Speckle and Conservation

The aim of this research is to demonstrate the usefulness of speckle, a trait of an expanded laser beam, for the non-destructive testing of artwork in both the imaging of subsurface structure and the quantitative detection of physical movement of canvas Laser Speckle Contrast Method (LSCI) is a useful method for the viewing of subsurface layers and movement. By investigating the statistical properties of dynamic speckle it is possible to reveal drawings that are hidden beneath scattering layers such as the primary layer of paint or adhered paper. This is achieved by taking a series of speckle images captured in a short time frame and applying one of a number of post processing algorithms. We explore the limitations of this method when applied to various paper samples that have a sketch executed in various media beneath the top layer. The ability to resolve gray scale images was examined as well as looking at the dependence of the contrast of the revealed drawings to the temperature of the surface. Current work is being done on using LSCI to reveal indentations in artwork caused by the application process. The successful use of Electronic Speckle Pattern Interferometry (ESPI) both in the laboratory and in-situ for the detection of in-plane movement of painted canvas due to humidity fluctuations and the out-of-plane movement of paint as it dries has also been demonstrated. Canvas paintings can be very susceptible to movement due to changes of the environment. ESPI is a non-destructive technique yielding sensitive results that can detect displacement on a surface of less than the wavelength of the illuminating coherent light source. While ESPI has been successfully applied to the in-situ study of painted frescoes, previous studies have employed tensile testers as a support for painted canvas. We have shown a portable version of ESPI to be of use in tropical environments in the Philippines, Malaysia and Singapore with original artworks where variations in humidity occur and the samples have not undergone special preparation before analysis, revealing significant directional movements. Furthermore, a simple variation in the direction of beams paths permits the characterisation of out-of-plane movement, specifically as the height of paint shrinks due to the drying process. We have used ESPI to view the drying process of alkyd resin paints over the time period of 24 hours. INTRODUCTION The classification and authentication of artwork is critical for artists, collectors, curators and museums. As forgers gain more insight and better tools for producing fraudulent pieces, it becomes even more important for conservation scientists to remain at the forefront of identifying and analyzing key elements of the artwork. Monitoring the structural integrity of paintings as they age is also important to avoid damage such as cracks and delamination. Here we explore two applications of laser based technology to conservation science; (i) LSCI (ii) ESPI. The first technique presented in this paper is a method to look beneath the primary surface of a work on paper to view inscriptions, alterations and underdrawings if they are present, this is known as laser speckle contrast imaging (LSCI) [1]. LSCI is a frequently used technique in the biomedical field to distinguish fluid flow and movement beneath organic tissue such as skin and bone [2]. Primarily used with organic materials, we demonstrate that it can also be utilized with static samples such as artworks with graphite drawings beneath them. As well as having specific identifiable attributes, canvas paintings are very susceptible to movement both initially during their construction and subsequently due to changes in the environment such as temperature and humidity. Electronic Speckle Pattern Interferometry 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65