The use of portable single-sided Relaxometry and laboratory imaging NMR devices in stone conservation

Two nuclear magnetic resonance (NMR) devices, a standard imaging apparatus and a portable single-sided relaxometer (with geometry that allows non-destructive in-situ relaxation measurements on samples of any dimensions), were used to assess the hydrophobic performances of an acrylic polymer (Paraloid B72) applied as a trichloromethane (chloroform) solution on a biocalcarenite (Lecce stone). The NMR images and relaxation time distribution functions of samples characterized by different treatments were acquired for different water absorption and measurement sequences. Samples were analyzed both fully saturated and during the capillary water absorption. The relaxation results have been compared with NMR images of internal sections of the same samples in the same absorption conditions. In spite of the instrumental and physical differences of the two devices (the single-sided device was tuned to detect the signal from a sensitive volume that is only a small portion of the entire sample, located in the first 2 mm below the surface), the experimental results of relaxometry and imaging are in good agreement. The comparison allows the assertion to be made that the single-sided NMR technique is a powerful tool for in-situ evaluation of water-repellent treatments used for consolidation and/or protection of stone artifacts.