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Platinum Palladium and paper humidity

hiroh

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Location
Lisbon
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When I'm printing platinum, I always measure the relative humidity in the room and the paper humidity before coating and before UV exposure.

My usual settings are:

Room humidity: 50-70%
Paper humidity before coating: 7-8%
Paper humidity before exposure and development: around 15%

With these settings, I'm getting rich blacks with the mix of Palladium Solution #3 + Sodium Platinum Solution (Na2) 5% in a ratio of 20:1.

I usually print a single print in one session. Sometimes I repeat the process if I'm not satisfied with the results.

Today, for the first time, I had prepared digital 6 negatives to print. The room was at around 65% humidity, the paper before coating was around 7%, but when I started developing, with 10-15 minutes between each print, the paper started drying, so I started at around 15% for the first print and ended up at 7% with the last print around 1.5-2 hours later. I noticed that with each print, the blacks are less and less deep. The first one was as I was expecting it to be, really deep blacks (as much as it can be on platinum prints), like I'm used to on my previous prints, but then all the other prints are kinda faded. Even the second print with 12% humidity has noticeably brighter blacks.

I'm wondering, could it be that the humidity of the paper affects blacks this much? I'd like to hear your experiences and make sure that this is the possibility of why the other 5 prints turned out faded. I'll repeat the process tomorrow again for these 5 prints, making sure that I'm exposing it always at around 15% humidity.

Bonus question: What is the best way to dampen the coated paper to increase the humidity by just a few percent? Steamed bathroom?
 

My knowledge of this is purely anecdotal. FWIW I think the issue is not paper moisture, but the moisture of the sensitizing solution. My experience (mostly with kallitypes) is that a freshly-coated paper will give better results than a coated paper that has been sitting in a paper safe for an hour. Others will surely have better answers grounded in science.
 

Hi, when I said "the humidity of the paper after coating," I actually meant the moisture of the sensitizing solution (coating). So, if this has been your experience with kallitype, then I guess that must be it. My experience also shows that freshly coated paper gives the blackest blacks, but I didn’t think that less than two hours would make such a difference (even in my quite humid room).
 
I always coat one sheet at a time, just prior to printing. It makes life so much simpler. I can't speak to the precise chemistry/science as to how long you can go, but humidity definitely makes a difference. You CAN re-humidify your paper and bring it back if you've let it sit in the darkbox, but unless you have access to better controls than I do, getting it right is harder and more time-consuming than just coat as you go.
 
I agree. Well, this was a lesson learned hard way. 5 platinum prints wasted
 
hiroh said:
Bonus question: What is the best way to dampen the coated paper to increase the humidity by just a few percent? Steamed bathroom?
Click to expand...

The best solution I've found came from Pradip Malde's book "Platinotype" where he describes a humidity chamber you can build. The internal humidity level is controlled by what is put into it; I use iodized salt which maintains a 70 - 75% humidity level. The level seems to vary slightly based on ambient room temp. Search the interwebs for Pradip and/or Mike Ware because I'm pretty sure I saw this chamber described on one of their websites.
 
I live in a rather humid climate but most of the time the AC is on. I never check the humidity but what I do is to only coat one sheet at a time. I first brush on distilled water and wait until it is almost dry to the touch before coating. I’m using Arches Platine paper if that matters. For my average 7x7 print I use (drops) 8pd, 3pt, 1au, 10ferricoxalate #1, 2#2, 4h2o2, 8 distilled water. When close to dry I finish drying to touch with a blow dryer. I feel the tone depends on the negative and exposure at least in my experience.
 
hiroh said:
Bonus question: What is the best way to dampen the coated paper to increase the humidity by just a few percent? Steamed bathroom?
Click to expand...

I've had success with using a cool mist room humidifier - I will waft the paper through the stream of vapor output by the humidifier at sufficient distance from the nozzle that water droplets will not form on the paper. I also heat my PotOx developer to 140 F with a sous vide gadget, so the water bath for that is mildly steaming - I can hold the paper over the water bath for a minute or two so the steam has a chance to penetrate the paper - if you're doing this to re-humidify an already coated sheet, (either with the steam or the cool mist) don't point the coated surface at the water source. Just keep waving it in and out of the vapor column until the paper feels cool and slightly limp in your hand.
 
hiroh said:
I'm wondering, could it be that the humidity of the paper affects blacks this much?

Bonus question: What is the best way to dampen the coated paper to increase the humidity by just a few percent? Steamed bathroom?
Click to expand...
Yes, sensitiser humidity is key to consistency of blacks.

The easiest way to boost the humidity of coated paper is to suspend it face down over a tray of water for a few minutes.
 
Ian Leake said:
Yes, sensitiser humidity is key to consistency of blacks.

The easiest way to boost the humidity of coated paper is to suspend it face down over a tray of water for a few minutes.
Click to expand...
Hi Ian, here a dumb beginner's question, the water in the tray should be at room temperature? 21C in my case
 
Raúl Aragón said:
Hi Ian, here a dumb beginner's question, the water in the tray should be at room temperature? 21C in my case
Click to expand...

Steam from the hot water will moisten the paper more quickly. Just make sure it doesn’t get wet—not even damp—just to the point it doesn’t feel completely dry.

I bought a moisture meter on Amazon, which I use to check the paper’s humidity. I print platinum at around 15% measured paper humidity. This is a very important step in my printing process because it gives me complete control over the paper’s humidity. As we know, paper humidity can significantly impact the final print, especially in how the blacks appear, based on my experience.

I generally avoid rehumidifying the paper. Instead, I plan my printing session so that I’m ready to print when the paper reaches 15% humidity. I’ve found this approach consistently gives me great results.
 
Oh, by the way, if I ever need to humidify paper, here’s how I do it. It’s very simple:
1. Fill the sink with warm or hot water.
2. Place a drying rack with mesh over the sink.
3. Lay the paper on the drying rack.
4. Cover the paper with an upside-down tray to trap the steam around it.

When I do this, I check the humidity every minute or so because it doesn’t take long for the paper to reach the desired humidity level. For me, it usually takes no more than a few minutes to reach 15% humidity.
 

Thanks Hiroh for the useful advice, definitely I'll search for the moisture meter
 
Steam from hot water can sometimes lead to mottled prints. Be careful.
 
lecarp said:
Steam from hot water can sometimes lead to mottled prints. Be careful.
Click to expand...

Don’t place the paper too close to the water. It shouldn’t be damp or wet, just not completely dry.

With experience, you’ll learn how to gauge the paper’s humidity by feel. I use a moisture meter simply because I have one, but based on my room’s relative humidity and the time since I applied the coating, I can usually estimate when the paper will reach the right humidity without needing to measure it.
 
It is important to understand that it is the humidity of the sensitiser that matters not the humidity of the paper (although the two are closely related, of course). There are a few different ways of controlling this, some mentioned above.

A good rule of thumb is to get the sensitiser so that it almost but not quite sticks to the negative. Always have a sheet of protective mylar between the print and your precious neg.

Raúl - to answer your specific question, yes, room temperature is fine. I don't think it's that critical, to be honest.
 
Thank you very much Ian
 
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