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

[fgorga]

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.

 

[shutterbags malone]

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?

 

[fgorga]

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!

 

[koraks]

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.

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

 

[fgorga]

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

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

 

[nmp]

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.

 

[largo]

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.

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