An in‐depth technical examination and conservation treatment of paintings by William Williams (Bristol 1727–1791 Bristol) has shed light on the artist's materials and technique. This investigation centered primarily on Williams's two 1766 portraits of William and David Hall. The paintings are considered the earliest life‐sized, full‐length portraits executed in the Philadelphia area. The analysis of the artist's palette indicated deliberate choices in the use of orpiment (As2S3). The mineral's tendency to oxidize to colorless and water‐soluble arsenic oxides likely caused color changes and degraded organic binder in the orpiment‐rich areas. μ‐XANES revealed orpiment photodegradation to arsenate species at the paint surface, with migration to the ground layers. Just below the paint surface, arsenic remains bound primarily as arsenite, with some associated with sulfur as orpiment. This As distribution suggests that the paint is liable to further degradation by photooxidation and use of moisture would be detrimental. Given this treatment‐critical degradation phenomenon, it was important to identify all arsenic‐containing areas of both portraits. Scanning XRF allowed rapid and accurate collection of maps from both portraits. Elemental maps of arsenic identified the orpiment‐rich areas of the painting, which would be susceptible to further degradation upon exposure to water during treatment. An aqueous adhesive was necessary to consolidate the cupped paint of the glue‐paste lined paintings. The arsenic maps guided the use of two different consolidants–BEVA 371 for the water‐sensitive orpiment‐rich paint and sturgeon glue for all other areas, striking a compromise between esthetic improvement and long term preservation.
In this paper, we consider the advantages and limitations of two existing techniques: structured light fringe projection (FP) and reflectance transformation imaging (RTI), and we propose a new hybrid shape measurement approach that combines their advantages. FP allows direct full field-of-view measurements of shape; however, due to the nature of the technique, the high frequency shape details of the object may not be captured. RTI is a recently developed technique that allows high resolution measurements of surface normal vectors, which are related to high frequency details of shape of the object; however, extracting the object shape from RTI data requires numerical integration, which leads to cumulative low frequency bias errors. We present representative results that demonstrate the ability of our approach to perform high resolution shape measurements and non-destructive testing of structures.Keywords Fringe projection • High-resolution measurements • Optical metrology • Reflectance transformation imaging • 3D shape measurements
IntroductionAs technological developments progress, components having more complex geometries and shapes are required. In addition, new manufacturing methods to produce these components as well as corresponding metrology tools to guarantee their dimensional specifications are needed. Also, 3D shape measurements of such components become critical in terms of investigating their geometry as well as for rapid prototyping of similar objects in future applications. Shape measurements using optical techniques have become popular because of their non-contact and non-invasive nature.In this work, a new hybrid high-resolution optical shape measurement method that combines structured-light fringe projection (FP) and reflectance transformation imaging (RTI) is developed. FP allows direct full-field-of-view measurements of shape. However, in this technique, the spatial details defining the shape of the object under investigation may not be captured at a sufficient resolution. RTI is a recently developed technique that allows high-resolution measurements of surface normal vectors, which can be related to the spatial details defining the shape of the object under investigation. To extract shape information from RTI measurements, numerical integration is required. However, numerical integration is prone to errors, which prevent the use of RTI to perform direct shape measurements. Our hybrid shape measurement method exploits the advantages of FP and RTI.We present representative results of the approaches, which demonstrate the ability of our hybrid method to perform high-resolution shape measurements.
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