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.

Platinum Process (platinotypes)

This is a printing process that produces deep blacks and silvery-grey mid tones of subtle gradation. Variations in processing could produce warmer, sepia prints and toning was sometimes used. Platinum images consist of metallic platinum lying on the surface of the paper, rather than in a gelatine emulsion, they are permanent, stable, and the prints are as durable as the paper used.

The process was formulated by William Willis (b. 1840, d. 1923) between 1873 and 1879 when it was commercially introduced, it remained in popular use until the First World War period when the increase in the price of platinum greatly increased the cost. Platinum paper (prior to use) has a coating of ferric oxalate and potassium chloroplatinite, on exposure to light the ferric salt is reduced to the ferrous state. In the presence of the potassium oxalate developer the platinum salt that is in contact with the ferrous oxalate is reduced to the metallic state and the ferrous oxalate dissolved. The image is fixed and the remaining salts dissolved by washing in hydrochloric acid.

Platinum paper was sold pre-sensitised and ready for use, it was very sensitive to moisture and therefore sold in sealed containers, unused paper could be stored in air-tight tubes containing a drying agent. Platinum paper was exposed under a negative in a printing frame until a faint brownish image was visible on the yellow paper. Development was normally by immersion in a bath and had to be carried out soon after printing. An alternative was to brush on the developer in the presence of glycerine which retarded the action of the developer and allowed local control of the process. Development was either hot or cold, different papers were sold for each process, the hot process produced warmer sepia toned prints, cold development papers were introduced in 1892.

The image on the right shows a tube of platinum paper.

Papers with different textures were produced from smooth through to rough and in different thicknesses, the surface is normally matt but a semi-matt 'Japine' surface was also produced. A 1908 catalogue lists 12, 10" x 8", sheets of platinum paper as costing 6/6, in comparison ordinary bromide paper was priced at 2/9.

An early platinum process worked on by Willis had the platinum in the developer rather than being part of the sensitised paper, variations on this were used as an alternative to using commercially available paper. Satista, a silver-platinum, paper was introduced in 1914 as a cheaper alternative to platinum.

Minor Processes

Garnier and Salmon

Henri Garnier and Alphonse Salmon developed a dusting-on process using iron citrate. The process depended on iron citrate becoming non-hygroscopic when acted on by light and on the adhesion of carbon powder to moistened iron citrate unmodified by light. A very strong solution of iron citrate was coated on a sheet of paper, placed under a positive print and exposed to light. On removal from the printing frame the paper was dabbed with carbon powder at the same time as breathing on it, the carbon powder would attach itself to the unexposed parts. Dipping in water would remove the unaffected citrate of iron.¹

Fargier's iron salts process

Fargier's process of c. 1874 used paper coated with iron perchloride and citric acid exposed under a negative. The paper was then coated with pigmented gelatine or gum containing potassium bichromate. The gelatine became insoluble where it was in contact with light-affected perchloride. No use was made of the process.²