gum printing / batch coating / emulsion "swollen"

[timeUnit]

I was printing like a madman yesterday, juggling 8 prints at the same time!

I thought I'd try batch-coating to save some drying time.

I did this:

1. get the prints I want to coat with the same emulsion (W&N New Gamboge yellow)
2. flourescent lights off
3. mix "pre gummed" pigment and pot. dichromate (in this case 1:1 ratio)
4. coat first print
5. mix a little more pigment and dichro
6. coat second print, with the same brush. found out that it was difficult to judge the amout of emulsion to be poured on the prints, as the brush was quite full of pigment at this time
7. coat third print with remaining emulsion and same brush
8. mix more pigment and dichro
9. coat fourth print, still struggling to not pour too much emulsion on print.
10. place prints to dry, wash up.

When coating, do you gum printers batch coat? Do you use one brush for each print? I suspect the use of quite different papers, Fab. 5 and Ruscombe Talbot, made it extra difficult to judge the amount of emulsion, as they are different in absorption.

As I exposed these prints I noticed that the emulsion seemed very "swollen" and hadn't stuck so well to the paper base. Some prints were worse than others. I wonder about the causes for this. These are my ideas:

1. Too much emulsion on papers, making exposure times longer, for which I didn't compensate. I expose for 6-7 minutes for a "medium" gum layer, 8-10 for a "highlight" layer, 3-5 for a "shadow" layer.

2. Pigment/gum ratio wrong. I used a trick I learned from my teacher Chia -- painting the pigment/gum mix on a newspaper page. If the text barely shows through, the mix is OK. Maybe I should make it thinner, ie more gum.

3. Something bad happens when using the same brush for succesive prints? Just a thought.

I'll have to judge the prints when dry, to see how the yellow layer looks.

I'm greatful for any input!

Thanks,
Henning

 

[donbga]

I was printing like a madman yesterday, juggling 8 prints at the same time!

I thought I'd try batch-coating to save some drying time.

I did this:

1. get the prints I want to coat with the same emulsion (W&N New Gamboge yellow)
2. flourescent lights off
3. mix "pre gummed" pigment and pot. dichromate (in this case 1:1 ratio)
4. coat first print
5. mix a little more pigment and dichro
6. coat second print, with the same brush. found out that it was difficult to judge the amout of emulsion to be poured on the prints, as the brush was quite full of pigment at this time
7. coat third print with remaining emulsion and same brush
8. mix more pigment and dichro
9. coat fourth print, still struggling to not pour too much emulsion on print.
10. place prints to dry, wash up.

When coating, do you gum printers batch coat? Do you use one brush for each print? I suspect the use of quite different papers, Fab. 5 and Ruscombe Talbot, made it extra difficult to judge the amount of emulsion, as they are different in absorption.

As I exposed these prints I noticed that the emulsion seemed very "swollen" and hadn't stuck so well to the paper base. Some prints were worse than others. I wonder about the causes for this. These are my ideas:

1. Too much emulsion on papers, making exposure times longer, for which I didn't compensate. I expose for 6-7 minutes for a "medium" gum layer, 8-10 for a "highlight" layer, 3-5 for a "shadow" layer.

2. Pigment/gum ratio wrong. I used a trick I learned from my teacher Chia -- painting the pigment/gum mix on a newspaper page. If the text barely shows through, the mix is OK. Maybe I should make it thinner, ie more gum.

3. Something bad happens when using the same brush for succesive prints? Just a thought.

I'll have to judge the prints when dry, to see how the yellow layer looks.

I'm greatful for any input!

Thanks,
Henning

Henning,

You can coat several sheets with the same brush, successively. You may wish to dampen the brush very slightly before coating your first print. The first time you use the brush it needs to charged with your gum mixture so a bit more is needed than for subsequent prints. For me about 2 ml is needed to brush coat an 8x10 ish sized print.

It's not unusual for me to coat 4 pieces of paper at once for printing. I expose all of the prints and then batch process them. You can also process them 1 at a time sucessively.

A couple of weeks ago I deliberately left two coated sheets in my paper safe over night to see what effect I could observe. They printed and processed normally the next day. Not something I'm likely to do often if ever but it seemed to indicate that dark reaction may not be as immediate as one might think. YMMV, of course.

Sounds like to me your gum/pigment mix is too thick. I coat very thin coats, always.

Practice make perfect,

 

[timeUnit]

Don,

thanks for the info, I really appreciate it!

A quick question...

When you write about the gum/pigment mix is too thick, do you mean I should use more gum in the mix, or do you mean I should coat each paper with less emulsion? (Or both! )

Thanks again!
Henning

 

[donbga]

Don,

thanks for the info, I really appreciate it!

A quick question...

When you write about the gum/pigment mix is too thick, do you mean I should use more gum in the mix, or do you mean I should coat each paper with less emulsion? (Or both! )

Thanks again!
Henning

Probably both. How do you mix your gum?

And how do you mix your gum/pigment stock solution?

 

[Katharine Thayer]

Hi Henning,
Every time you ask a gum question you'll probably get as many different answers as there are gum printers in the vicinity; here's mine:

I always coat and expose one print at a time, but I use a hair dryer to dry the coating, so it's very fast, usually less than a minute, from coating to exposure.

The main issue for coating and air drying multiple papers is the dark reaction, which means that as soon as the gum and dichromate are coated into a thin film, a reaction starts between them which eventually renders the gum insoluble. The speed of this reaction is very dependent on humidity, but also on the concentration of the dichromate. I've heard of people living in very dry climates who use a fairly low dichromate concentration, who can coat multiple papers and leave them for days without fogging the paper. I've also heard of people in climates where it's hot and humid in the summer, who can't coat and print even one print in the summertime because the dark reaction is so immediate. When I tested the dark reaction, I was living at the beach, in other words a humid climate, and the coating I set aside in the dark was completely insoluble, the entire layer, within just a few hours. That's why I don't keep coated paper around. But this is something you have to test for your own situation. At any rate, from your description, I doubt that's what's causing your problem.

If you used the newspaper test, it's quite unlikely that your mix is overpigmented; I wouldn't add any more gum if you were able to barely see text through the coating. In my experience much more pigment than that can be used successfully, but I do think the newspaper test is a good way to keep beginners from overpigmenting their mixes. So I don't think that's the problem here.

Instead, I'm more inclined to think what's causing the "swollen emulsion" is that you're coating too thick, possibly by using too much coating and by not brushing and smoothing the coating out thin enough after "pouring" the emulsion onto the paper. (I never pour anything onto the paper, I just charge the brush from the mixed solution and brush it on).

A separate comment: You mention mixing the dichromate and gum/pigment before coating each paper; that is not necessary. There's no "dark reaction" to worry about in the mixed gum/dichromate solution before it's spread into a film (dichromated colloids in solution and dichromated colloids in film behave very differently, chemically) so you don't need to worry about mixing enough for several prints and using it over a printing session. I've used the same mixed solution for as long as five or six hours, in the same type of climate where I got a dark reaction in the film in a very short time.
Hope any of that is useful,
Katharine

 

[Katharine Thayer]

Addendum re dark reaction

I've heard of people living in very dry climates who use a fairly low dichromate concentration, who can coat multiple papers and leave them for days without fogging the paper. I've also heard of people in climates where it's hot and humid in the summer, who can't coat and print even one print in the summertime because the dark reaction is so immediate.

I might qualify this last statement a bit. It's true that I've heard of people who live in very hot and humid summertime climates who claim that they can't print at all in the summertime because of fogging, and who attribute this to the dark reaction. And it's also true that in graphs of lab tests of the dark reaction in gum, the time required to harden gum through the dark reaction alone does cross the x-axis at something like 82% RH, indicating that the time required for the reaction to go to completion can approach zero if the humidity is high enough. So it is certainly possible, maybe even probable, that these reports are correct and that the dark reaction is so extreme and immediate in these conditions of heat and humidity as to make gum printing impossible, or at least very difficult, in their environmental situation.

But at the same time, I've never been entirely sure whether other possible explanations had been ruled out. For example, I don't know whether it was established in these cases that it was dark reaction and not simple overexposure. Humidity also has a very strong direct relationship with speed of the emulsion, and I've never been able to confirm that the people who report this "dark reaction" have adjusted their exposure downward to allow for the increased humidity, and it is possible that the fogging is a simple result of overexposure. So I don't have enough information to evaluate the claim about immediate dark reaction, although given the above facts and lab evidence, I have no reason to doubt it either. I've never seen it, but I'm lucky enough to live in a cool humid climate rather than a hot humid climate.

 

[timeUnit]

This is very interesting Katharine, and I'm quite sure that I can forget about the coated print I didn't have the time to expose yesterday... When I get to expose it tomorrow evening (it's close to midnight here right now) it'll probably be too late. Great info! I think the RH is quite low here right now though, not above 50%, that's for sure.

On a different note: you mentioned the use of a hair dryer for drying the emulsion coat. Do you use it hot or cold? I'll definitely buy a cheap dryer tomorrow, as the drying times are slowing me down very very much.

Do you use the dryer on the freshly developed and washed prints as well?

Thanks again,
Henning

 

[timeUnit]

Probably both. How do you mix your gum?

And how do you mix your gum/pigment stock solution?

I mix my gum (this is the first time for me) by pouring 300 ml of distilled water into an old marmalade jar, and then adding 90 grams of gum arabic bought from an art supply store around here. The brand is LeFranc & Bourgeois. Probably quite old, but seems to work. This is the first time I mixed my own gum solution, so I used 45 grams of gum first, which was too little, and added 45 grams more the day after. Then I added three crushed tablets of acetylsalicylic acid (aspirin) as a preservative. Finally, after the solution was thick and lump-free, I filtered the solution through a folded, clean cloth. It's now as clear as the W&N jar I bought from the art supply store.

Mixing the gum/pigment I use the trick I learned from my teacher. Paint the mix on a newspaper. If you can barely see the text through the mix, it's good. I think I'll dilute the yellow pigment a bit more, though. It seemed a bit too thick to me yesterday.

 

[Katharine Thayer]

This is very interesting Katharine, and I'm quite sure that I can forget about the coated print I didn't have the time to expose yesterday... When I get to expose it tomorrow evening (it's close to midnight here right now) it'll probably be too late. Great info! I think the RH is quite low here right now though, not above 50%, that's for sure.

On a different note: you mentioned the use of a hair dryer for drying the emulsion coat. Do you use it hot or cold? I'll definitely buy a cheap dryer tomorrow, as the drying times are slowing me down very very much.

Do you use the dryer on the freshly developed and washed prints as well?

Thanks again,
Henning

Actually, if your humidity is low, then chances are quite good that your paper will still be okay tomorrow evening. The relationship between humidity and speed of the dark reaction is almost vertical; the graph of the data from a lab study shows the time for completion of the reaction to a particular degree of hardening of dichromated gum goes to zero at 82%, but at about 45% the time required for hardening to the same degree is 140 hours. So I wouldn't throw it away, by any means. Like I said, this is so variable according to conditions in different places that you really have to test it for your own situation.

I almost hesitate to admit using a hair dryer because there are so many gum printers who say unequivocally that you must never use a hair dryer because it will harden the gum and fog the print. But nevertheless, I have always used a hair dryer (set on warm) and have never had a fogged print in my life, at least not from that cause. And yes, I dry the hardened gum after development the same way. After the gum layer is dry, I hang the print with clothespins on a line to dry the paper.

But be aware that this is counter to conventional gum wisdom so proceed at your own risk should you decide to follow me in this scandalous behavior.

 

[Katharine Thayer]

Mixing the gum/pigment I use the trick I learned from my teacher. Paint the mix on a newspaper. If you can barely see the text through the mix, it's good. I think I'll dilute the yellow pigment a bit more, though. It seemed a bit too thick to me yesterday.

Henning, I looked up your yellow pigment; PY 153 (nickel dioxine) isn't a pigment I've used myself, except for testing, but it looks to be a fairly transparent pigment, as yellows go. I'd trust your judgment about adding more gum whether I'd checked this out or not, but just to back you up in your decision, and to somewhat contradict what I said before, I'd say that if you'd used enough semitransparent yellow that you could barely see the text, then it may well be too much. I think the newspaper test may work better for more opaque pigments, and darker pigments, but that's just a guess; I've never used it very much myself.

kt

 

[scootermm]

But be aware that this is counter to conventional gum wisdom so proceed at your own risk should you decide to follow me in this scandalous behavior.

no matter your methods with gum printing (or gum "over" printing in my case) you are always breaking someones rules... but then again, I always see them as guidelines more than rules

 

[donbga]

I mix my gum (this is the first time for me) by pouring 300 ml of distilled water into an old marmalade jar, and then adding 90 grams of gum arabic bought from an art supply store around here. The brand is LeFranc & Bourgeois. Probably quite old, but seems to work. This is the first time I mixed my own gum solution, so I used 45 grams of gum first, which was too little, and added 45 grams more the day after. Then I added three crushed tablets of acetylsalicylic acid (aspirin) as a preservative. Finally, after the solution was thick and lump-free, I filtered the solution through a folded, clean cloth. It's now as clear as the W&N jar I bought from the art supply store.

Mixing the gum/pigment I use the trick I learned from my teacher. Paint the mix on a newspaper. If you can barely see the text through the mix, it's good. I think I'll dilute the yellow pigment a bit more, though. It seemed a bit too thick to me yesterday.

Well that gum mixture certainly doesn't sound too thick but it could be mixed a little thinner.

I use a drink blender to mix the liquid gum from powder, works great. I think I would thin the gum/pigment mixture just a little but experience will guide your decisions. Just keep working.

The first rule of gum printing is that there are no rules for gum printing!

Don

 

[Katharine Thayer]

no matter your methods with gum printing (or gum "over" printing in my case) you are always breaking someones rules... but then again, I always see them as guidelines more than rules

Well, no matter what you do, you'll never be breaking any rules of mine, because I don't believe in setting out rules for gum printing. My only rule is to never take anyone's word for anything, to not follow anyone else's recipe but to experiment with your own materials, your own environmental conditions, your own equipment, until you find the way that gum works for you. It's not that hard, and gum is its own best teacher. Whenever anyone starts making dogmatic statements about how to and how not to print gum, that's usually a dead giveaway that they don't know much about it, IMO.
kt

 

[z-man]

hair dryers

Well, no matter what you do, you'll never be breaking any rules of mine, because I don't believe in setting out rules for gum printing. My only rule is to never take anyone's word for anything, to not follow anyone else's recipe but to experiment with your own materials, your own environmental conditions, your own equipment, until you find the way that gum works for you. It's not that hard, and gum is its own best teacher. Whenever anyone starts making dogmatic statements about how to and how not to print gum, that's usually a dead giveaway that they don't know much about it, IMO.
kt

in comercial production situations used a dryer in each hand set at highest heat-dryers were comercial salon dryers with higher output than consumer units-

my newest dryer has an ionizing switch to counteract the positive ions normally generated by the process of using a resistance electric heater

the variables of the process are so many and so influential that once the system has been tightened up enuf for predictability any disclosure of ones methods is usually of no use to any one else

vaya con dios