Nail Your Base Exposure Time for Cyanotype

Mar 10, 2020 / Photography / cyanotype process

So you coated your paper. What’s next? This article discusses one of the preparatory steps that will optimize your cyanotypes. What I mean by “optimize” is the cyanotype will as dark as it can go. I already gave tip #1 for making cyanotypes darker (how you apply the emulsion affects how dark it will be). Tip #2, the subject of this article, is to nail your base exposure time. I got this term from Precision Digital Negatives by Mark Nelson. However, my testing procedure described below is not from that book. The method is simplified. This term refers to the minimum exposure time that produces the darkest cyanotype possible, given your workflow.

Materials Needed

For this step, you will need your coated cyanotype paper, a transparency film, a UV light source, and a timer. The transparency I currently use is from Rite Film. I use rolls of this film, but you could get them in smaller sheets like 8.5 x 14 inches. This transparency is a cheaper alternative to Pictorico transparency film.

You will also need a UV light source for exposing cyanotypes indoors. I have a fancy vacuum UV box meant for screen printing. I went overboard when I purchased this unit years ago. I plan to make large cyanotypes in the future. You can buy smaller exposure units of course (try eBay). You could also create your UV lightbox. You can use the sun as your UV light source, but in my experience, it’s challenging to have a consistent process using sunlight because it depends on the time of day and extent of cloud cover. Sometimes, the sun is too intense; sometimes it’s too weak. While it is possible to use sunlight, I find it hard to work with. What if you had to develop your cyanotype at night? Then you’re out of luck.

How to Determine Base Exposure Time

Place your transparency film on top of your coated cyanotype paper, leaving a portion of the paper naked and uncovered, as demonstrated in my video. You can tape the transparency film to the paper so it doesn’t move. Now take a thick cardboard and place it over the entire setup.

Expose this setup in your UV box in one-minute increments. After the 1st minute, slide the cardboard upward by one inch and expose again for one minute. Repeat this process 19 times or until you run out of paper. This segment is the most time-consuming part. It is a pain in the ass to do, but you’ll be rewarded later in terms of consistent cyanotype prints from one printing session to the next.

By the end of the process, you will have a piece of paper with 20 levels of exposure, representing 1-minute increments of exposure. It’s useful to write down the number of minutes each wedge represents so that you know what the exposure times were for each level. Wash your cyanotype for 10 minutes in several changes of water. Let it dry naturally.

cyanotype print being tested for its base exposure time for the darkest print possible
Base Exposure Time. Testing Process for the darkest cyanotype possible

Choosing Base Exposure Time

Now that your cyanotype paper is dry, look at the step wedge. Your base exposure time is the first instance in the bottom part of the cyanotype where its tone value is visually equal to the darkest tone of the uncovered portion of the cyanotype print. As shown in the video, my base exposure time is 12 minutes.

Your Base Exposure Depends on Your Process

Choosing the base exposure time is partly a subjective call. Don’t obsess on whether you should have chosen 10 minutes or 12 minutes, for example. If you can’t tell the difference between one level to the next (in terms of tone value), then just choose one.

Another thing to remember is that -your- time will be different from mine. In my experience, there are many variables that affect the base exposure time: the paper, the cyanotype emulsion drying time, the thickness of the plastic transparency, the water pH level, the dissolved minerals in your water, the water temperature when rinsing your cyanotype, etc. For example, if you read the cyanotype literature you will encounter advice like prepare your cyanotype emulsion one day in advance or wait one day after coating cyanotype paper. I’m too impatient. I mix my cyanotype emulsion when I need it, and I only wait 30 minutes for my cyanotype paper to dry before using it. You might choose to follow this advice, in which case, your base exposure time will reflect your process and therefore, will be different. That’s why it’s critical that once you are happy with your cyanotype prints, you should never change your workflow. Everything has to be the same.

This article gives tip #2 for ensuring that your cyanotype will be as dark as possible. All these steps will come together as we go through the entire cyanotype printing process and you’ll be printing cyanotypes as dark as possible. Try the process above and let me know!

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