Metal oxides

  • Ammonium ferric citrate - Iron ammonium citrate - the base material for making cyanotype prints

What is Cyanotype? What Materials Do I Need for Cyanotype Printing? Where Can I Buy materials such Ammonium ferric citrate and potassium ferricyanide?


Ammonium ferric citrate is a chemical compound complex formed by the combination of iron (Fe³⁺) and citrate, which is derived from citric acid. This compound has several applications, and its use can vary depending on the specific form of ammonium ferric citrate (green or brown) and its intended purpose.

Here are a couple of common uses:

Medicine: Ammonium ferric citrate is used in the pharmaceutical industry, particularly in the preparation of iron supplements. Iron is an essential element for the human body, and iron supplements are often prescribed to individuals with iron deficiency anemia.

As a food ingredient, it has an INS number 381, and is used as an acidity regulator. Most notably used in the Scottish beverage Irn-Bru.

Water purification

As a reducing agent of metal salts of low activity like gold and silver

Photography: Ammonium ferric citrate, especially the green form, is used in the field of photography. It is employed in the preparation of cyanotype solutions, which are used to create blueprints and blue-toned photographs.

Cyanotype is a photographic printing process

that relies on the light-sensitive properties of ammonium ferric citrate when combined with potassium ferricyanide.

It's important to note that while ammonium ferric citrate has these specific uses, it should be handled with care, and its use in medical or photographic applications should adhere to appropriate guidelines and safety precautions.

The cyanotype process is one of remarkable simplicity, producing a blue and white print on paper or cloth. It was invented in 1842 by Sir John Herschel. The first book of photographs was printed using this method! From 1870 to sometime in the 20th century, it was widely used as the "blueprint" method to copy technical drafts until photocopy methods made it obsolete.

Cyanotypes aren't silver-based as most other photo-chemical photographic processes, Cyanotypes are based on the chemical reaction that produces "Prussian Blue", between Ferricyanide and Iron (III) molecules, this reaction is accelerated by UV light, such as the UV in sunlight or "black light" lamps. Traditional cyanotype involves ferric ammonium citrate (NH4)5[Fe(C6H4O7)2] and potassium ferricyanide, K3[Fe(CN)6] potassium dichromate is used as an additive, to increase sensitivity and contrast. Newer versions of this process use Ammonium Iron(III) Oxalate as ferric salt which gives it a more sensitive and longer life. Traditional cyanotype is very sensitive to alkali and it fades over time in darkness, a reexposure to light makes it reappear.

The paper can be prepared under incandescent light (no UV), by applying the sensitizer solution directly to the paper, cloth or other media, which can then be printed on dry. Applying more than 1 coat of sensitizer makes the image a deeper tone of blue.

It is a printing-out process: the image formed by the light is visible, not latent, and the exposure can be estimated by the changes in color of the emulsion. There is no strict "development" stage, instead, excess emulsion is washed away by running water. Washing in a diluted acetic acid solution (0.5-1.5%) increases the midtone density and the color looks brighter. Toning can be applied to increase or decrease the intensity of the blue and/or the contrast.

A common use is to make photograms: opaque objects with interesting shapes are laid on the sensitized paper, and then the paper is put in direct sunlight or under an ultraviolet lamp, for a time usually measured in minutes (quality of sunlight or brightness of the UV lamp is a factor.)

Of course, this can also be done with large or medium format negatives, or ink on tracing paper. The latter is how blueprints were made when the process was in vogue. A drafter would draw the plans on paper in pencil, then trace it in ink on tracing paper. This would then be used as a negative, to produce positive copies made of white lines on a Prussian Blue background. This was done for several reasons, the main being that it was cheap, could make any number of copies, and was very difficult to alter once printed.

Cyanotypes can be toned using several methods, a common one is to use tannins, such as the ones found in tea or coffee. This has the effect of increasing the permanence of the images. In the direct method, the image turn gray-blue and the paper will be stained as well, in the indirect method the amount of bleach determines the final color, in hues from gray-blue to brown.
As cyanotype is a cheap way of making contact prints, it still survives to this day under the heading of an "alternate process." Unfortunately, it is extremely difficult to make enlargements on cyanotype: most enlarger lenses are not designed to transmit ultraviolet light, and most enlargers do not take any common ultraviolet bulb. Thus, it is somewhat limited to the realm of contact prints and photograms.
More about Cyanotype can be found on Wikipedia

Formula: (NH4)5[Fe(C6H4O7)2
Molecular Weight: 265.00 g/mol
Form: green color solution
CAS Number: 1185-57-5
EC Number: 214-686-6
E Number: E381 (antioxidants)
Density: 1.8 g/cm³
Synonyms: Iron ammonium citrate, FERRIC AMMONIUM CITRATE, Ammonium ferric citrate, Ferri Seltz, Ferric ammonium citrate green, Ammonii ferri citras, Ammonium iron(3+) citrate, Ferriseltz, Iron ammonium citrate green, Ferri-ammoniumcitrat

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Cyanotype is a photographic printing process - Ferric Ammonium Citrate

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