history, forensic, veterinary, and medical context. Unfortunately, most mounting media of these slides deteriorate over time, with the reason for this not yet understood at all. In this study, Raman spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, and different types of light microscopy were used to investigate the ageing behaviour of naturally aged slides from museum collections and the experimentally aged media of Canada balsam and Permount™, representing a natural and a synthetic resin, respectively, with both being based on mixtures of various terpenes. Whereas Canada balsam clearly revealed chemical ageing processes, visible as increasing colouration, Permount™ showed physical deterioration recognisable by the increasing number of cracks, which even often impacted a mounted specimen. Noticeable changes to the chemical and physical properties of these mounting media take decades in the case of Canada balsam but just a few years in the case of Permount™. Our results question whether or not Canada balsam should really be regarded as a mounting medium that lasts for centuries, if its increasing degree of polymerisation can lead to a mount which is no longer restorable.
The study, dedicated to Beautiful Pietàs conserved in South Tyrol (Northern Italy), aims to establish, for the first time, a connection between Austroalpine raw materials and the high-fired gypsum mortars constituting the Gothic figure groups in question. The origin and chronology of this stylistically and qualitatively differing ensemble have been subject of art historical debate for nearly a century. The discourse is dominated by three main hypotheses: itinerary of an Austrian artist versus itinerary of the work of art created in an artist’s workshop in Austria versus itinerary of the stylistic vocabulary via graphical or three-dimensional models. The comparison of the δ34S values and the 87Sr/86Sr ratios of the gypsum mortars and Austroalpine sulphate deposits (in a compilation of own reference samples and literature data) points to the exploitation of sediments in the Salzkammergut and possibly also in the evaporite district of the Eastern Calcareous Alps, thus evidencing the import of the sculptures and not the activities of local South Tyrolean or itinerant artists. Two geochronological units are distinguishable: The Pietà in the Church St. Martin in Göflan can be assigned to Upper Permian raw material, whereas the metrologically consistent sculptures in the Church of Our Lady of the Benedictine Abbey Marienberg and in the Chapel St. Ann in Mölten correlate with deposits of the Early Triassic (or the Lower-Middle Triassic transition). The medieval gypsum mortars also differ in their mineralogical characteristics, i.e. in their geologically related minor components, as in the first case, characterised by a significant proportion of primary anhydrite, natural carbonate impurities mainly consist of calcite (partly converted to lime-lump-like aggregates), whereas in the second group dolomite (or rather its hydration products after pyrometamorphic decomposition) predominates, accompanied by celestine, quartz and potassium feldspar. The Pietà in the Cathedral Maria Himmelfahrt in Bozen turned out to be made of Breitenbrunn calcareous sandstone (Leitha Mountains, Burgenland, Austria), which is why the sample is not considered in the geochemical analysis.
In the second half of the 19th century, Roman and Portland cements played an essential role as active hydraulic binder material in building construction and façade ornamentation. Size and heterogeneous phase assemblage of unhydrated cement clinker remnants in historical cement stone differ significantly from those of remnants occurring in modern Portland cement clinker burnt in rotary kilns due to limitations of the production technology available in the 19th century (e.g., comminution and homogeneity of the feedstock, burning temperature and regime in the intermittently operated shaft kilns, grinding machinery). In the common analytical approach, thin sections and fracture surfaces of historical Roman and Portland cement mortars are characterised regarding their mineralogical composition and microstructure using optical and electron microscopic imaging techniques. Raman microspectroscopy can be additionally employed for petrographic examination, overcoming some limitations of the methods used so far. The determination of the phase content of residual cement clinker grains in the hydrated matrix allows for the differentiation of Roman and Portland cement binders. As marker phases, we propose the calcium aluminates CA, C12A7, C2AS and C3A – besides the commonly used calcium silicates C2S and C3S – because of their different formation temperatures and stability fields. This study focuses on the identification of different calcium aluminate and aluminoferrite phases in clinker remnants in samples of cast ornaments of three buildings in Switzerland raised between 1875 and 1893; the obtained Raman spectra are compared with fingerprint spectra of the corresponding pure, synthesised clinker phases collected with the same instrument for an unambiguous data interpretation. In addition to these phases, mainly minerals showing no hydraulic activity, such as, wollastonite CS, rankinite C3S2, free lime, portlandite, iron oxides, garnets, augite, albite and feldspathoids have been identified in the sampled historical cement stones by Raman microspectroscopy. As there is a strong relationship between coexisting clinker phases and the chemical composition of the raw meal as well as the burning and cooling history during clinkering, the results can help in understanding the physical and mechanical characteristics of historical cement mortars. This knowledge is fundamental for the choice and the formulation of appropriate repair materials with tailored properties employed in the field of restoration and preservation of the architectural heritage of the 19th and early 20th centuries.
Raman microspectroscopic imaging was just recently introduced into the analysis of cement stone. Here, we demonstrate this approach on 19th-century Roman and Portland cement mortars and extend it to gypsum-based samples originating from a medieval stucco sculpture (high-burnt gypsum) and a stucco ornament prefabricated at the beginning of the 20th century (plaster of Paris). Furthermore, the distributions of dolomite and Calcite were mapped in an accessory mineral grain with approx. 500 nm lateral Resolution demonstrating the ability for studying alteration processes such as dedolomitisation. As we would like to make this approach accessible to other researchers, we discuss its present status, advantages, limitations and pitfalls.
Raman spectroscopy provides vibrational fingerprints of chemical compounds enabling their unambiguous identification. The assignment of Raman spectra to minerals is straightforward, if appropriate reference data is accessible. Modern couplings of Raman spectroscopy with microscopy (Raman microspectroscopy) merge the high structural specificity with down to sub-micrometre spatial resolution. This analytical tool has high potential not only in the identification of minerals from natural sources but also for studying the complex microstructure and mineral distribution of both ancient and modern man-made materials. In addition to the chemical identity of minerals, Raman spectra are affected by crystal orientations (varying relative Raman band intensities); (sub)stoichiometric compositional changes (e.g., in solid solution series), traces of foreign ions, strain (the latter three shifting Raman bands); and crystallinity (changing Raman band widths), enabling a comprehensive physico-chemical characterisation of minerals. Thus, Raman spectroscopy – including its in situ measurement capabilities – provides possibilities to study mineral paragenesis in both, natural and man-made samples at the micrometre scale. While in 1928 the first experimental evidence for inelastic light scattering was provided by C. V. Raman and K. S. Krishnan by using sunlight for excitation, filters for selecting the inelastically scattered light, and their eyes for detection (later, photographic plates for acquisition of spectra were employed), modern Raman spectrometers make use of laser excitation, dispersive spectrographs and charge coupled device (CCD) detection. This Special Issue includes technological developments and applications in the field of modern Raman spectroscopy of minerals in a broad sense, from natural mineral deposits and archaeological objects to inorganic phases in man-made materials. The studied minerals include fossil resins, typical rock-forming minerals (calcite, quartz, forsterite), iron-sulphur species (e.g., mackinawite), a range of sulphates (gypsum, bassanite, anhydrite III, anhydrite II, celestine, barite, ternesite), as well as silicate minerals like garnets (e.g., almandine).
Raman microscopic imaging was just recently introduced into the analysis of residual Roman and Portland cement grains in 19th century cement stone, displaying evidence of the experimental adaptation of contemporary technological knowledge and practice to local circumstance. Beyond calcium ferrites, this study deals with ferruginous clinker phases that are atypical compared to present-day commercial conditions of manufacture, such as iron oxides, clinopyroxenes or pyroxenoids. Analog, microtexture, mineralogy and chemical composition of pulverized ferrosilicate slag used in the course of the 19th century as mineral additive in lime mortar reflect local resource utilization, recording the melting history within the furnace and the effectiveness of the reduction process of a single smelting event. In the case of the discussed example, chemical imaging by Raman microscopy allowed deducing a lime-rich, low-silica melt exposed to fairly reducing conditions because of the detection of the pyrometallurgic phases fayalite (Fe2SiO4), kirschsteinite (CaFeSiO4) and calcioolivine (Ca2SiO4) in zoned olivine laths and (with the melilite gehlenite, Ca2Al2SiO7) in the Interstitial matrix, cross-cut by dendritic wuestite (FeO). The presented analytical approach faces the high spatial complexity of such mortar samples by microspectroscopic imaging with micrometer lateral resolution and their chemical complexity by extracting the rich chemical information content from Raman spectra. Intensity maps of marker bands provide spatial phase distributions. Furthermore, maps of peak positions can give access to the dissemination of spectroscopically similar phases of solid solution series (e.g., olivine and calcium Aluminate ferrite) as well as varying amounts of incorporated foreign cations (e.g., in hematite).
Even though being the subject of natural scientific research for many decades, the system CaSO4–H2O, consisting of the five crystalline phases gypsum, bassanite, and the anhydrites III, II, and I, has left many open questions for research. Raman spectroscopy was used because of its structural sensitivity and in situ measurement capability to obtain further insight by studying phase transitions in both ex situ and in situ experiments. The findings include significant contributions to the completeness and understanding of Raman spectroscopic data of the system. The dehydration path gypsum–bassanite–anhydrite III was shown to have strong parallels to a physical drying process, which depends on many parameters beyond the burning temperature. Raman band width determination was demonstrated to enable the quantitative discrimination of α-bassanite and β-bassanite as well as the postulated three sub-forms of anhydrite II (AII), which are all based on differences in crystallinity. In the latter case, the observed continuous structural variations over increasing burning temperatures were elucidated as a combination of decreasing surface areas and healing of crystal lattice defects. We propose an only two-fold sub-division of AII into reactive “disordered AII” and much less reactive “crystalline AII” with a transition temperature of 650°C ± 50 K.
Measuring the Burning Temperatures of Anhydrite Micrograins in a High-Fired Medieval Gypsum Mortar
(2017)
Typical feature of high-fired medieval gypsum mortars is a compact microstructure of squat gypsum crystals containing firing products as remains of the calcination process. So far, the burning history of the binder is estimated based on morphological characteristics of the latter. A novel Raman microspectroscopy approach provides access to the calcination temperatures of individual anhydrite grains based on quantifiable spectroscopic changes appearing due to gradual variations of crystallinity, as independently confirmed by X-ray diffraction analysis of anhydrites synthesised at temperatures between 500°C and 900°C. The approach was successfully applied to the high-fired gypsum mortar of a South Tyrolean stucco sculpture of a pieta dated around 1420. Microparticles of burned anhydrite II with firing temperatures scattered around 650°C and clusters of thermally damaged natural anhydrite II crystals from the raw material were identified and imaged.
The use of high-fired gypsum as binder for masonry and joint mortars or stuccowork in Central Europe in the Early and High Middle Ages was a regional specific as it depended on local gypsum deposits. The calcination technology possible at the time resulted in an assemblage of calcium sulphate phases dehydrated to different degrees and partly thermally damaged accessory minerals of the raw gypsum. Not hydrated clusters of firing products preserved in the binder matrix are a typical feature of such mortars. A novel Raman microspectroscopic approach, providing access to the burning history of individual anhydrite grains, was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for tracing and visualising pyrometamorphic reactions in natural impurities of the kiln run. In the discussed examples mineral thermometry indicates process temperatures above 800°C: the breakdown of magnesium-rich chlorite led to the formation of forsterite Mg2SiO4, while the thermal decomposition of dolomite CaMg(CO3)2 to periclase MgO and lime CaO yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Hydration of periclase in the mixed gypsum paste containing sulphate ions also resulted in magnesium sulphate hydrates, here identified in the form of hexahydrite MgSO4·6H2O. Lower burning temperatures left the accessory minerals in their pristine form, but can be traced by measuring the spectra of individual anhydrite crystals in grains of firing products and evaluating Raman band widths. Throughout the present study, calcination temperatures ranging from approx. 600°C to 900°C were determined.
Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars
(2019)
When used as a mineral binder, gypsum is thermally dehydrated and mixed with water, resulting in a paste hardening in the backreaction to calcium sulphate dihydrate (CaSO4 · 2 H2O). Although nowadays mainly hemihydratebased (CaSO4 · ½ H2O) binders are employed, higher firing temperatures in medieval kilns yielded anhydrite II (CaSO4). Except for the discrimination of the metastable phases anhydrite III and I due to different crystal structures, variations within the production temperature range of anhydrite II (approximately 300 to 1180°C) were not analytically accessible until recently. This study describes the development of an analytical technique, which is based on steady changes of band widths in room‐temperature Raman spectra of anhydrite II as a function of burning temperature. Raman microspectroscopic mapping experiments enable to pinpoint individual unreacted grains of thermal anhydrite in mortars and to discriminate them from natural anhydrites originating from the raw gypsum. The determination of band full widths at half maximum of down to 3 cm−1 and differences between them of a few tenths of wavenumbers is not a trivial task. Thus, a focus of this work is on peak fitting and strategies for correction of instrument‐dependent band broadening, which is often neglected also beyond the field of mortar analysis. Including other potential influences on band widths, burning temperatures of 400 to 900°C can be retraced in high‐fired medieval gypsum mortars with an uncertainty of approximately ± 50 K, as demonstrated with sample material of a stucco sculpture dated around 1400.