Salt crystallization is a major agent of deterioration in buildings, especially important when belonging to a city’s cultural heritage. The study of this process is therefore essential to understand the decay evolution and to establish a correct preservation protocol to avoid future interventions. KNO₃ is a salt found commonly associated to other salts in weathered areas of buildings. In order to recognize the presence of this salt without sampling, the crystallization of KNO₃ was assessed by means of a new non-destructive tool, the infrared thermography. The supersaturation necessary for the nucleation of crystals was obtained by evaporation of solution droplets that allowed the recording of all the processes with an infrared thermography camera. The droplets were tested at different temperatures by placing them on an electric plate at 20 °C and 50 °C to simulate real conditions of building stones placed outdoors and also at 75 °C to emulate extreme situations of stones in arid environments. The droplets evaporated from four different substrates all of them with different interstitial properties: a black 3M tape frequently used as a reference substrate in similar works, a glass slide, and a marble plate with two different artificial finishes, polished and sawed. Thus, the contact angle was around 30° on the glass substrate, over 60° on the 3M tape and the polished marble, and close to 90° in the case of the sawed marble. Results highlighted the usefulness of the infrared thermography in the study of crystallization processes. The exothermic reaction associated to crystallization was too low to be observed, and only punctual heat release spots were recorded. However, a creeping process that creates efflorescence crystallization was clearly observed as an intermittent decrease in the thermographic signal preferentially at high temperatures. The contact angle of the droplet with the surface played an important role in the crystallization type (crystal shape, location, spreading length) when comparing different substrates (i.e., glass and black tape). The different test temperatures revealed different behaviors related to the substrate. The solution spread more at 20 °C on the black tape creating efflorescences while at 50 °C this kind of crystals formed on the stone with both finishes. At 75 °C, the solution spread uniformly over the surface of the glass slide while crystals grew on the top of preexisting ones for the three other substrates with higher contact angle.

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基板和温度对红外热成像技术研究KNO 3液滴结晶的影响

盐的结晶是导致建筑物恶化的主要因素，在属于城市的文化遗产时尤其重要。因此，对该过程的研究对于理解衰变的发展以及建立正确的保存规程以避免将来的干预至关重要。KNO ₃是一种盐，通常与建筑物风化区域中的其他盐相关。为了在不取样的情况下识别出这种盐的存在，KNO ₃的结晶通过一种新的非破坏性工具红外热像仪进行了评估。结晶成核所必需的过饱和度是通过蒸发溶液液滴而获得的，该液滴可以用红外热像仪记录所有过程。通过将液滴放置在20°C和50°C的电板上以模拟室外放置的建筑石材的真实条件，并在75°C以模拟干旱环境中石材的极端情况，在不同温度下测试了这些液滴。液滴从四种具有不同间隙特性的不同基材上蒸发：黑色3M胶带经常在类似作品中用作参考基材，玻璃载玻片和带有两种不同人造饰面的大理石板，经过抛光和锯切。从而，在玻璃基板上的接触角约为30°，在3M胶带和抛光大理石上的接触角超过60°，对于锯切的大理石，接触角接近90°。结果突出了红外热成像在结晶过程研究中的有用性。与结晶有关的放热反应太低而无法观察到，仅记录了准时放热点。但是，可以清楚地观察到产生风化结晶的蠕变过程，这是热成像信号的间歇性下降，优先在高温下。当比较不同的基板（例如玻璃和黑胶带）时，液滴与表面的接触角在结晶类型（晶体形状，位置，扩展长度）中起着重要作用。不同的测试温度揭示了与基材有关的不同行为。溶液在20°C的黑色胶带上散布得更多，产生了风化；而在50°C的情况下，这种晶体在两种表面上均形成于石材上。在75°C时，溶液均匀地散布在玻璃片的表面上，而晶体在其他三个具有更高接触角的基板的原有晶体的顶部生长。