[reference] Cyanotype – Ammonium ferric citrate vs. Ferric ammonium oxaclate

when researching the cyanotype process, two mixtures for the emulsion are most commonly found. Its either Ammonium ferric citrate, or Ferric ammonium oxalate that gets mixed with Potassium ferricyanide. The percentages of each solution differs from source to source (examples).

After I used Ammonium ferric citrate for my first tries, I got some Ferric ammonium oxalate later and compared it to the results is got with the first salt.

For this I mixed one part 8% Potassium ferricyanide with another part of either 20% Ammonium ferric citrate (Let’s call this Emulsion A), or 30% Ferric ammonium oxalate (Emulsion B) and coated a piece of Fabriano Designeo 5 watercolor paper with a thin layer of each of the emulsions.

Then I applied the same test strip with black values between 0 and 60% to the paper and exposed it to UV light for 10 minutes in the UV box. Afterwards I thouroughly washed the paper and dried it.

The first thing I noticed, is that for emulsion B the image is overexposed after 10 minutes (60% black value does not lead to complete white, as with emulsion A). The second finding is, that the gradient in the lower half of the chart is much smoother with emulsion B. With eyes the roughness of the gradient with emulsion A is even clearer to see, than in the scan above. Finally it seems like the resolution of the contrast is much higher for emulsion B. In the upper pictiure the values for 0 an 10% Black and for 50 and 60% black look almost the same, while they are clearly distinguishable in the lower picture.

After these findings, I concentrated on the properties of the Ferric ammonium oxalate emulsion. I took out the test stips with black values between 0 and 100%, that I used all the way at the beginning of my experiments with cyanotypes, and used them for exposures between one and four minutes on the same paper.

After one minute of exposure all the values are already clearly separated, but the prussian blue is not yet as dark as it could be. Between 2 and 3 minutes the difference of the darkest value is almost not noticable at all and after 4 minutes 0 and 20% black value at the negative (the two leftmost rectangles) look the same. This information combined means, that an exposure time of 2, mabye 2:30 minutes is already enough to give the darkest value of prussian blue possible, while maintaining a high dynamic range.

Another thing I noticed is, that the emulsion with Ferric ammonium oxalate stains the paper yellow, on the not exposed parts (See the numbers in the first picture. On the lower test tey are stained yellow). I could not get rid of that yellow tint, even after watering the paper for about 20 minutes. Depending on the motive this could be a nice effect, but it is a thing to consider, when planning a cyanotype. I have yet to check if I can see this effect on different paper as well.