U of S | Mailing List Archive | alt-photo-process-l | Re: Measuring humidity

Re: Measuring humidity

Returning to part of my Kosar post of now years ago (point 5-9 of Kosar's Top 10--it's interesting that we are now using Kosar as an authority because when I quoted him years ago I got all sorts of flak about his information! My how times change...speaking of never being heard, HA! Mark I have you beat on that in spades):

5. PH, temperature, and moisture all affect printing speed.

6. Adding an alkali to the gum/dichromate mix: if so much is added it is converted into a monochromate, and the light sensitivity drops to 25%. The higher the pH of the layer, the> longer the required exposure. Chromates, thus, are slower than dichromates.> With ammonia, you may start out with a high pH in solution, but due to> the volatility of ammonia, it evaporates during drying and the pH of the coated layer returns to a lower pH. If a solid alkali is used (sodium hydroxide or carbonate) the alkalinity of the dried layer remains the same.> The useful life of a sensitizing *solution* is greatly increased with addition of ammonia. If pH is 8 or higher, deterioration of solutions is practically nonexistent (note: not coated paper).>

7. Humidity: The presence of a certain amount of moisture in a coated and dried layer is necessary for the hardening reaction. When dry, the moisture> remaining varies with relative humidity. Completely dehydrated or fully swollen coatings do not show any light sensitivity at all, but in between the sensitivity is high when the humidity is high. Sensitivity doubles with> increase of 30% humidity.

8. Paper will keep, coated, for even 70 days in the ice box, or 3 days at room temp. If paper is dried at room temp high enough to dehydrate coating,> dark reaction does not occur and consequently shelf life is very good.

9. Raise in temp increases rate of chemical reactions, and for each 10 degree centigrade raise there is a 3x dark reaction rate, if rH is constant.>

Since none of us, I presume, are going to run hundreds of tests to prove which has a greater effect on dark reaction (what a waste of time) maybe we can be assured of saying that pH, temp, and humidity both in the air and within the coated paper has an effect on dark reaction. In my book, Kosar comes closest to scientifically testing this all, but true, he was NOT a gum printer.

Mark Nelson aside, we are talking here not of spontaneous hardening (which I take, in the old lit, to mean "instant" hardening that was said to occur with some pigments, or instant untimely hardening as in Mark's case) but a hardening that occurs over time to a speedier or slower degree. And yes, Mark, it seems to me when you brought up the humidity issue a number of months back, you were dissed, and now you aren't. Hmmm, your lucky underwear must be working......

My temps were consistent in SC but the humidity was consistently greater down there than even the most humid day in MT will be. When I did a series of gums--9 was the most I could seem to do successfully at one time--the 7-9th ones were recalcitrant in development. Here I don't see a problem with 13. For me, humidity messes with development a bit, but not enough to make printing impossible, just less predictable. I just have to work harder and say more swear words.

I can't necessarily apply my experience straight across the board to another gum printer because one would have to take into account exposure time, source, pigment layer, layer thickness, and all those other things, too. I can only apply it to my own practice because there I can compare apples to apples. When someone down South says gum "doesn't work" I can understand that feeling totally. And I can suggest these three culprits to be at fault, but certainly can't quantifiably suggest one is more important than the other unless I am willing to run 100 little tests.

Sandy, when you teach carbon at the Photographer's Formulary do you adjust your practice to suit the dryness there?


Temperature certainly is a variable, but my home is 70 degrees year round and it's the humidity that can vary a lot. Humidity has a LOT of influence—at least with PT/PD. But then, as you say, I am holding my temperature constant—and wearing the same lucky undershorts

I guess I didn't make myself clear; I didn't mean that humidity
doesn't have an influence; it should be clear from my earlier post
and my posts over time that I think humidity is very important;
higher humidity makes for faster printing and easier coating, for
example. When I say "it's not the humidity, it's the heat" I'm
answering people who say high RH creates problems for gum, makes it
"not work," produces that spontaneous hardening that has been
plaguing you so much. My point is that obviously it's not true that
it's the humidity that's causing the problem, as I print in very high
humidity without these problems, and Keith's post also confirms that
humidity is not detrimental to gum printing. Hence my comment that
it's not the humidity by itself that's the problem, it's heat added
to humidity. Keep those shorts on,