[Student Projects, Photography]
The effects of varying light source, exposure time, chemical ratio, and paper type when working with cyanotypes
History and background information:
"The cyanotype process was invented in 1842 by Sir John Herschel, after he had first discovered the photosensitivity of ferric (iron +3) salts. In this process, a suitable (both chemically and physically) sheet of paper is made sensitive to high energy ultra violet (UV) light by coaing it with a solution of ferric ammonium citrate and potassium ferricyanide. Exposure to UV light reduces a portion of the ferric (Fe +3) salt to the ferrous (Fe+2) state, and a portion of the ferricyanide (Fe+3 in Fe(CN)63-) to ferrocyanide (Fe +2 in Fe(CN)64-), resulting in the formation of a pale yellow-blue image consisting of ferrous ferrocyanide. Washing removes the soluble, unreduced (unexposed) salts, leaving behind a deep blue and white image. The image intensifies upon drying as ferrous ferrocyanide is slowly oxidized to a deep blue color that results from a mixture of ferric ferrocyanide and ferric ferricyanide. The oxidation can be hastened by treating the image in an oxidizing bath of either hydrogen peroxide of potassium dichromate."(Chem. and Art: Cyanotypes and Van Dyke Prints, Malde/Bordley, p1)
When making the perfectly colored cyanotype print several things must be considered. In order to achieve the deep blue hue desired in a cyanotype there must be an adequate light source, carefully prepared chemical solution, a sufficient exposure time, and paper that will yield the highest possible gradation from light to dark. In this project conducted in Chemistry and Art, a class concerned with explaining chemistry method at work in various forms of art at the Universtiy of the South, these factors were varied in several tests and the resulting cyanotypes created were compared in order to produce the most asthetically pleasing print possible.
Materials: 25% cotton paper, 100% cotton paper, "unknown" paper, Ferric ammonium citrate in solution, Potassium Ferricyanide in solution, light test wedges, UV light box, Sun, pipets and labeled jars, solution "spreader", stopwatch
Two light sources were chosen to provide the UV rays necessary for the chemical reaction to occur in cyanotypes. The project involved a UV light box and the sun. Chemical solutions were then prepared to have the desired ratios of the chemicals used. Solutions were made to have 1:2, 1:1, and 2:1 ratios(sol. A to sol. B). For the initial stages of the project the 1:2 solution was discarded because it was seen as insufficient in its results. It was later reincorporated as a point of reference when comparing the other two ratio samples.
After coating eight sheets of the unknown paper samples with the 1:1 and 2:1 solutions, two were left exposed in the sunlight for six minutes and two more were placed in the lightbox and exposed for six minutes. Then the same conditions were set up but the exposure time was doubled to that of twelve minutes. After the samples were exposed, they were rinsed of all unexposed ferric salts. When the samples had dried and oxidized completely the results were compared. After the initial results were compared it was decided that the light box would be used on account of the sun being an unreliable light source when working with a deadline.
At this point it was decided that the 1:2 chemical solution would be used again. Additionally, different paper types of 25% and 100% cotton paper were introduced along with the unknown sample and the experiments were repeated in order to factor in all variables of the project. The preparation method of the scattered test strips were "combined," as it were, onto a single sheets of different paper. A twelve minute exposure time was chosen for all new samples because six minutes proved insufficient in fully exposing the print.
After the experiments were completed the desired combination of cyanotype variables were quite subjectively concluded. When this had been done a beautiful cyanotype of a lace doily was made illustrating the graceful transition from light to dark areas and the deep "Prussian Blue" color characteristic of cyanotypes. Colorimeter measurements of the difference in lightness between each step were also taken down.
It was determined that a twelve minute exposure time was better than six because at six minutes not enough ferric ammonium citrate was sensitized, therefore sacrificing potential steps on the test wedge. In other words, a six minute exposure time failed to yield the desired gradation from light to dark blue. If a print were to be made that required a gradual and lengthy transition from light to dark in order to produce good detail, six minutes would prove inadequate.
After comparing the test strips exposed in sunlight and those exposed in the light box it was decided that the light box would be used for the rest of the project. The sun, while it is free and has no competition as far as amount of UV light emitted, is rather unreliable because it is impossible to control inclimate weather and hold atmospheric conditions constant. The lightbox produced a more even and consistently colored print.
Traditionally, the 1:1 ratio of ferric salts is preferred over all others. The reason for this was discovered after the experiments were conducted. With equal parts ferric ammonium citrate and potassium ferricyanide yields the best conditons for the reduction reaction to occur and produce the deepest blue possible of the three ratios tried. However, while the number of steps on the test wedge remained constant on each different paper sample the steps were pushed farther up on the scale indicating that as you move further toward a 2:1 ratio the print's sensitivity to light increases. From this it can be concluded that the ferric ammonium citrate, solution A, is the chemical sensitive to light, as its proportion is increased more of the chemical is sensitized. Since this is true it can also be concluded that the potassium ferricyanide, solution B, is the chemical allowing the blue color to come through.
Additionally, it was discovered that 100% cotton paper yielded 9 steps on the test wedge whereas the 25% cotton paper only gave up 8. Since the object of the project was to produce a cyanotype that not only looked superior but had as much gradation from light to dark as possible, the 100% cotton paper was deemed the best. The unknown sample was comparable to the 25% cotton paper as it too yielded only 8 steps. According to Ron Connelly, a contributer to a cyanotype email distribution list, there is a paper called Arche Platine which, if used correctly, is capable of producing prints having from 12 to 15 steps of color. Paper such as this would be necessary for highly detailed prints.
From the colorimeter measurements another characteristic of cyanotypes was discovered. The first three steps showed some change as they got darker, but when the fourth step was exposed the color began to shift to a lighter contrast at a more rapid rate. With each other step drastic changes occured as it got lighter and lighter. With the first three steps the lightness increased by no more than one point on the colorimeter at a time. At step four, the lightness increased by 2; at five the it increased by 5; at six by 9 and so on. It was concluded that this was because the test strip allowed less and less light through as the steps increased in number.