Re: gum and humidity (a summary)
I promised Kate I'd write a quick summary of that discussion, which was long and took some detours and hopped around on two or three different threads. There's really not very much to the issue, when it's boiled down. This summary is of course culled from my own recall and understanding of the problem, and subject to debate itself if anyone cares to debate it. There were two questions that fueled the debate, one about whether humidity causes difficulty with development (long and difficult development/clearing) and one about whether humidity is related to contrast, whether prints made in conditions of high humidity will have lower contrast than those made in conditions of lower humidity. I think the two people were probably describing a similar problem, but using different words to describe it: a print that's mostly DMax and is hard to clear the highlights and open up the midtones and shadows, thus apparently low in contrast. I asked whether people had adjusted the printing times to compensate for the increased sensitivity of gum with increased RH, since lab tests have shown that, all other things being held constant, there is a fairly steep relationship between RH and sensitivity in gum. I described some preliminary data from my own shop that support quantitatively what I've known experientially; I've found that when the humidity is high, I have to shorten printing times, and lengthen them when the humidity is low. If I don't shorten the exposure time to compensate for the increased sensitivity, then I get overexposed prints that resemble the prints people were describing, but if I do adjust the printing times to the humidity, then I get prints of optimal contrast (for that particular coating mix) at all humidity levels. I also commented I've done a lot of printing at close to 100% RH, for example the year and a half that I worked in a dank unheated cellar with a river running through it, without encountering the problem described, so I don't think it's humidity (RH) by itself that causes the problem. There's also the fact that one can print very nicely on wet paper, which Keith described beautifully, and which I had demonstrated earlier, which rather casts doubt on the idea that water by itself causes the problem. Then Francesc suggested that in fact, the relevant variable for gum printing is probably not RH but the water content of the air (silly me, I always thought that's what RH measured) which has to be calculated using the air temperature and the dew point along with the RH. Then Jeffrey added that one should also take into account the vapor pressure, which will determine how fast the moisture moves into or out of the coating. So it's more complicated than just looking at the RH. So there's that issue, the relationship between humidity and sensitivity, to be considered, with its complications. And then there's a second possible issue, that of some spontaneous hardening process, either a thermal hardening from the heat or a dark reaction that causes overall hardening, which is so accelerated by the heat/humidity that it happens immediately as soon as the coating is dry. There's some evidence that this could be the case with the dark reaction, since there is a strong relationship between humidity and speed of the dark reaction, and even given the presumed standard temperature used in lab studies of the dark reaction, the line representing the time required to harden the gum to a certain degree without exposure does cross 0 at about 82% RH, as I mentioned during the discussion. Some people brought up the dark reaction in terms of how long you can keep the coated and dried paper, mentioning that in humid conditions you can't keep the coated paper as long as in drier conditions. This is fairly well established and generally assumed to be true (although Keith's observation that he can keep damp coated paper in a humidifier all day undermines it somewhat) but this is a different issue and complicates the question at hand unnecessarily. The question under discussion wasn't whether you can keep the coated paper long enough to print 12 prints or 20 prints before the dark reaction starts becoming noticeable; the issue here was, does the dark reaction go to zero under certain combinations of heat/humidity so that even if you dry the coating fast and print immediately on drying the coating, the dark reaction will still mess up your print. As far as I know, no one has done the experiments to separate the issues from each other. There are a couple of different ways this could be approached: indirectly, by first adjusting the exposure to accommodate the increased sensitivity, by finding ways to accelerate the drying of the coated paper (such as a fan), by printing immediately on drying. If after doing all that there's still some overall tone that's difficult to remove, then it's more likely that, by default, the explanation is some spontaneous hardening process such as a thermal reaction or a dark reaction. Or you could address the question more quickly and directly by coating a piece of paper, drying it as quickly as possible, and then putting it directly into water. If there's a stubborn overall tone remaining on the paper after a reasonable development time, then you've probably got some spontaneous hardening process going on. This is assuming it's not stain, of course; throughout these discussions I've assumed that the problems occur with coating mixes that don't cause any kind of problem under normal conditions. I think that pretty much sums it up: no answers, but I hope that's a fairly clear summary of the issues. Katharine On Oct 9, 2006, at 9:09 AM, Katharine Thayer wrote: Okay, I'll try to do that in the next few days, before I forget what it was all about. I've been a little feverish myself, and also working on an entirely different project, related to educational policy. So now I'm the one who is behind on reading the list.
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