U of S | Mailing List Archive | alt-photo-process-l | Re: humidity and gum coating

Re: humidity and gum coating

On Sun, 3 Sep 2006, Tom Sobota wrote:
As to why some papers develop spots when coated wet, that's a good question, I haven't the slighest idea. Differences in surface porosity come to mind, but these papers have been double-coated with gelatin and hardened ...

For the sake of "experiment," I tried coating the gum emulsion on pre-wet paper a couple of times a while back, and found some spotting, but mostly staining. Serious staining. I theorize that even though the surface was blotted off, the interior remained wet and that the emulsion soaked down & settled in there -- in other words, got tangled into the paper fibers, as it doesn't when it just sits on the gelatin-sized surface.

I gather the reason for coating wet in the 1st place is to get an even coat. IME, coating with a fairly wide foam applicator (the kind with the wood handle that has been very slightly dampened & then blotted dry) and NOT NOT NOT hake brush, spreading swiftly and THEN smoothing with dry hake, solves any problem with coating.

It's important however to move across the surface quickly, meaning in wide free strokes. Trying to "smooth" as you go (instead of later) slows you down, and (I find) by the time you get across, it's started to get tacky & stripe-y where one stroke passes the next. In other words, the risk of streaks is with the initial dispersal. Once that's done, you can "buff" at relative leisure.

However, this works best with a relatively thin emulsion, at the very least equal parts am di solution & gum...