I was doing a quick classic cyanotype print to verify a paper (Schut Laurier) indeed works on this process. I knew it did as I’ve used it before in this capacity, but I just wanted to make sure before recommended it. This also create the opportunity to plot the response curve, and since I had a salted paper print on my desk as well, a comparison is easy enough to make!
The cyanotype in question is a classic cyanotype – the kind made with potassium ferricyanide and ferric ammonium citrate. Here’s the print:
It’s a poor scan of a quick & dirty print, but it does allow for some measurement at least. The salted paper print is one I made a few months ago and took some new measurements of with an i1Pro photospectrometer. The salt print happened to be on Fabriano Artistico. Both prints included a Stouffer T2115 step wedge.
The exposures were likely quite similar, but I did not specifically control for this variable. This means that no inferences can be made about how these processes relate to each other in terms of speed. Practically speaking, they’re both pretty slow. My salt print exposures are usually around 10 minutes or so. Previously I used UV-BL tubes and currently 365+395nm LED. The speeds are about the same; the LEDs just print sharper, which is especially relevant for carbon transfer.
Anyway, here are the curves – cyanotype in blue, salted paper in brown:
Optical density on vertical axis, logE exposure on the horizontal axis. Let’s see what we can make of this:
Classic cyanotype is a short-curved, high-contrast process. Its dynamic range in this example is about 3 stops, which corresponds to a variable contrast paper ISO-R of around 100. This means that a negative that will print nicely on grade 3 should also produce an acceptable cyanotype print. However, since the shoulder of the curve is quite long, the shadows have a tendency towards blocking up, which is also nicely visible in the actual print. With good shadow separation, the process is limited to the variable contrast paper equivalent of around grade 3.5 or 4.
Salt print is a long-scaled process with lower contrast. The ISO-R would be something like 210 or so. There’s no variable contrast grade that corresponds with this, which means that a negative that would print OK at grade 0 or 00 would not be long-scaled enough for a good salt print. Ideally, you need something like a 2.10 or 2.25logD tonal scale for a good salt print. This is also why some of the salt prints I see being made from inkjet negative look somewhat lifeless, since it is difficult or even impossible on some inkjet printers to make sufficiently dense negatives (let alone with pleasing tonality).
Moreover, the salt print has a long and mostly symmetrical toe and shoulder to its curve. This means that delicate highlights always contain some detail (but fog also bites us easily!) and shadows don’t block up easily. This gives a nice smoothness to such prints, while cyanotype is much more harsh in its rendering.
Note how, interestingly, both processes end up producing about the same visual dmax, despite being different processes on different papers. This is a bit of a fluke, since evidently, the hue of an untoned cyanotype is entirely different from a salt print. This reflects in the cyan, magenta and yellow reflectance curves:
Here, the respective colors of these prints manifest themselves clearly. The salt print curves remain bunched together, consistent with the fairly neutral hues. A salt print is a bit warm in tone, which is reflected in the lower cyan optical density at the top of the curve. Note also how the yellow curve sits a little above the others; the paper I used here is Fabriano Artistico and its paper base is a little warmer than the more neutral Schut Laurier. This is one reason why I like the Laurier paper so much! The cyanotype CMY curves clearly show the cyan-blue tones, with lots of cyan density, less magenta and only limited yellow density.
Coincidentally, the Lab* values of the maximum density on these prints is [24.7, 1.74, 2.15] for the salted paper print and [25.78, 2.08, -26.66] for the cyanotype. If you synthesize those colors in e.g. GIMP, you’d get this:
I may mix up some New Cyanotype sensitizer one of these days to do a comparison with the classic formula. It’s been years since I printed New Cyanotype, but the curve was more similar to salted paper in terms of contrast.