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

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.

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