Cyanotypes and pH
Cyanotypes are a relatively low-toxicity photographic process combining ferric ammonium citrate (C6H8FeNO7) and potassium ferricyanide (K3[Fe(CN)6]), which are combined in equal amounts and then exposed to produce a distinctive blue image. Neither of these two chemicals are considered toxic: ferric ammonium citrate is often found in supplements, potassium ferricyanide is a stable compound that only becomes a risk to your health if it’s heated beyond 300°F. If you are worried about exposure, or if you happen to be allergic to either of these chemicals, use protective gear.
You can change some properties of a cyanotype print by altering the pH during processing. After printing, you want the paper to be a neutral pH to remain highly archival. Many cyanotype prints from 100 years ago are still as vibrant and stable as when they were printed. It’s important that you understand the effect that various environmental conditions will have on your print both during processing and during storage.
Your print will be influenced by the pH of the paper you choose. I only use Hahnemühle Plat Rag and Fabriano Artistico because I’ve tested both of these papers and they print cyanotypes consistently.
Acids (pH below 7)
During processing, you will notice a lower contrast and greater tonal range in your prints if your water is slightly acidic. This is great for your highlights, but can also can block up your shadows. If you’re going to bleach and tone your print, you probably want this… I do. I want more detail in my print so I have more to work with as it bleaches away.
For cyanotypes that will be part of a tri-color print, I don’t always add acid to the rinse water, but for single-color, toned cyanotypes I add 1tsp. citric acid to the two liters of water in my tray until the pH drops to 5 or so.
Bases (pH above 7)
If you introduce a base (alkali), your prints will gain contrast but lose detail in highlights. With enough exposure to an alkaline environment, your prints will start to “bleach,” fading to yellow and then eventually to white. The higher the pH, the faster the Prussian blue fades out to yellow and the faster and more you lose detail.
Bleaching Prints with Bases
I originally tried bleaching with sodium carbonate (Na2CO3), but found that the alkalinity rose too quickly. I set a timer for 3 minutes, but as I put my paper into the water the blue turned to a surprising purple and then immediately started to fade to yellow. I had barely started the timer before my print had bleached past yellow and into a ghostly white. I pulled it out quickly, but instead of giving it a good rinse to stop the process it needed about 10 minutes of washing to stabilize. I had lost a lot of detail, so that was a bust.
I created a video to show what this looks like.
However, calcium carbonate (CaCO3) maintains a pretty stable pH of around 8, which allows me more control over how much my print is bleached. I used 1 tsp for every two liters of water, though I’m not precise with this chemical. Calcium carbonate is cheap and adding more won’t raise the pH more. I don’t need to be skimpy. I also created a video of this process. I think you’ll agree that bleaching with calcium carbonate gives you more control over the final result.
A Final Note…
The thermostat and hygrometer I keep in my dimroom.
As with all alt processes, I note the temperature and humidity on the back of my prints so that I can recreate exactly the ambient environment that existed for a successful print. I have been surprised by how much of a difference a 5% change in ambient humidity can make! I aim for slightly above 50% humidity for cyanotype work and around 40% humidity and for all my gum bichromate work. I keep my dimroom a consistent 70º-76ºF (20º-24ºC).