Darkroom variations for the classic cyanotype

Paolo Saccheri tests different variations of citric acids, washing and printing variables for the cyanotype process and shares his results with us. Please note that this is for the advanced cyanotyper. If you are a beginner, please start with Cyanotype – the classic process.

Writer and photography / Paolo Saccheri

Paper white and citric acid variations for cyanotypes

Normally, when you print in cyanotype (or any of the other alternative photographic processes), you have to face a step in the negative calibration where you want to reach “paper white” in your test print, and only in the last steps of the step tablet you printed.

Paper white is the lightest color you can have on a print that is the color of the paper itself. If your paper is creamy, your “paper white” will be that creamy color. With a limit, because even with the best coating and washing practices there will always be a residue of emulsion on the paper causing a barely visible fog, so you’ll have a creamy+fog color as your paper white.

Having paper white in your print depends on the density of your negative. If the negative is dense enough (dark enough), light won’t pass through it during exposure; therefore, under the darkest part of the negative, your print will be paper white. If the negative is not dense enough and you can’t raise the ink level you probably need to reduce your exposure time (or raise density with a color).

You want to have paper white only in the last steps of your test print. If you are having a lot of paper white in your test print (typically a 256 step tablet), you can compensate in two ways: by reducing the ink density level (if your printer driver allows that) or, in cyanotype, by using citric acid in the first tray when developing the print.

The paper white on your print depends on how long your exposure time will be and, at the end, how dense your negative is, so the printer you use is crucial for this experiment. A printer with pigment based inks will give you a denser negative than a printer with dye based inks. Also, a printer where you can change the ink density (quantity) level from its driver will  give you the chance to change the maximum density of your negative to match the exposure scale that you need. That is, you can change the ink density to move the paper white only in the last steps of the step tablet you printed, not before, nor after.

If your negative is very dense and you can’t or don’t want to change the ink density level, and so you have a lot of paper white in your test print, you can develop your print in citric acid to extend the exposure scale of your print and so have paper white only in the last steps. You will have great results if the acid is used in the correct amount. It degrades in high staining (no paper white) if too much acid is used.

“I did a few tests using citric acid on Arches Platine and sulfamic acid with Canson XL Watercolor paper, the former paper being pure cotton without alkaline reserve, and the latter a fully buffered paper that needs much more to be happily printed with.”

To have the acid ready to use, I prepare a 10% solution, stored in bottles, starting from the raw crystals. I use a syringe to measure the quantity I need and put it into my water tray, I stir the water for a while until I am sure all the acid crystals are already completely dissolved and the acid is completely mixed into the developing water, for consistency that is the base for a perfect digital negative.

Cyanotype variations tested on Arches Platine

For my process (sum of all my variables including paper type, relative humidity, type of coating, printer model and settings, etc.) my best results were between 5 mL/L (very good) and 10 mL/L (less paper white) as you can see from the images. I decided to go with 5mL/L in the first developing tray for a perfect digital negative.

P.S. The tap water where I live is desalinated from the sea, and comes with a pH of 9 – 9,5 — very alkaline for cyanotype prints that would be bleached if washed into that water. To compensate for that alkalinity, it is only necessary to add a 0.3 mL/L (10% solution) of citric acid; so, a mere 0,03 grams of citric acid salts dissolved into one litre of water are able to make that water neutral. You want to make sure to have neutral tap water before washing your cyanotype into it.

P.P.S. As a side discussion, I know that the pH paper test strips have a shelf life of two-three years in general (and you don’t know for how long your provider had kept them in his shelf). Also, depending on the accuracy used by the producer, you need to be careful when using any pH tester for cyanotype (also the electronic devices). I use a liquid pH test for aquariums that comes with an internal color reference scale printed in good quality. Testing and one’s eyes can be a good tool.

Washing variables

Washing your cyanotype in the best way is very important in order to have a print that will last beautifully.

If you don’t wash enough, some emulsion will remain inside the paper and it will show up in time as a yellow cast in the highlights.

If you wash too much, you risk washing off a bit of the darker areas of your print and so reducing its overall contrast.

Water temperature, water pH, washing time, and speed of agitation (rocking) are all factors that may give you a difference. You want to find your own way to develop and wash and keep it consistent, from the first test prints during the digital negative calibration to your real photo prints; all this depends on the reliability of your digital negative calibration, and at the end the quality of your print.

I found that for my process with Arches Platine (a process consisting of the sum of all the darkroom and printing variables including emulsion type, paper type, relative humidity, type of coating, etc) the best washing time was 20 minutes in total.

My tap water is alkaline and to compensate it I need to use a little citric acid as follows. This is valid only for me; you need to check the pH of your tap water. For this, I suggest you use the liquid tests for aquariums which are more reliable than the paper strips. Also, I use a certain quantity of citric acid in the first development tray to widen the exposure scale in my prints, but I’ll write more on that in a specific post.

For a 30×40 cm (about 12×16 inches) print on Arches Platine and classic cyanotype, my 3 trays are configured this way:

• First tray: 2 litres of water with 5 mL/L (in 10% solution) citric acid, constantly rocking by hand for 3 minutes, print face up to avoid air bubble formations, with a continuous and rather quick (but not too much) rocking at least in the first minute when developing occurs to prevent staining from bleeding.
• Second tray: 10 litres of water with 0.3 mL/L (in 10% solution) citric acid; I use a small submerged aquarium pump because I get bored, (hence the 10 litres, otherwise 5 litres are enough if my pump can stay beside the print inside my tray) for moving water for 3 minutes, print face down, to clear most of the emulsion.
• Third tray: 5 litres of water with 0.3 mL/L (in 10% solution) citric acid, gently rocking by hand from time to time for the last 14 minutes, print face down.

For smaller prints like 25×25 cm (10×10 inches) I use 2 litres, 5 litres, and 5 litres respectively in my 3 trays and if the prints are smaller I reuse the water of the second and third tray for washing more prints. I always change the water in the first tray for each print.

Washing for a total of 15 minutes was also good but the paperwhite was best at 20 mins, so that’s my washing variable defined for a perfect digital negative.

Waiting time from coating to printing cyanotypes

• I coated a sheet of paper using a rod and very little emulsion (0,0025 ml/cm2) down to where I drew the pencil lines. I cut the sheet in four strips when dry.
• Room temperature was 22 deg C and Relative Humidity 74%.
• I waited 20 minutes from the coating and printed the first quarter of the coated sheet (Test strip 415c) for 1 minute (a shorter exposure time than my standard so as to better see any color variation when comparing it with the other prints).
• I covered a part of the coating with an opaque material to have a non-exposed area where to check fogging.
• I waited 40 minutes from coating before printing the second quarter of the sheet as above (Test strip 416c).
• I waited 60 minutes from coating for the third quarter as above (Test strip 418c).
• I waited 80 minutes from coating for the fourth quarter as above (Test strip 417c).

P.S. Yes, I mismatched the strips already numbered when printing so they are not in sequence, but correct.

At the end of the test I could see there was no difference in dMax (color density) in the four prints and the fog under the covered part was pretty similar and almost absent.

In my conclusion, there is not much difference if you wait for 20 or 80 minutes to print after coating.

P.P.S. I always suggest not to wait too long from coating to printing, otherwise fogging may become evident in some circumstances (depending on the quantity of emulsion used and the R.H. of your location) and your paper white may be less white or gone.

I tend to always wait from 45 to 60 min for consistency in my darkroom variables, the base for a perfect digital negative.

Waiting time from printing to washing the cyanotype

• I coated a sheet of paper. Room temperature was 22 deg C and Relative Humidity 73%.
• I waited 60 minutes from coating and printed it with a negative, keeping all my darkroom variables consistent for getting a perfect print. I covered a part of the coating with an opaque material to have a non-exposed area to check for fogging.
• I cut the exposed paper in 4 strips and developed/washed as follows:
• First strip (419c) immediate wash after printing.
• Second strip (420c) waiting 15 minutes between printing and washing.
• Third strip (421c) waiting 30 minutes between printing and washing.
• Fourth strip (422c) waiting 60 minutes between printing and washing.

There is no visible difference between an immediate wash and waiting 15 minutes.
There is a slight difference between an immediate wash and waiting 30 minutes; the print is very slightly lighter.
There is a visible difference between an immediate wash and waiting 60 minute;, the print is lighter.

To measure and better show you the difference I scanned the two farthest strips and created a grayscale image, so it is more visible that the difference lost in dMax = 4%.

In my conclusion, you should always develop/wash your cyanotype as soon as you expose it. If you print a lot of photos and want to wash them all together, you may have a visible difference from the first print you exposed (it will be lighter) to the last one (correct).

I always wash my prints immediately after exposure for consistency in my darkroom variables, the base for a perfect digital negative.

Alkaline buffered papers (= acid-free)

I used only acid-free papers for quite a long time before I started buying Arches Platine, a pure cotton rag with no alkaline reserve that gives me wonderful results with blue and turquoise shades.

Acid-free papers such as watercolor papers often give very dark blues, even darker than Arches Platine and they are much cheaper, but they tend to bleach out when printing in cyanotype unless you adopt some countermeasures.

You have two ways to do so:

• pre acidify your sheets of paper before using them for printing
• compensate for the alkaline buffer when developing and washing your print.

I never pre-acidify my paper because I don’t want to spend time working twice on a sheet of paper, but it might be the best practice. In fact, if you pre-acidify, you end up printing with a neutral paper and this makes things good and easy.

If you coat your buffered paper the way it comes and print on it when the emulsion has dried  let’s say after one hour or so), you can perfectly compensate for the alkaline buffer when you develop and wash your paper immediately after printing.

In my tests, I found that the alkaline buffer of Canson XL Aquarelle (or Watercolor, the name depends on on the market where it is sold) can be perfectly compensated using 0,3 g/L of sulfamic acid put in all the dev and washing baths, for at least 15 minutes, but 20 minutes is better for a complete wash.

Before Canson, I also used other buffered papers such as Fabriano Accademia and Caballo 109, and I always compensated for their alkaline buffer during the wash.

Sulfamic acid has good properties when used with paper because it doesn’t interact badly with the paper itself and has no smelling fumes (unlike chloridric acid that I used when I began printing). But sulfamic acid is not super soluble so you need to warm up some water, up to 60-70 degrees Celsius (140-160 F), and then pour it on your acid crystals inside a capable container, then stir for a few minutes until complete dissolution.

I make a rough 10% solution (not exact in chemical practice) using 90 g of sulfamic acid crystals and 900 ml of water (0,9 liter).

When developing and washing the print that you have made on Canson XL Aquarelle (Watercolor) you need to put 3 mL of the sulfamic acid solution you  prepared previously for each litre of water you put in your trays.

For 20×30 cm (about 8×12 inches) prints on Canson and classic cyanotype, my 3 small trays are configured this way:

• 2 litres of water with 3 mL/L (in 10% solution) sulfamic acid, constantly rocking by hand for 1 minute (there is a lot of wash off).
• 2 litres of water with 3 mL/L (in 10% solution) sulfamic acid, rocking by hand for 4 minutes.
• 2 litres of water with 3mL/L (in 10% solution) sulfamic acid, rocking (but less) by hand for 15 minutes.

You need to change the water in the first tray for each print (it gets very dirty) and you can use the water inside the second and third tray (with two liters of washing water each) for a maximum of three prints. This is because, during washing the acid is slowly neutralized while compensating the alkaline buffer of the paper, so you need to have fresh water and acid to be always effective in washing the print and getting rid of the alkaline buffer.

If you want to use another type of acid, like citric acid (the worst option is chloridric acid), you’ll need to experiment with the quantities I suggested, since each acid has a different pH when diluted in water and so a different compensating ability.

If you want to use white vinegar you need to experiment too; you’ll need a larger amount.
This is part of the Digital Negative video course, an external resource for cyanotype printing.
Everything must be tested, defined, and then kept consistent in your printing process; that is the base for a perfect digital negative.

Paolo Saccheri has been a passionate photographer since a very young age. He later attended a photography course at Istituto Europeo di Design in Milan. His intense work in cyanotype brought together his technical abilities and his vision. He has also released the video course Digital Negative for Alternative Photography.