U of S | Mailing List Archive | alt-photo-process-l | Re: Gum Humidity Question

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,