Non-invasive imaging techniques for the investigation of cultural objects

© 2017 Dr. Elaine Robyn MilesLaser speckle is used in cultural materials conservation to study changes in the physical condition and movement on the surface and within the structure of objects. This thesis investigates three speckle methods, Laser Speckle Contrast Imaging (LSCI), Dynamic Speckle Imaging (DSI) and Electronic Speckle Pattern Interferometry (ESPI) to: image obscured text, examine paint application methods, record physical transformations during paint drying processes, and monitor canvas strain due to climate variations in- situ. It extends the boundaries of use of speckle-based methods in conservation through a set of four case studies that cover a broad range of cultural conservation problems faced in Australian collections. These studies were contextualised within an examination the properties of viscoelastic materials, a mathematical description of speckle, an explanation of the speckle-based methods and the experimental equipment and design considerations that provide information about the challenges and considerations required when deploying speckle-based methods. In the first case study, LSCI was used to image obscured text on books that had been repaired with layers of paper. The quality of the results, which was dependent on the weight of the paper, colour of the subsurface medium, and temperature of the object, was similar to that of Infrared Photography. In the second case study LSCI was used to monitor the curing process of modified alkyd resins over 120 hours to investigate whether the application method used by the artist was robust to typical ageing conditions. The results correlated well with traditional gravimetric results, which only provide information about the mass changes to the samples with no spatial information. LSCI was able to demonstrate where and when the alkyd resin surface was moving, most likely due to off gassing processes. The third case study used DSI to monitor the drying process of artists’ acrylic paints, allowing for the novel non-invasive detection of when the paint was touch dry, a task that normally involves destructive methods or has only been applied to rapid drying industrial paints. The final case study demonstrated the in-situ application of ESPI to monitor two archetypical colonial easel paintings in assessing physical movement due to variations in the display environment. ESPI was used in-situ to record changes in the strain of the canvas. Furthermore, it was shown that DSI analysis could be applied to ESPI data to monitor for vibrational disturbances that can decorrelate results The speckle-based techniques used in this thesis were shown to satisfy six criteria for preferred conservation methods: all information is gathered non-invasively and can be analysed in a variety of ways; off the shelf optical components are used, thus it is highly suitable for fields in which specialised optical components are not available; no prior analysis about the sample is required; experimental challenges can be diagnosed and overcome through the analysis of data sets and/or computer simulations; the techniques can be applied to the investigation of a range of materials with different optical properties; and multiple speckle-based methods can be combined to provide complementary information. The case-studies explored in this thesis further demonstrated that speckle is capable of non-invasive characterisation of many physical properties and can be applied both in the laboratory and in-situ. Through the assessment of theory and case studies this work has extended the application of speckle analysis within conservation studies, delivering practical outcomes to aid conservation efforts