I’ve always been interested in the chemistry behind photographic processes (as long as my limited chemistry comprehension allows me to grasp the reactions). For the chemistry geeks out there, here’s the reaction that takes place:
· Ferric Ammonium Citrate and Potassium Ferricyanide. (A liquid cyanotype kit containing both solutions can be obtained online from Photographers’ Formulary for $18.00. The kit can produce an estimated twenty-four 8x10’s.)
· Watercolor paper (recommended, but other paper works, as do textiles containing at least 50% cotton)
· Teaspoon (or a graduated cylinder or pipette for those fortunate enough to work in a laboratory setting.)
· Scotch tape
· Hake brush (recommended, but paint brushes or foam brushes are fine)
· Transparency sheets ( used for printing negatives on) or objects for creating silhouettes (e.g., leaves, ferns, paper cut-outs)
· Glass plate large enough to cover image to be printed
· Running water (from a sink, for instance. Note that hard water will affect the final image color)
· UV light source (the Sun works great and is free, but a UV light allows for greater predictability of exposure times)
1. Prepare the negatives or objects you wish to use to create the print. If using objects, objects must be flat enough to be sandwiched beneath a glass pane; creating these silhouettes (photograms) is a great introduction to the cyanotype process, since it is nearly impossible to overexpose the image. If using a negative to reproduce a photograph, create a negative by inverting the color of a digital image (e.g., "invert color" in Microsoft Paint) and printing on transparency sheets; make sure to choose "transparency" for the paper type under printing options.
If desired, it is possible to change the typical blue cyanotype to a sepia tone by first using a bleaching agent (sodium carbonate or ammonia), followed by a toning agent (tea). During the bleaching phase, the Prussian blue lattice is broken up by the hydroxide ion, and the
ferric iron is converted into an insoluble pale yellow-brown ferric hydroxide,
while the soluble ferrocyanide ion is washed away. Next, the toning process is performed by soaking the print in tea, which contains tannins--based on my limited understanding of the chemistry, by mixing tannin with the iron hydroxide, an insoluble, dark ferric tannate pigment is formed.