prussian blue (pb) and its analogues are widely studied because of their interesting and promising magnetic and optical properties. the pigment prussian blue, found in different types of artworks (paintings, watercolors and photographs), is also studied in the area of heritage science, where its capricious fading behavior under light or anoxia treatment poses problematic conservation issues. pb fading is due to the reduction of iron(iii) to iron(ii) and depends significantly on the artefact. this paper focuses on the roles of the substrate in affecting the pb structure and modifying the redox process. in particular, x-ray absorption experiments at the fe k-edge of unfaded and faded pb\textendash paper samples show that changes in the pb structure can happen by simple contact with the substrate, prior to the fading treatment. spectrophotometric measurements on a series of model pb\textendash paper samples further demonstrate the multiple influences of the substrate and show that not only its chemical composition but also its role as a dispersion and textured medium significantly alter the fading behavior of pb. a potential roadmap is proposed to rationally investigate the complex fading process of prussian blue on a substrate.

this activity demonstrates catalysis of chemical reactions by ultraviolet (uv) light using one of the earliest photographic processes, the cyanotype process. the photographic paper for cyanotype photography is easily prepared in the classroom, giving students the opportunity to see that the photographic image is a result of chemical treatment of ordinary drawing paper. the sensitized paper can be handled in visible light. the image is produced by exposure to uv light from direct sunlight or fluorescent black lights.

this laboratory experiment is intended for a chemistry course for non-science majors. the experiment utilizes one of the earliest photographic processes, the cyanotype process, to demonstrate uv catalysis of chemical reactions. in addition to making photographic prints from negatives, the process can be used to test the effectiveness of sunscreens and the relative efficacy of the spf (sun protection factor) rating of sunscreens. this is an inexpensive process, requiring solutions of ammonium ferric citrate and potassium ferricyanide, with options to use hydrogen peroxide and ammonium hydroxide solutions. students can prepare their own uv-sensitized paper with the indicated chemicals and watch the photographic image appear as it is exposed to sunlight or fluorescent uv lamps in a light box designed for use in this experiment. the laboratory experiment should stimulate discussion of uv catalysis, photographic processes and photochemistry, sunscreens, and uv damage to biological organisms. the chemicals used are relatively nontoxic, and the procedure is simple enough to be used by groups of diverse ages and abilities.

engaging students and the public in understanding uv radiation and its effects is achievable using the real time experiment that incorporates blueprint paper, an ``educational toy'' that is a safe and easy demonstration of the cyanotype chemical process. the cyanotype process works through the presence of uv radiation. the blueprint paper was investigated to be used as not only engagement in discussion for public outreach about uv radiation, but also as a practical way to introduce the exploration of measurement of uv radiation exposure and as a consequence, digital image analysis. tests of print methods and experiments, dose response, spectral response and dark response were investigated. two methods of image analysis for dose response calculation are provided using easy to access software and two methods of pixel count analysis were used to determine spectral response characteristics. variation in manufacture of the blueprint paper product indicates some variance between measurements. most importantly, as a result of this investigation, a preliminary spectral response range for the radiation required to produce the cyanotype reaction is presented here, which has until now been unknown.

whether it is more apt to describe cyanotype as invented \textemdash{} or discovered \textemdash{} by sir john herschel in 1842 is a question the reader may decide by the end of this essay. whatever the verdict, it is well known that this process of photographic contact-printing in prussian blue was brought to light just three years after louis daguerre and henry talbot had announced their independent inventions of photography in silver, using metal and paper substrates, respectively. their successes in finally securing images of silver represent separate fruitions of an idea that had been gestating for more than a century in the minds and laboratories of several noted natural philosophers.1 in contrast to this well-known history, the birth of cyanotype was totally unexpected, coming both literally and metaphorically `out of the blue', to a single parent. the question that this essay will address \textemdash{} at least, by implication \textemdash{} is: why should the circumstances. of these two photographic innovations have been so different?

the accidental discovery over 300 years ago of the artists' pigment prussian blue, iron(iii) hexacyanoferrate(ii), opened up a whole new area of chemistry\textemdash that of the cyanide radical. the variable composition of prussian blue has tantalized chemists, until investigations by x-ray crystallography in the late 20th century determined its structure. the open 'zeolytic' lattice can act as host to small molecules and ions, making the substance useful as an antidote to certain poisons, notably thallium and radiocaesium. the redox chemistry of the 'mixed valence' iron atoms in prussian blue confers the property of electrochromism on the solid, which can therefore serve in electronic digital displays. photochemical production of the pigment is the basis for the cyanotype or blueprint reprographic process. inclusion of redox-active species in the prussian blue lattice can 'fine-tune' its color, by shifting the electronic charge transfer band in the visible absorption spectrum. thus, photographic artists unknowingly employ coordination chemistry to tone the hues of their cyanotypes.