Traditional Cyanotype - Effect of Humidity (I think) on Sensitivity

This observation relates to the traditional cyanotype sensitizer.

I make cyanotypes in my basement which is unheated most of the time. Much of the year the temperature is between 55 and 65 degrees F and the humidity is around 40%. This time of year the temperature is in the mid 40's F, too cold for me to work unless I fire up the wood stove which I did a couple of days ago. Running the wood stove allows me to get the basement to the mid 60s F but the humidity is very low... about 30%.

My usual exposure time for traditional cyanotype (using a LED UV source) is 7 minutes. To my surprise I have had to double that time to get my normal "blue" since I started up the wood stove. The prints turn out just fine otherwise.

Everything else is the same (i.e. I am using the same stock solutions, the same paper, the same processing)... seemingly the only thing that has changed since last time I printed cyanotypes (without the woodstove running) is the humidity.

Has anyone else found an effect of humidity on the sensitivity of cyanotype?

A quick search of this site and the web in general for "cyanotype humidity" turns up a few hits talking only about coating problems related to high humidity and nothing about exposure time.

I know that both Ziatype and the printing out version of platinum printing are sensitive to humidity so there is some precedent that photochemistry on paper can be sensitive to humidity.

This is a very old thread, but one of the first results that comes up in google.
I believe I am experiencing the same issue in the UK. During normal damp weather I calibrated my cyanotype process and was able to achieve an exposure time of 8 minutes using a 1% Acid Bath for developer. In the last two weeks we've had unusually hot, dry weather and I noticed that to get full highlight details I need 12 minutes with the same negatives, and 15 minutes to get the same level of rich deep blue, like you this is almost double the time. I noticed that the test prints I made in dry weather have much more grain showing than the smooth prints I made a few weeks back on the same paper, I have read this is also related to paper which is too dry.

Did you ever find out if there is any truth behind this low humidity/exposure time link?

I have not followed up on this issue in any systematic way. I have simply stopped making alt process prints when the humidity goes below 40%.

From what I understand both long exposure times and speckling/graininess at low humidity are both problems with other hand coated paper alt processes, especially printing out processes.

One solution is to humidify your paper before coating/exposing it. There is plenty of information about this out there on the web, but you can take a look at https://www.alternativephotography.com/massive-paper-chart/ to start. I have no experience doing this, as I said I simply find other things to do in the short periods when low humidity is a problem in my dim room.

My educated (I'm a retired chemistry professor) guess about the cause of the long exposure times is that the transfer of electrons between metals during exposure requires a certain amount of water present, either as free water or more likely in a hydrogen bonded network with the cellulose of paper. If the water content is too low, the rate of electron transfer (and thus exposure) slows down. I have no evidence for this idea and am not likely to test this experimentally. Being retired, these days I prefer to do art rather than chemistry!

fgorga said:

My educated (I'm a retired chemistry professor) guess about the cause of the long exposure times is that the transfer of electrons between metals during exposure requires a certain amount of water present, either as free water or more likely in a hydrogen bonded network with the cellulose of paper. If the water content is too low, the rate of electron transfer (and thus exposure) slows down. I have no evidence for this idea and am not likely to test this experimentally.

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AFAIK this is the common working hypothesis also offered by Mike Ware and likely others.

koraks said:

AFAIK this is the common working hypothesis also offered by Mike Ware and likely others.

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Glad to hear that. I guess that it takes more than a decade away from chemistry to erase four decades of experience!

I kind of go the other way. For coating, I equilibrate the paper in a saturated salt box which is kept at 25 C for at least overnight to humidify it in a consistent manner, only to be taken out when ready to coat. Then after coating, the print is left alone for 10 minutes to let the sensitizer soak in, followed by a trip to a toaster oven kept at 50 C with convection fan on for another 10 minutes which makes it very dry. I found that while there might be some benefit to having a damp paper before exposure in terms of speed, there are other outweighing advantages of a bone-dry paper, i. e. a) no sensitizer transfer to the negative so the it stays pristine for multiple prints, b) there is less "bleeding" (so I have observed) which makes exposed to unexposed areas transitions less diffuse - consequently increasing the overall sharpness and shadow details, and finally c) it is far easier to be consistent when you thoroughly dry the paper than to try to nail a particular water level so the your print-to-print variability improves. With these conditions, I get about 10 minutes for exposure to max Dmax, so speed is not a great issue for me. All my experience is based on Canson XL watercolor paper.

:Niranjan.

nmp said:

I kind of go the other way. For coating, I equilibrate the paper in a saturated salt box which is kept at 25 C for at least overnight to humidify it in a consistent manner, only to be taken out when ready to coat. Then after coating, the print is left alone for 10 minutes to let the sensitizer soak in, followed by a trip to a toaster oven kept at 50 C with convection fan on for another 10 minutes which makes it very dry. I found that while there might be some benefit to having a damp paper before exposure in terms of speed, there are other outweighing advantages of a bone-dry paper, i. e. a) no sensitizer transfer to the negative so the it stays pristine for multiple prints, b) there is less "bleeding" (so I have observed) which makes exposed to unexposed areas transitions less diffuse - consequently increasing the overall sharpness and shadow details, and finally c) it is far easier to be consistent when you thoroughly dry the paper than to try to nail a particular water level so the your print-to-print variability improves. With these conditions, I get about 10 minutes for exposure to max Dmax, so speed is not a great issue for me. All my experience is based on Canson XL watercolor paper.

:Niranjan.

Click to expand...

Interesting. Also I think some of the transparencies that we use for the negative and print in inkjet printers have a special coating that is very hygroscopic (to absorb the ink). I wouldn't be surprised if it could suck up some of the sensitizer if not bone-dry.

Of course this does not apply to traditional negatives