gum and humidity (was: Measuring humidity)
(This discussion seems to be ranging across several threads; I pity anyone who tries to follow it later)
On Sep 28, 2006, at 9:11 PM, Sandy King wrote:
Yes, but this example confounds humidity with heat again. For gum at least, sensitivity is also higher at 60F and 90% RH than it is at 60F and 65% RH (35% is a range I never encounter, so I stayed within a range I'm familiar with so I can speak from my own observations). If the graphs are correct, and I assume they are because different graphs from different labs and for different colloids show about the same relationship, then it's clear not only from my own experience but also from printing industry literature and from Kosar that if you hold temperature constant, there's still a steep relationship between RH and sensitivity. (Temperature doesn't enter very much into my picture since the temperature here doesn't vary that much from season to season.) I agree that humidity should be factored into printing, although not many gum printers do, and there's at least one gum printer who has disagreed with me when I've suggested that humidity is a factor, and that there's no difference between printing times between high humidity conditions and low humidity conditions.
My question, which I've asked several times over the years and have never yet got an answer to, is whether the people who have reported problems with gum under high humidity conditions have taken the increased sensitivity into account and adjusted their printing accordingly. Without doing that, it's hard to say whether the problem is really some kind of spontaneous overall hardening (due to a thermal hardening process or an accelerated dark reaction) or whether it's simply overexposure that wasn't recognized and compensated for by extended development; they can be hard to tell apart.
As I've reported here a number of times, I adjust my printing times as the humidity fluctuates. I don't have a set algorithm for doing that; my goal for these tests was to try to establish an algorithm, which now seems more complicated given the information Francesc provided. Up until now, I've just done the adjustment by trial and error: do a test print and adjust the exposure by the results of that. One test print is usually sufficient to get a good exposure on the second try. But, as I reported 2 or 3 weeks ago, when the local humidity suddenly plunged to 17%, it wasn't that simple. I doubled and tripled the "normal" exposure and the gum was still just running off in development. Since the humidity reverted to normal before I found the right exposure for 17% RH, and everything went back to normal, I don't know what it would have turned out to be, but given what I'd seen so far, I'll just give a wild guess at 15 minutes.
Going this direction (high to low) you can see that the problem is obviously underexposure; the gum is just simply running off the paper as soon as it hits the water. There's no danger of mistaking it for something else; that's simply all it can be. More radiation necessary. But let's say I'd gone the other way; let's say 17% was the usual humidity here and I'd calibrated my practice to print for 15 minutes, and then suddenly the humidity had shot up to 90%. If I kept printing at 15 minutes, I would obviously grossly overexpose the gum layer, since 2 minutes is about right for 90% humidity (for the pigment mix I'm using in all these trials and a Stouffer 21-step, in my shop and for my practice and temperature range).
In these trials, I printed at several set times at each humidity level and found that a 2-minute exposure worked well for over 90%, RH, with a development time of around 45 minutes. When I exposed for 3 minutes at over 90%, even a 2-hour development hadn't begun to open up the tonal scale. I drew the line at 2 hours and took the prints out; those prints have very little contrast (they are mostly DMax with perhaps one or two other tones). But I didn't assume that these prints were "low contrast" for some mysterious reason or that there was some sort of spontaneous hardening going on or that gum printing doesn't work at 90% humidity; I simply assumed they were overexposed and underdeveloped. On the other hand, 3 minutes exposure was perfect for humidity in the 70% range, opening up the tonal scale completely with a 30-minute development.
And just to tie things together, I'll repeat what I reported in another thread, that so far in these trials I've found that the optimal exposure at each humidity range produced 7 tonal steps and a visually similar DMin and DMax, for this particular coating mix, so I don't agree that greater RH necessarily makes for lesser contrast.
However, if you use water content rather than RH as a measure, then things might look quite different, but no one's done that, and I'm not going to be able to do it here because I don't very often have heat and humidity in combination with each other, so I wouldn't be able to get into the higher ranges of water content. But someone should do that experiment.
I'm also not discounting the possibility that with high heat and humidity in combination, there's some spontaneous hardening process going on. All I'm saying is that so far no one has told me that they've actually tried adjusting the exposure to the increased sensitivity of the higher humidity, and so as far as I'm concerned we still have to consider the possibility that it's simply a matter of overexposure.