Comparison of induced thermal change to climate chamber simulated environmental change in mosaic model by Digital Holographic Speckle Pattern Interferometry (DHSPI)

Ancient wall mosaic decorations can be exposed to uncontrolled environmental conditions. The main mosaic conservation problems, induced by external factors, are related to temperature and humidity variations, which may result in moisture, salt crystallization and accelerated material deterioration within the underlying layers. Though such deterioration in the subsurface is not immediately visible, it can strongly affect the preservation of the entire structure and the decoration layer in the short or long-term future. In this paper, we present and discuss laboratory experimental results of Digital Holographic Speckle Pattern Interferometry (DHSPI), applied to the subsurface diagnostic of a mosaic model after artificially induced thermal change and under simulated environmental conditions in climate chamber. The tests were performed on a custom-built wall mosaic model with known construction and known defects, which are simulating voids, detachments and deteriorated, restored or reshuffled areas. Two types of induced stress were used: 1) low thermal excitation emitted by IR lamps directly to the surface; 2) simulated environmental conditions inside the climate chamber. In regards of the in-situ diagnostic investigation any structural alteration occurs as a natural reaction to the surrounding environment and monitoring of surface reactions and defect detection under natural fluctuations it could be considered as a very important aspect of implementation to optimize range of applicability for non-destructive remote portable instrumentation. The obtained DHSPI results on the detection of known defects in the mosaic under artificially induced thermal alteration and under simulated environmental conditions are compared and discussed.