The Ferro-Prussiate, Or Blue Process
The preparation of the paper by this process is very simple, and requires very few manipulations, while the results, when prints are made, are pleasing and lasting. Prints in blue are so easily made as to be extremely convenient for making proofs from negatives, and they are well adapted for mottoes, plans, drawings, manuscript, circulars, and for representations of scenery, boats, machinery, etc., etc., and for engravers' use.

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Note added at 12 mins (2019-06-22 15:37:29 GMT)
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THE FERRO-PRUSSIATE PROCESS 622. Industrial Papers for Copying Tracings. Ferro-prussiate papers and fabrics are used in considerable quantities for making "blue-prints " from engineers' and architects' tracings. They In all cases where the differentiating reagent gives a coloured precipitate with the ferrous salt only, the reagent can be mixed with the ferric salt before sensitizing the paper, the reaction then taking place partly during the exposure to light and partly in the first wash water (or in a common solvent of the salts used). A positive image is obtained in this way if the print is made under a negative. If the differentiating reagent forms a coloured precipitate with the ferric salt, it should be used separately as a kind of developer in which the print is placed without intermediate washing. In this way a positive picture is obtained when the print is made under a positive.

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Printing Processes using Iron Salts
Cyanotype, Ferro-prussiate Paper
This ferric-salt process was announced by Sir John Herschel in 1842, it made little impact at the time (with the exception of Anna Atkins' work) but was later widely used to copy plans and drawings. The usual method of working was to coat a sheet of paper with solutions of ferric ammonium citrate and potassium ferricyanide, when dry the paper was exposed under the negative or drawing. After exposure the print need only be placed in water and then dried. In this form ferro-prussiate paper was available commercially. An alternative was to delay the application of the potassium ferricyanide until after exposure.

The light-sensitive salt, ferric ammonium citrate, was reduced to the ferrous state on exposure, this reacted with the ferricyanide resulting in the formation of Prussian blue. Clear parts of the drawing appeared blue in the cyanotype, a modification of the process, due to Pellet, reversed this to give a white background.

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