Influence of substrate and temperature on the crystallization of KNO3 droplets studied by infrared thermography

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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.