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

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.


> -----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