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Welham used a photogram printing process called wet cyanotype, where ammonium citrate and potassium ferricyanide are mixed to create a photosensitive solution which is then painted on to watercolour paper and left to dry.

In 1842 Herschel sent Children a copy of his paper describing his refinement of Talbot’s process, an invention he called the cyanotype, involving two chemical compounds, ammonium ferric citrate and potassium ferricyanide, which were applied to a sheet of paper with a brush.

After a secondary fixation in 1% osmium tetroxide, 1.5% potassium ferricyanide, samples were dehydrated through a graded ethanol series, and embedded in situ in an epon analogue resin.

When I was growing up my chemistry set had a lot of potentially harmful chemicals like copper sulfate and potassium ferricyanide.

Cyanotype is a photographic medium that uses ferric ammonium citrate, potassium ferricyanide, and simple sunlight to make an image.