Color Gum Bichromate Printing - color management?

[jag2x]

Hi all,

I havent as yet done any basic gum printing. Though I would like to understand all the processes fully before I begin.
At the moment I print B&W prints using the ilford multigrade darkroom paper, using imagesetter negatives. I followed the Yule/Neilson Density table supplied by Huntington Witherill (www.huntingtonwitherill.com). Which has been very valuable to me in trying to figure out with a densitometer, what the corresponding density values are from my print to the percent values in photoshop "K Values". E.g my Ilford paper Dmax is 2.04, Dmin 0.00, and the 50% K Value reading from the chart corresponds to 0.66 in density. Once I figure out all the values I make the curve and apply it to the image in Photoshop and get the resulting imagesetter negative.

Alright that I understand, now the next phase is trying to print color seperation negatives for color gum bichromate printing.
From what I've been reading I can produce these negatives easily with the cmyk channels, producing 4 B&W negatives.
Say I want to know what my densities are for K channel is going to be, so I use a black watercolor paint and use a step wedge to print. Now I presume I use the same method as I did before. Using my densitometer and figuring out what my DMAX and DMIN is for the paper(following the Yule.Nelson Density Table). Then creating the curve for that K channel to get a good tonality from 0 to 100% "K Value".
Now the questions are:
Once I get this curve do I use this curve on all the other channels? C, M , Y?

Would exposure time be different or the same for the K channel print if I printed C,M or Y?
Do the watercolors I use for the other channels have to be taken into consideration?

Excuse my ignorance but I know very little about color targeting/profiling, is there a way to use a method to match a base watercolor cyan (Thalo blue) (or any other pigments that I use for the other chanells)print pigment to correpond to actual value in Photoshop? I understand scanning it would be one method, would a color densitometer help in this instance or those Gretag Macbeth type systems work here? Though I rather not scan and match by eye, I would like to use a denistomter if possible?

So the main purpose is to get the actual color image on the computer to look almost like the final print.

Am I way off target, are these valid questions to ask? What other questions should I be asking when it comes to color printing?

I understand printing color Gum Bichromate is riddled with a million factors, but I just want to get a better picture (pun intended) of all the processes and being able to control each step.
Ta
Jacek

[DOYLE THOMAS]

--------------------------------------------------------------------------
Once I get this curve do I use this curve on all the other channels? C, M , Y?
--------------------------------------------------------------------------
I as yet have little little experience but I have attempted some tri-color Gum Dichomate printing. I understand what you want to do, ie fixed exposure and development time per layer = correct color balance. It won't be that easy if my experince is any indication.
--------------------------------------------------------------------------
Would exposure time be different or the same for the K channel print if I printed C,M or Y?
--------------------------------------------------------------------------
depends on the pigments used, see below.
--------------------------------------------------------------------------
Do the watercolors I use for the other channels have to be taken into consideration?
--------------------------------------------------------------------------
Yes, there are many factors including the pigment density and the pigments opacity to UV light.

As you said, Gum Bichromate (dichromate) is riddled with a million factors. I have been looking out for a good class/workshop for about a year now. Let me know if you hear about one.

I will look into the Yule/Neilson Density table you mentioned.

Doyle

[timeUnit]

Gum printing is very very different from silver gelatin printing. It will take years of practice to be able to produce coats that are consistent in Exposure Scale, Density, Hue, Saturation and Opacity. As each coat you make will be different, there is no use trying to create negs with carefully compensated/linearised curves (IMO).

I would suggest you try to match the ES of each of your gum pigment mixes with the DR of your neg. Maybe you'll make general compensations in the curve(s) to match/separate certain values better, but this is probably something you'll do later on, when you've gotten the hang of things.

From there on, it's more of an improvisation. "Maybe this print would benefit from an extra cyan layer." "Maybe I will choose a lower contrast gum/dichromate mix for the magenta layer in this print." Try and fail, try and succeed.

Getting perfect color balance with gum prints is achievable, but very difficult. IMO the beauty of gum printing is in the unlimited possibilities of interpreting an image into a print. Making perfect color balanced prints is just not it's strength -- it is the exquisite cross between photography and painting, the way one works with the pigments while wet, etc. And of course how the end result is on a beautiful art paper, and will stand unfaded for hundreds of years.

[Katharine Thayer]

Gum printing is very very different from silver gelatin printing.

Amen, brother.
kt

[Don Bryant]

It will take years of practice to be able to produce coats that are consistent in Exposure Scale, Density, Hue, Saturation and Opacity. As each coat you make will be different, there is no use trying to create negs with carefully compensated/linearised curves (IMO).

TU,

Sorry but I have to disagree with you. You can become profficient at gum printing in a few weeks. Also digital negatives can be made that work predictably <sp?>.

I would suggest you try to match the ES of each of your gum pigment mixes with the DR of your neg.

This may not be possible depending on the DR of the negative.

Regards,

Don Bryant

[Katharine Thayer]

TU,

Sorry but I have to disagree with you. You can become profficient at gum printing in a few weeks. Also digital negatives can be made that work predictably <sp?>.

I agree with Don that it doesn't take years to become proficient and consistent with gum printing. Look how fast Denise is picking it up, for example.

But I also agree with doyle and henning in their responses to the OP, when they say that precise color matching in full color printing isn't as simple as he was hoping to make it; it's at least as much a matter of understanding the pigments you're using and how they interact with each other (see the discussion about tricolor gumover in the silver gelatin thread for an example) and balancing their proportions correctly, as it is about dialing in curves for each of the individual colors.

What I was agreeing with in my earlier response to Henning's post was the opening sentence specifically: gum printing is very different from silver printing; I responded to that before I even read the rest of the post, because it resonated so plangently with my thoughts from the other thread where I argued that it doesn't make sense to lump gum in with other alternative processes, such as when making generalizations about matching print tones to the tones on the monitor.
kt

[timeUnit]

OK, I'll back down a bit from "years of practice" statement. I meant that it is quite difficult to create two gum prints that are indistinguishable from each other, ie with a very high degree of consistency in mixing, coating and exposure. At least that's my own impression after doing the process for a little while (and my teacher seems to agree after 20 years of experience). And, to me, that's the beauty of it.

[timeUnit]

This may not be possible depending on the DR of the negative.

Regards,

Don Bryant

The gum process seems to need a DR of around 1,2, or so I've heard. Unfortunately I don't have a densitometer and have no means or checking my negs that way. It's trial and error for me.

Anyway, most digineg-processes should be able to produce a neg of that density, right? At least my Epson 1290 has no problems with it.

But then again, the ES of gum bichromate is very short, and with a low Dmax compared to other alt. processes. So one cannot always expect to get all tones on a full neg onto paper in one layer. One might need an extra high contrast layer for that punch.

In my limited experience, anyway.

[dwross]

Hi All:

I've been looking in on this thread and the "curving out emulsion" one with delighted interest. Every time someone made a comment I went, 'Yeah, me too'. I think it was Katherine who made the comparison of the blind men and the elephant to gum. I've decided that is spot on.

I want to believe that it is possible to get the gum process down to where one could reliably make near identical copies, but I'm sure not there. So many variables! Light output, heat, humidity, what looks like a degree of self-masking dependent on how thick you get a coating... well, you gum veterans know far better than I. It's fun, though. I'm convinced there's a lot of potential in crafting the color separation negatives. Lot of work, too.

Katherine: I think I have basically worked out a (as in one) color set that works. I have deliberately printed heavy so that I can see the combinations in three coats. Blue: one to one Phtalo blue and Carb.Violet DS water soluble inks, Yellow: one to one Hansa Yellow and Lemon yellow DS watercolors, and Red: one to one DS Rhodonite Genuine and Quinacridone Red. I like the results (although I'm printing too strong for my personal taste) but I'm going to try to get away from the inks. Yuch! smelly nuisance stuff.

I going to try to post attachments here. Let's see if it works.

d

[dwross]

Well, I don't think that worked. Try, try, and try again. Sheesh.

Ok, it's to the Gallery.