Non-destructive analysis of Nineteenth century Scottish calotype negatives and salt prints

Abstract

ABSTRACTNineteenth 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.