Non-destructive analysis of Nineteenth century Scottish calotype negatives and salt prints
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Non-destructive analysis of Nineteenth century Scottish calotype negatives and salt prints Katherine Eremin, James Tate, Alison Morrison-Low, James Berry1 and Sara Stevenson1 National Museums of Scotland, Chambers Street, Edinburgh, EH1 1JF. 1 National Galleries of Scotland, The Mound, Edinburgh, EH2 2EL. ABSTRACT Nineteenth century negatives and positives in the collections of the National Museums of Scotland (NMS) and the National Galleries of Scotland (NGS) were analysed non-destructively to identify the techniques used in their manufacture. Modern positive and negative images prepared using known nineteenth century processes were also analysed for comparison. Air-path energy dispersive x-ray fluorescence analysis and controlled pressure scanning electron microscopy with energy dispersive microanalysis enabled the images to be divided into groups based on the levels of bromine, iodine and silver, and the likely processes used inferred. An early group of positives were probably sensitised with either silver chloride or silver bromide and fixed with potassium bromide. However, most positives were probably sensitised with silver chloride and fixed with sodium thiosulphate. Most negatives were probably sensitized with silver iodide and fixed with potassium bromide (predominant), sodium thiosulphate or potassium iodide. Cobalt and arsenic are present due to the use of smalt in the production of white paper. Copper and zinc are attributed to incorporation of fragments of brass buttons left on the rags used in paper production, observed as small blue spots. The presence of iron, sometimes visible as orange spots, may be from rust off the paper making machines. INTRODUCTION In 1839 William Henry Fox Talbot announced the photogenic drawing process, which fixed the image seen in a camera obscura, and in 1841 patented the calotype process in England [1, 2]. The calotype process was introduced to Scotland through his correspondence with Sir David Brewster, and John Adamson took the first successful Scottish portrait photograph in St Andrews, probably in 1842 [3, 4, 5]. John Adamson and his brother Robert, who worked with David Octavius Hill from 1843 to 1847, used the technique to produce a large corpus of negatives and positives [6, 7, 8]. John Adamson received some information from Talbot, and the Adamsons probably based their methods, at least initially, on Talbot’s. Surviving documentary evidence shows Talbot used several processes [2, 9]. However, if the Scottish photographers documented their processes, the information has not survived. The Scottish images vary significantly in colour and tone, suggesting experimentation with different chemicals and processes. Production of images involved two main stages: sensitisation of paper with silver halide followed by fixation with a solution of halide or sodium thiosulphate to ensure the final image was insensitive to light. Treatment with halide modified the residual unexposed silver halide to a less reactive halide, whilst treatment with thiosulphate removed the residual hal
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