Abstract
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.
Highlights
Cultural heritage built from limestone is prone to deterioration by chemical weathering, a natural process, that is enhanced by pollution
In order to achieve this on a tungsten scanning electron microscope (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
Surface topographic and compositional contrast images were generated on a standard Zeiss EVO MA10 tungsten emission scanning electron microscope (SEM) equipped with variable pressure (VP) air admittance, a 5 segment HDBSD backscattered electron detector (BSE) and a beam deceleration (BD) kit to put a bias voltage on a single sample holder
Summary
Visualization of the effects of a treatment is the key to understand processes and not the least to convince whether theory meets the real world. This, in part, is likely due to the difficulty of imaging non-conductive samples at sufficiently high quality with minimal penetration depth (low kV), which, essentially is required to account for the sensitivity of organic material to the electron beam and the thinness of the newly formed material. The key would be to be able to observe non-conductive surfaces of rock samples at low acceleration voltage with reproducible positioning. We present a strategy that is capable of delivering SEM images at various (dry) stages of the process—original (cleaned) surface, after the consolidation treatment, and after outdoor weathering—at the same location with a standard tungsten cathode variable pressure VP-SEM and a beam deceleration accessory
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