Cyanotypes, known as photographs and architectural plans made by photo-reproduction from the 19th and 20th centuries, are subjects for conservation. Wet cleaning for conservation treatment has been reported to be unsuitable for cyanotypes because Prussian blue on cyanotypes is thought to move physically with the application of water. The manner in which Prussian blue is fixed onto the paper substrate is important for determining the treatment method. This study is the first step toward clarifying this mechanism. The presence of Prussian blue in cyanotypes was first confirmed using X-ray diffraction analysis (XRD). Then, the location of Prussian blue in the fibre was confirmed using optical microscopy and micro-Raman spectroscopy analysis, by observing the blue colour and by detecting its cyanide bond. With field-emission scanning electron microscopy (FE-SEM), particles approximately 20–100 nm in size were observed on the surface of cyanotype paper fibres, and particles approximately 20–50 nm in size were observed from the cross-section of the paper fibres. The location where the particles were observed agreed with the location where the blue colour was observed and cyanide bond was detected. The fact that the sensitiser solution soaked into the paper fibres and formed Prussian blue within the paper fibres when exposed to light is thought to be important for the blue fixation of cyanotypes.

Cyanotypes are known to de-colourize with alkalis; however, not much research has been done to regain the lost information. This research examines two possibilities to bring back the information: one is to utilize elemental mapping by X-ray fluorescence spectroscopy analysis to regain the image non-invasively, and the second, to treat the cyanotype with acidic solutions. Both methods brought back the image to the extent that the information could be obtained. As photo reproductions, one of their greatest values is the information they convey. This research could be significant in that it opens possibilities to regain the lost information.