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

Re: humidity and gum coating

The RH here stays more or less put at 20% during the summer months, but I see that you have rather fast daily changes.

To answer your question: in summer I use one part of gum, one part of water an half part of amm. dichromate 35% (saturated).

This mixture coats fine, but it is slower than the usual one part gum/one part dichromate. This actually suits me well because I recently built myself an exposure unit based on 12 BL tubes which is rather fast, at least much faster than the sunlamp I used before. A less sensible emulsion allows more control.

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 ...

Tom Sobota

Katharine Thayer wrote:
On Sep 2, 2006, at 3:53 PM, Tom Sobota wrote:


"The paper will be more easily coated if it is immersed in water and blotted before being placed on the board."
Neblette, "Photography,its principles and practice", London 1927

Surely, people have tried this before.
Thanks for the citation. I would have thought people had tried it, but it's not something I'd ever seen recommended, so I was curious about others' experiences, even though I had tried it myself and found it worked well enough.

Me too. Madrid, where I live, is very dry, so I have tried all kind of tricks to make coating easier. Wetting the paper works with some papers but with others you get irregular spots. What I do now is simply to add water in the mixture.
I can believe it about different papers behaving differently; I'll try it with my current favorite, Arches bright white, and see how that works. Unfortunately I didn't note what paper I used for that earlier experiment, but I would guess that was from the days I was printing on Arches Aquarelle.

I didn't want to add liquid because I don't like to dilute either the pigment or the dichromate, but you probably wouldn't have to add a whole lot of water to get the coating to smooth out. How much do you add? At any rate, we're back up to 65% here today, even though it still feels fairly dry, and the one print I've done so far today, the coating went on like a charm, so it looks like my immediate emergency is over.


Katharine Thayer wrote:

Chris, this makes me wonder if anyone has ever tried humidifying paper for gum coating, to make the coating easier in dry climates. The reason I'm wondering this is that my humidity, ordinarily >90%, has been way down for the last couple of days (17% yesterday afternoon) and I was having a heck of a time getting a smooth coat with my usual straight gum (no added water) mix. I decided that if this keeps up (unlikely) I'll need to start adding water to the mix, but your troubles with palladium gave me to wonder if anyone has reported humidifying the paper to make the coating easier. I do know that I once coated and printed on wet paper, just to see if it could be done, and it coated beautifully and printed fairly well, except for a sort of mottled effect in the background. Here's the test print I posted at the time:
At any rate, that combined with your explorations around palladium have led me to musing about whether humidifying might give a better result.
On Sep 1, 2006, at 10:10 PM, Christina Z. Anderson wrote:


Happy Labor Day weekend everyone! It may be a true "labor" weekend for this
household after all, because my daughter is having her baby in my bathtub
any day (or hour) now. Hmmm...that sounds strange....we do have hospitals
in MT....ohhhh, never mind...

At least I got somewhere today with the bleeding issue. My Platine shipment
came in and I was ever so excited to see if the new batch would be different
and not bleed. Alas...it still bled like crazy.

SO, it helped that Mark Nelson said that at the Formulary (also in
MT--similar humidity) they had to do two things to get good prints on Cot
320 this summer: humidify 30 minutes before coating and 30 minutes after.

I've watched the humidity all summer and it has hovered around 30%.

I messed around with the humidity and Everclear variables--using Everclear
or not, humidifying before and not after coating, humidifying after and not
before coating, humidifying both before and after. I built myself a
makeshift humidity box with a couple of trays and stuck a gauge in there,

(Of course in the meantime throughout these tests I am racing first to the
hardware store for screening and then to the liquor store for my flask of
Everclear...I thought of downing the bottle on my way home.)

What I found was it was most important to humidify after coating, more
important than Everclear or having the paper humidified before. If I
humidified before coating, the paper tone was greyer and duller. If I
humidified after coating, for 30 minutes (70% humidity) while the paper was
drying, there was no bleeding. So it must be as Clay suggested--the stuff is
drying too quickly on the paper surface and not sinking in enough. It was
so bad on a couple of my test sheets tonight that I could literally take my
finger, wipe the wet surface of the print, and have black stains on my
fingertip. I was losing quite a bit of density in the print all over,
including highlights, and even had serious staining/bleeding into the
highlight area, too.

So tomorrow I am going to have to develop a new set of curves with this
after-coating humidity factor, and will continue this procedure to see if,
in fact, the bleeding completely stops. I just have to find someone to
build me a drying/humidity rack, now....

I'm still puzzled, though, why during the last couple years I never
experienced this, and now I do. If someone has a friend at Arches, could
you ask if their paper sizing has changed in the last year?

BTW, that green ink on the Epson 2400 is sure as heck dense.

The end.