Monitoring changes in hydric properties of treated stone material with conservation products by time-sequential IR thermography

Archaeological and Anthropological Sciences (2024) 16:91 https://doi.org/10.1007/s12520-024-01995-6 RESEARCH Monitoring changes in hydric properties of treated stone material with conservation products by time‑sequential IR thermography Natalia Perez‑Ema1 · Monica Alvarez de Buergo1 · Miguel Gomez‑Heras2 Received: 18 January 2024 / Accepted: 3 May 2024 / Published online: 27 May 2024 © The Author(s) 2024 Abstract A methodological approach for a semi-quantitative non-destructive testing (NDT) of the effects on hydric properties after the application of different conservation treatments (commercial ethylsilicate and siloxane compounds with consolidant, water repellent or consolidant + water repellent properties) is presented in this study. The NDT used for this purpose is a simplified method of time-sequential infrared thermography (IRT) on stone (granite and marble, treated and untreated) from the Roman theatre of Merida (Spain), which was listed by UNESCO as World Heritage Site in 1993. As water evaporation modifies temperature intensely, IRT enables monitoring the response of the stone samples during water absorption-evaporation processes. The comparative analysis between treated and non-treated specimens was performed by calculating the rate of surface apparent temperature change (ΔoC/h) value. The capillary rising test, monitored by IRT, clearly showed a different response of the samples treated with water-repellent treatments, proving their efficiency. Samples treated with organosilicic compounds showed a large and rapid drop in apparent surface temperature, indicating that water accesses rapidly. Results from the evaporation test also show this difference. The surface apparent temperature increased immediately in samples with water-repellent treatments, in which water does not enter inside stone, but remains on the surface. Samples with ethyl silicate, again, show a greater difference with respect to untreated samples, taking longer to recover surface apparent temperature, because the water is retained inside the stone for longer. This study proved simplified time-sequential IRT as a reliable technique for assessing the efficacy of conservation treatments applied to stone materials in laboratory. Keywords Time-sequential infrared thermography · NDT · Merida · Stone decay · Archaeological heritage · Restoration treatments · Consolidants · Water-repellents Introduction The evaluation of changes in hydric properties of stone materials caused by the application of restoration treatments is a key task when analysing the effectiveness of conservation interventions, as water is probably the most dangerous agent of degradation regarding archaeological remains. Usually, techniques that require sampling are employed for this kind of tests, such * Natalia Perez‑Ema 1 Institute of Geosciences-IGEO, Spanish Research Council and Complutense University of Madrid (CSIC, UCM), C/Doctor Severo Ochoa 7, 28040 Madrid, Spain 2 Department of Geology and Geochemistry, Faculty of Sciences, Universidad Autónoma de Madrid, c/ Francisco Tomás y Valiente 7, 28049 Madrid, Spain as capillarity water absorption (UNE-EN 15801:2010 standard, AENOR 2010a) or water saturation tests (UNE-EN 1936:2007standard, AENOR 2007), which must be performed in laboratory. Other NDT, such as Karsten tube absorption test (UNE-EN 16302:2016 standard, AENOR 2016a) or water-substrate contact angle (UNE-EN 15802:2010 standard, AENOR 2010b), as well as the sponge test method (Ribeiro et al. 2022; Bison et al. 2023), may be useful in this sense, although providing only superficial and not bulk information. Restoration treatments, such as consolidating and protective ones, can cause adverse effects resulting in damage of the substrate caused by the creation of a non-breathable surface barrier (Pérez Ema and Álvarez de Buergo 2013). In this case, the consolidant product does not penetrate adequately into the porous system creating a plastic barrier or crust that prevents the internal moisture to get out. Same effect can arise from the application of protective products, especially when they are not Vol.:(0123456789) 91 Page 2 of 15 permeable to water vapour (Charola 2003; Laborde Márquez 2013; UNE-EN 16581:2016 standard, AENOR 2016b). Thus, the crystallization of salts would occur in the layer immediately below the surface and could cause detachment. IRT is a non-destructive and contactless technique, traditionally used for civil engineering and architecture, as well as, particularly, for cultural heritage. In this sense, applications are wide: from diagnosis studies to conservation interventions and monitoring tasks (inspection of moisture distribution; detection of cracks, delamination, detachment or voids; to verify the presence of different building materials, etc. (Avdelidis and Moropoulou 2004; Moropoulou et al. 2001, 2013; Sansonetti et al. 2012; Paoletti et al. 2013; Menendez 2016; Forestieri et al. 2017; Forestieri and Álvarez de Buergo 2018, Kirimtat and Krejcar 2018, Vázquez et al. 2015). An IR camera detects the heat emitted by the observed objects in form of IR radiation and converts it into an electronic image considering the objects as “black bodies”, i.e., hypothetical perfect absorbers and radiators of energy, with no reflecting power. Although some corrections can be made to calculate absolute surface temperature calculations for very homogeneous materials, most application of IRT are based on relative changes of apparent surface temperature. There are two methodological approaches regarding IRT investigation: single-shot passive thermography and multiimage thermography. In passive thermography, an image of the object is taken, registering differences of apparent surface temperature at a given moment. Multi-image thermography aims to analyse temperature changes as a function of space and, particularly, time. One of the methods used is active thermography, in which an external heat source is used to “excite the sample” and obtain a series of consecutive images after the application of the stimulus (Gómez Heras et al. 2014) to enhance the differences in thermal behaviour during cooling or heating. Sequential thermography focuses on studying how a certain process is reflected in spatial and temporal changes of apparent surface temperature. Specifically, time-sequential thermography is often used to determine heat fluxes from either buildings or the ground, by computing series of images taken at a certain time (Hoyano et al. 1999; Meier et al. 2010). Water has a very high specific latent heat of evaporation. Therefore, the evaporation process lowers intensely the surface of the evaporating material, even more if this is porous. Hence, IRT is a very suitable technique for studying evaporation in porous materials (Chauvet et al. 2010; Avdelidis et al. 2003; Ludwig et al. 2004, 2018; Sansonetti et al. 2012). In this work, a simplified approach regarding the application of time-sequential IRT for t (...truncated)