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

Re: Gum Humidity Question

I use a hygrometer I purchased at Home Depot for about $20.00.  It works
quite well for my purposes.  I can tell you that much above 55% RH,  the
process does not work.  I think what happens is the solution leaches into
the gelatin and causes major stain.  This is from my personal experience.

As far as low RH, the RH dropped to 27% last winter here in Memphis and the
solution set up way to fast to be able to work it properly.  (I had to buy a

I have a dehumidifier for the summer months and keep it in the 49-52 range.
We had our first cold front of the fall and the humidity dropped
significantly.  The results were rather unexpected-more contrast that I had
been getting from the same neg.  At first I chalked it up to the vagaries of
gum, but then I thought about it and started thinking about the drop of
about 7% I had experienced.

I have only been aware of the effect of RH on gum for the past year.  This
will be the second winter cycle.  I will see what happens.  I was wondering
what others have experienced.

Thank you for your input.  Good luck finding a decent hygrometer.

Candace Spearman

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, September 26, 2006 5:11 PM
Subject: Re: Gum Humidity Question

> Candace, Kosar's information generally refers to dichromated gelatin
> or other colloids, rather than gum; the information I was referring
> to was specific to gum arabic.  They do show a similar rather steep
> relationship, but it's hard to say whether the graphs can be directly
> compared, since the graph referred to in Kosar uses "relative
> sensitivity" as an outcome measure, without specifying how "relative
> sensitivity" is measured,  while the graph I was referring to uses
> equivalent exposure units required to achieve a specified degree of
> gum hardening.  At any rate, it is well enough established that for
> dichromated colloids  there is a direct relationship between moisture
> content and speed of the coating, so we can say that in general at
> least.
> As to the effect of humidity on contrast, there's no information
> about that that I'm aware of, but that's one of the things I'm
> curious about and am looking at in a series of trials I started a
> couple of weeks ago, looking at the effects of humidity on gum
> printing.  So far I don't have an answer, but I'll report it as soon
> as I do.  I decided a week or so into the experiment that I wasn't
> sure my humidity readings were accurate enough, so the experiment has
> been on hold while I've been searching for a good cheap hygrometer.
> I found one, but it was too cheap and turned out to be worthless, so
> I'm still looking.
> Katharine
> On Sep 26, 2006, at 2:31 PM, ericawd wrote:
> > According to Christina Z. Anderson and Kosar:
> >
> > 7. Humidity:  The presence of a certain amount of moisture in a
> > coated and
> > dried layer is necessary for the hardening reaction.  When dry, the
> > moisture
> > remaining varies with relative humidity.    Completely dehydrated
> > or fully
> > swollen coatings do not show any light sensitivity at all, but in
> > between
> > the sensitivity is high when the humidity is high.  Sensitivity
> > doubles with
> > increase of 30% humidity.
> >
> > In my experience, I have found the above to be true.  My question
> > is, in the experience of the gum printers, what effect might
> > humidity have on contrast?  Lower humidity equals higher contrast
> > or vice versa?  All other things being as equal as they can be with
> > gum.
> >
> > Candace Spearman
> >
> >
> >
> >
> >
> >