U of S | Mailing List Archive | alt-photo-process-l | Re: two questions about gum

Re: two questions about gum

(1) Charles, this makes sense; there's enough size in most watercolor paper as manufactured to keep the first layer from staining, but subsequent layers on unsized paper (depending on the paper) could well stain.

(2) I don't know that there's a rule of thumb for how to deal with this; people do whatever works for them, I suppose. If cutting the exposure time works, great. I can imagine that a drastic reduction in exposure might a similar effect to a reduction in pigment, but I've never tested that. I've just observed that many people who calibrate the emulsions individually find that when they print these emulsiions as layers in a tricolor, the tricolor ends up too dark. But it depends entirely on how people choose their pigment concentration in the first place, whether this is true or not. Since the pigment depth that people use for tricolor is entirely arbitrary and individually chosen, the depth chosen may be too much or too little; there's no way of telling until you print it as a tricolor. The fact that you've already established the best exposure for the mix while calibrating suggests to me that it's pigment and not exposure that's the problem. But as I said, whatever works, is the main thing.

I'm struggling with a similar issue, but finding that I have a different problem than many people have reported. After happily printing tricolor gum for decades by simply inverting the channels and printing greyscale separations, I've been experimenting with a more "sophisticated" approach, with less than satisfactory results. But the problem isn't that the print is too dark when the colors are layered over each other, it's that it's too light and too muddy, compared to the same image printed with the same emulsion by my old habitual method of simply inverting channels and printing,

But that's a subject for another post; I'll upload an examples and then maybe you can help me figure this out.

On Sep 25, 2007, at 3:43 PM, ryberg wrote:

I have noticed that my Daniel Smith Perylene Red stains the margins fairly heavily EXCEPT when it is the first layer. Is this kind of behavior common? Am I missing something? It took me quite a few prints to determine this since I have been printing that layer 2nd or 3rd and bemoaning the stain. Then I realized my 10x10 test chart showed no stain and made a print with the Magenta layer first--no stain. I am printing on unsized Fab Aristico.
On a related topic: After determining the exposure time and curve for each of the three colors I made a test print which was WAY too dark. Cutting the exposure time by nearly 2 stops helped. Katharine Thayer confirmed off-list that it is customary to have to cut back, though if I remember right she suggested using less pigment rather than cutting exposure. Is there any rule of thumb for cutting either the pigment or the time?
Thanks. Charles Portland, Oregon