Cyanotype-

The recording of light took various forms over the course of history. Some were expensive while others used dangerous chemicals. The cyanotype is a process that is inexpensive and uses common chemicals to produce the rich blue images which are more common to Blueprints. The basic cyanotype recipe has not changed very much since Sir John Herschel introduced it in 1842. Unlike photographs set in silver, like in black and white photography, cyanotypes are using a solution of iron compounds.

The Cyanotype Process at a Glance

The cyanotype process is simple. It can be done easily in a few steps:

Mixing chemicalsThe cyanotype is made up of two simple solutions.

• Potassium ferricyanide and Ferric ammonium citrate (green) are mixed with water separately.
• The two solutions are then blended together in equal parts.

How does the cyanotype process work?

The classical form of cyanotype is that the paper is coated with a solution of a light-sensitive mixture of two chemicals – the light-sensitive iron(III) complex with a readily oxidizable polybasic carboxylic acid, usually citric acid or oxalic acid, and hexacyanoferrate(III). The resulting paint is allowed to dry and then the sensitized paper is covered with a negative original or various objects or opaque originals and exposed to UV light (either from the UV lamp or sunlight).

Figure 3: Formation of Prussian blue

The light-sensitive iron(III) complex represents the core of the whole process. The central iron(III) atom coordinates six carboxylate groups contained in the anions of the dicarboxylic acid (or tricarboxylic acid). In the case of oxalic acid, iron coordinates three oxalate anions and forms an octahedral potassium tris(oxalato)ferrate(III) complex, which is a stable, crystalline, relatively water-soluble green crystalline compound.

Figure 4: Structure of the light-sensitive complex

The original recipe of Sir Herschel employed the ammonium salt of citric acid while later industrial formulations have, however, prefered the oxalate process, since it allows a faster printing with better contrast. In addition, such formulations are less prone to mold attack (ammonium citrate supports mold growth because it contains both carbon and nitrogen in a well bioavailable form).

The violet and ultraviolet portions of the light spectrum have a sufficiently high energy to excite the tris(oxalato)ferrate(III) complex. The excited complex is unstable and undergoes an internal redox reaction – the central FeIII atom as an oxidizing agent oxidizes the nearby oxalate anion to the carbon dioxide and gets reduced to FeII. The newly formed Fe2+ cations combine directly with the ferricyanide in the interior of the paper fiber to form the insoluble precipitate of Prussian blue. The light-sensitive reaction can be described in a simplified form as follows:

Figure 5: Photochemical decomposition of tris(oxalato)ferrate(III) complex

Taken from http://www.chemistryandlight.eu/index.php/cyanotype-process/

Preparing the canvas

• Paper, card, textiles or any other naturally absorbent material is coated with the solution and dried in the dark.

Printing the cyanotype

• Objects or negatives are placed on the material to make a print. The cyanotype is printed using UV light, such as the sun, a light box or a UV lamp.

Processing and drying

• After exposure the material is processed by simply rinsing it in water. A white print emerges on a blue background.
• The final print is dried and admired.

The process is described in detail in the article Cyanotype the Classic Process. Below are the details of how the paper was treated before the prints were made. The paper being used is watercolor paper, cold pressed with a rough surface and 200 GSM.

1. Mixing the chemicals

Dissolve the chemicals in water to make two separate solutions. Add Ammonium ferric citrate to water into one container and Potassium ferricyanide to water in another. Stir with a plastic spoon until the chemicals dissolve. Mix equal quantities of each solution together in a third container. Unused solutions can be stored separately in brown bottles away from light, but will not last very long once they have been mixed. Dispose of any unused chemicals in a sensible and environmentally friendly way!

• Solution A: 25 grams Ferric ammonium citrate (green) and 100 ml. water.
• Solution B: 10 grams Potassium ferricyanide and 100 ml. water.

2. Preparing the canvas

Using a brush, simply paint the chemicals onto the material. Paper, card, textiles or any natural material can be used to print on. Decide how big your print is going to be, and cut your material to size. Make sure your working area is dimly lit, or lit with a low-level tungsten bulb. Once the material is coated, leave it to dry in the dark.

3. Printing the cyanotype

Print a cyanotype by placing your negative (to reproduce a photograph) or object (to make a photogram) in contact with your coated paper or fabric. Sandwich it with a piece of plexiglass. Expose the sandwich to UV light. Natural sunlight is the traditional light source, but UV lamps can also be used. A photogram can also be made by placing items on the surface. Plants, decorative items or other objects can be used to create silhouettes or interesting shapes. Exposure times can vary from a few minutes to several hours, depending on how strong your light source is or the season where you are printing. I left mine out for 15 minutes.

4. Processing and drying
When the print has been exposed, process your print by rinsing it in cold water. The wash also removes any unexposed chemicals. Wash for at least 5 minutes, until all chemicals are removed and the water runs clear. Oxidation is also hastened this way – bringing out the blue color. The final print can now be hung to dry and be admired. Hydrogen Peroxide can be used to enhance the contrast between the blues and whites. It will bring out more details. Below is my finished piece.

MAKING YOUR OWN PRINTS FOR CLASS
You will need to follow steps number 3 and number 4. You will need to predict how transparent your object is to determine an accurate time to get finer details. The longer the exposure the more detail. Manufactures recommend 10 to 30 minutes and that depends on the weather since the paper is UV sensitive. You will need to make sure to put your name in pencil on the back of your paper. Although you should easily recognize your print based on the object(s) you used.

To download the reading of pages 1-22 of Blueprints to Cyanotypes please click HERE. You will need to read pages 7-22 to answer the worksheet below as well as use the resources on this tab.

To download the Cyanotype Worksheet click HERE.

Cyanotype Worksheet by Meredith Giltner on Scribd

Blueprint to Cyanotypes p1-22 by Meredith Giltner on Scribd