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 humidifier.) 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 > > > > > > > > > > > > > >
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