[from William Crawford, The Keepers of Light, p.163-165, 177-180 and Catharine Reeve and Marilyn Sward, The New Photography, p 55-63]
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 coating 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 ferric ferrocyanide. The oxidation can be hastened by treating the image in an oxidizing bath of either hydrogen peroxide or potassium dichromate.
Prepare stock solutions-
NB: prepare these solutions at least 24 hours before coating paper. Label bottles with dilution ratios, batch numbers and names of chemicals, and the date. Store both solutions in air-tight brown bottles.
distilled water at ~21 °C 100 ml (50 ml)
Ferric ammonium citrate (green) 20 g (10 g)
distilled water at ~21 °C 100 ml (50 ml)
Potassium ferricyanide 8 g ( 4 g)
A Note on the paper base
The following properties in the paper contribute towards good image resolution:
a smooth ‘hot pressed’ (HP) surface, unless you deliberately want to produce a print on highly textured paper;
a high cotton or alpha-cellulose content;
an internal sizing agent, ideally with a pH of 7-7.5;
there should be an absence of alkaline buffering agents;
good wet strength, hence the heavier weights of paper are more suitable.
All papers have a ‘felt’ or ‘blanket’ side and a ‘wire side. The former generally has a more random texture and has a less intrusive texture.
It should be noted that paper is a chemical entity in its own right. Each type of paper will react in a different manner with the sensitizer – and at times results can also vary from batch to batch.
Sensitize and expose the paper- work in tungsten light only, not fluorescent or daylight.
1. Paper should be cut so that there is a margin of at least 20 to 30 mm around the area to be coated. This allows for handling.
2. Make guide marks delineating the area to be coated on the sheet of paper. These marks should be lightly penciled in and can be best located by using a cardboard template that corresponds to the size of your negative. Write a print serial number on the sheet thus: [your initials].n – e.g. PM.1
3. Mix the following amounts from your ‘stock’ solutions (which should be at room temperature, 18-22 °C) in a 10 ml flask:
The total volume of sensitizer depends on the negative size, the type of paper, and absolute humidity.
There should be enough sensitizer to make 3-5 passes with the coating rod. Adjust as necessary while keeping the 1:1 ratio of sol.A to sol.B
solution A 0.2 ml
solution B 0.2 ml
Use a separate syringe for each solution (label them). After dispensing the solutions into the 10 ml flask, mix thoroughly by passing the liquid in and out of a syringe two or three times. Use the syringe to pick up this sensitizer solution.
4. Tape the marked paper onto a sheet of glass. Arrange glass coating rod (make sure it is free of any dust or grease), blotting paper, and loaded syringe nearby. Clean the paper surface with a blower.
5. Coating should be done at a room temperature of between 18 and 22 °C and a RH of around 75%. (If conditions are too cold or dry, surface crystallization will occur, and too high an ambient temperature or RH may cause the solutions to penetrate too deeply into the paper). Gently eject solution from the syringe along the top of the coating area. Promptly pick up the spreader and place it on the paper just above the liquid line. Draw the rod into the solution, pause briefly (about a second), allowing the fluid to distribute itself evenly along the length of the rod, and then smoothly pull the spreader down the length of the area to be coated. The surface may look blotchy at this stage, but it will even out in the next few moments. One ‘pass’ for a 10×15 cm area will take about 3 to 5 seconds. Apply only a slight pressure, otherwise the paper surface will get abraded. At the bottom of the first pass, ‘hop’ the spreader over the line of solution and smoothly push it back up to the top of the sheet, thus repeating the process, but in the opposite direction. Make 3 to 5 passes over the paper in this way. It is best to stop before all the solution has been absorbed into the paper. The excess is then soaked up with a strip of blotting paper.
5.5 An alternative method is to use a brush to apply the solution.
6. Place paper in drier for 10 minutes. Drier temperature should be between 35-40 °C (95-105 F).
7. As soon as drying is complete, place paper in contact with negative (sensitized surface to emulsion) in printing frame and expose to UV light source. Printing times will vary according to negative density. The image is partly formed during the exposure and the construction of the printing frame allows you to examine the print by opening one half without disturbing the registration of the negative to the print. Make sure that this ‘inspection’ is carried out in an area shielded from the UV source, otherwise the print will be fogged.
Caution: do not look into UV sources while they are on.
8. Clear print in a water bath for about 1 minute. Wash in running water for 5-10 minutes, or until highlight (white) areas in print have cleared.
9. Hang up to dry.
Van Dyke Prints:
Ferric Ammonium citrate
Mix each solution separately.
Combine solutions A and B and slowly add solution C, stirring constantly.
The sensitizing solution is already mixed and prepared. Follow essentially the same steps as for the Cyanotypes.
There is one additional fixing step, after the developing/initial washing for about one minute. Fix for five minutes in a 5% thiosulfate bath. Then wash.
1. Using the negative or other high contrast object (e.g., lace doily or lace ornament; dried flowers and leaves also make interesting prints) and paper supplied make 3 cyanotypes and 3 Van Dyke prints; the first print might should be a test strip to see the effect of exposure time.
2. For each process make changes in one of the variables so that you can study the effect of that variable on the process.
Sunlight if shining
A (single) negative or pseudonegative
Use a 21-step wedge
Paper – several kinds
3. Turn in all prints, labeled. Either attach to lab report pages or (and perhaps better) put them in an envelope so I can spread them out.
1. Choose the three or four areas (one dark, one white, one or two grays) that you used with your best black and white print -and mark these same locations on your best cyanotype and best Van Dyke print.
Use the colorimeter to measure the L*a*b* values (and others if you are interested).
Compare and comment on the results, i.e., compare L* values among types and a* and b* (and/or chroma and hue) within types.
Be sure to check calibration at the start and again at the end. A graph/chart of L*, a*, and b* values might be good.(That doesn’t mean to recalibrate at the end! – rather check that the calibration is still correct.). Record those two points on your Excel sheet too.
2. Compare the same 3 or 4 areas on your negative. (You may have done this last week with the silver prints).
3. Weather permitting, do at least one exposure using the sunlight rather than the UV lamps or vice versa.
4. Use handmade paper – sizing is an interesting variable.
5. For at least some of your colorimeter readings, do duplicate readings.
A table something like the following should be used for each sample:
ml solution A
ml solution B
# of passes for coating
Chemistry and Art: Cyanotypes and Van Dyke Prints.