Simone Bugani

Investigating morphological changes in treated vs. untreated stone building materials by x-ray micro-CT

AbstractCalcareous stones have been largely used to build historical buildings. Among these, the calcarenites are usually characterized by a high content of calcite and a high open porosity, which make them very sensitive to the weathering caused by physical and chemical agents. In order to prevent their deterioration and to retard their decay, different protective products—mainly polymers—are applied on the stone artefact surfaces. In this work we apply the methodology tested in a preliminary study to investigate the morphological changes of the internal structure of a biocalcarenite (Lecce stone) by micro x-ray computed tomography (μ-CT). The porosity and other morphological parameters of the rock before and after the conservation treatment were calculated on a significant number of samples. The Student’s t test was applied for statistical comparison. The results reveal that the treatment with Paraloid B72 (PB 72) is homogenously distributed and causes small changes to the natural properties of the rock, whereas the application of a fluoroelastomer (NH) causes an appreciable decrease in porosity and variation in terms of wall thickness distribution, probably resulting from its inhomogeneous distribution. FigurePorosity and other morphological parameters of Lecce stone were investigated by μ-CT: the effect of conservation treatment with fluoroelastomer on wall thickness distribution is illustrated

Study on the impregnation of archaeological waterlogged wood with consolidation treatments using synchrotron radiation microtomography

In favourable conditions of low temperature and low oxygen concentration, archaeological waterlogged wooden artefacts, such as shipwrecks, can survive with a good state of preservation. Nevertheless, anaerobic bacteria can considerably degrade waterlogged wooden objects with a significant loss in polysaccharidic components. Due to these decay processes, wood porosity and water content increase under ageing. In such conditions, the conservation treatments of archaeological wooden artefacts often involve the replacement of water with substances which fill the cavities and help to prevent collapse and stress during drying. The treatments are very often expensive and technically difficult, and their effectiveness very much depends on the chemical and physical characteristics of the substances used for impregnation. Also important are the degree of cavity-filling, penetration depth and distribution in the structure of the wood. In this study, the distribution in wood cavities of some mixtures based on polyethylene glycols and colophony, used for the conservation of waterlogged archaeological wood, was investigated using synchrotron radiation X-ray computed microtomography (SR-µCT). This non-destructive imaging technique was useful for the study of the degraded waterlogged wood and enabled us to visualise the morphology of the wood and the distribution of the materials used in the wood treatments. The study has shown how deposition is strictly related to the dimension of the wooden cavities. The work is currently proceeding with the comparison of synchrotron observations with the data of the solutions viscosity and with those of the properties imparted to the wood by the treatments.

Evaluation of manganese-bodies removal in historical stained glass windows via SR-μ-XANES/XRF and SR-μ-CT

The speed and effectiveness of a conservation treatment used for stained glass windows have been investigated. Dark-coloured Mn-rich stains can be found in the alteration layer of ancient glass artefacts and cause the surface to turn brown/black: this phenomenon is known as Mn-browning or Mn-staining. While in glass manganese is present in the +II or +III oxidation states, in the Mn-rich bodies, manganese is in a higher oxidation state (+IV). In restoration practice, mildly reducing solutions are employed to eliminate the dark colour and restore the clear appearance of the glass. In this paper the effectiveness and side effects of the use of hydroxylamine hydrochloride for this purpose are assessed. Archaeological fragments of stained glass windows, dated to the 14th century and originating from Sidney Sussex College, Cambridge (UK), were examined by means of synchrotron radiation (SR) based microscopic X-ray Absorption Near-Edge Spectroscopy (μ-XANES) and microscopic X-Ray Fluorescence (μ-XRF) and with high resolution computed absorption tomography (μ-CT) before, during and after the treatment. The monitoring of the glass fragments during the treatment allows us to better understand the manner in which the process unfolds and its kinetics. The results obtained reveal that the hydroxylamine hydrochloride treatment is effective, but also that it has a number of unwanted side effects. These findings are useful for optimizing the time and other modalities of the Mn-reducing treatment as well as minimizing its unwanted results.

Insights into the macroporosity of freeze-cast hierarchical geopolymers

Geopolymer monoliths with controlled lamellar macroporosity and total porosity ranging from 60% to 70% were prepared by ice-templating a partially geopolymerized slurry. Both the maturation treatment of the starting mixture and the water specifically added for freeze-casting were tailored to modify both the geopolymerization and viscosity of the slurry, and, consequently, its freezing behavior, in order to optimize the final lamellar architecture. Following a room temperature maturation treatment, a 50% water content added for freezing developed thick lamellae and wide pores. A lower water content (30%) and curing at 80 °C after maturation at room temperature (for both 50% and 30% H2O) was conducive to a narrow lamellar pore width distribution in the 30–130 μm range. However, the consumption of water due to geopolymerization in samples cured at 80 °C led to a decreased length and thickness of the lamellae. Lastly, the interparticle meso- and macropores (0.003 to 1 μm) within the geopolymer lamellae were only slightly modified by the maturation treatment.

Micro-XANES study on Mn browning: use of quantitative valence state maps

Historical glass, especially non-durable medieval glass, can undergo corrosion. This sometimes results in the formation of dark-coloured manganese-rich inclusions that reduce the transparency of the glass. While unaltered bulk glass contains manganese mainly present in the +II valence state, inside the inclusions Mn is present in higher valence states (+III to +IV). Two different strategies may be considered by conservators when aiming to improve the transparency. One is based on the reduction of highly oxidised black/brown compounds using mildly reducing solutions, while the other focuses more on the extraction of manganese from the inclusions by the application of chelating agents. In this paper, a method for quantitative mapping of the Mn speciation inside partially corroded historical windowpanes based on X-Ray Absorption Near-Edge Structure (XANES) spectroscopy is discussed. The calibration of such Mn valence state maps based on the combo method, a fairly reliable way to determine the oxidation state, is described in more detail. This method is used to evaluate the effect of reducing treatments on historical glass, dated to the 14th century and originating from Sidney Sussex College (Cambridge, UK), suffering from Mn browning. Glasses were examined by means of Synchrotron Radiation (SR) based microscopic X-Ray Absorption Near-Edge Structure (μXANES) spectroscopy and microscopic X-Ray Fluorescence (μXRF). X-Ray elemental distribution maps of glass cross-sections are recorded at different energies, while Mn K-edge spectra are used to convert these into Mn valence state (VS) maps. Such valence state maps will allow evaluation of a reducing treatment.

Non-destructive 3D measurements of sandstone's internal micro-architecture using high resolution micro-CT

Calcareous stones such as Lecce stones have a high porosity which results in a readily uptake of rainwater. Due to the atmospheric pollutants dissolved in the water these stones, used in a lot of historical buildings, are constantly under attack which leads to a decay of the stone [1]. Different kind of organic hydrophobic products such as Paraloid B72 (PB72) and fluorinated rubber (NH) are often applied as protectives with the aim to reduce the corrosion of the material. In order to study the manner in which these treatment products fill the pores a desktop X-ray microtomography system was used. This technique allows the 3D investigation of the internal structure of the stone in a non-destructive way [2,3]. In this research morphological parameters such as the total porosity (as a percentage of the enclosed empty spaces on the volume of interest), pore size distribution, surface-to-volume ratio (which gives an idea of the complexity of the internal structures) and structure model index (SMI) (giving an estimation of the average shape of the pores (0 = ideal plate, 3 = cylinder and 4 = sphere)) were calculated before and after treatment in order to evaluate the changes induced by the polymer application. The 2D reconstructed cross-sections, shown in Figure 1, confirm that Lecce stone has a very complex internal structure. Several different inclusions such as shells with different shapes and sizes (from a few µm up to 1mm, foraminifera in Figure 1) can be clearly distinguished. The 3D rendering of a small portion of the pores network (Figure 2) gives an idea of the complexity and interconnectivity of the internal structure. The pore size distribution (Figure 2) shows that almost 90% of the pores range from 8 to 29µm.