RE: Gum Humidity Question
OK, you live on the coast and it's foggy outside so the humidity is near 100%. When you get a hygrometer ($30 from radio shack - been using them for years), it would be interesting to know what the conditions are in your darkroom/work area. We get several 100% humidity days where I live, but the humidity in my work area is more like 50 to 60% in those conditions. The 80% and 80 degrees we had on Sunday were measured IN the work area. I've read that these inexpensive hydrometers read a bit on the low side, so it may have been a little higher than that. This is all good information, though. Weather it's speed or an effect on the sizing or something else, something is definitely going on here. Kerik > -----Original Message----- > From: Katharine Thayer [mailto:kthayer@pacifier.com] > Sent: Tuesday, September 26, 2006 7:51 PM > To: alt-photo-process-l@usask.ca > Subject: Re: Gum Humidity Question > > > On Sep 26, 2006, at 4:32 PM, ericawd wrote: > > > > I can tell you that much above 55% RH, the process does not work. > > This made me smile, not at you, but at the futility of > making categorical statements about gum, since most of my > work is printed at between 90 and 100% RH, and the process > works just fine. Obviously there's something else going on, > some other variable, that must account for the difference > between our observations about humidity. > > Curious whether I could see any difference in contrast, I > looked at the data I'd gathered so far on my little > experiment. I've used the > same pigment mix throughout (Prussian blue), mixed 1:1 with > saturated ammonium dichromate. For the humidity readings for > these test strips, I used the current reading from the noaa > statiion at the local airport. My exact readings here will > vary somewhat from the airport, which is why I decided I > should get a hygrometer to get accurate readings in my > workroom, but the airport readings shouldn't be more than a > few percentages different from mine, as I'm near a large body > of water and so is the airport, and it's just on the other > side of the hill from me. > > I had the test strips sorted into envelopes by humidity > range, so it was easy to line up the test strips from the > different envelopes and see if there's any difference in > contrast between the test strips from the different envelopes. > > At each of the humidity ranges (60-70%; 70-80%; 80-90%, and 90-100%) > this particular coating mix printed an optimum 7 steps. But for > each humidity range, the exposure needed to print those 7 > steps was different. > > There is possibly a slight difference in the separation > between the steps at the different humidity ranges; the test > strips at over 90% seem to have slightly more apparent tonal > separation between the steps than those printed at lower > humidity, but not enough to make > the darkest and lightest tones seem noticeably different. So I > guess at this point I would have to say tentatively, no, I > don't see a difference in contrast at different humidity > levels, at least within this range of humidity (60-100%). > > > Thank you for your input. Good luck finding a decent hygrometer. > > > > Thanks too. What I'm finding is that hygrometers tend to > measure less and less reliably the farther they get from > 50-60%, so it's going to be difficult to find one that will > give me accurate readings in my normal humidity range. Thanks > for posing an interesting question, Katharine >
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