Questions about tricolor gum using QTR separation negatives

[Vivaldi]

I have just started trying to make tricolor gum prints and Im hoping some of you with experience might be able to help guide me in the right direction.

My first few tricolor gum prints seem to be mostly yellow, even though I am using calibrated correction curves for each color. For example, bushes and leaves on trees are yellow in the print rather than green. I believe the QTR separation negatives are accurate in color separation, because the magenta, yellow, and cyan colors seem to be going in the right places in the print, but my third and last printing with the Cyan layer seems to mostly come off in the wash, even though it has been previously calibrated properly as a single color. My printing order is Yellow, followed by Magenta, and Cyan is last. I am not sure what variable I should be changing at this point to make a color correction.

The following is an outline of my process workflow so far:

I am making digital separation negatives using QTR with an Epson 3800 by inverting an RGB file in photoshop, splitting channels, converting each channel to gray gamma 1.8, applying my adjustment curve in QTR, and printing digital negatives on OHP transparency for Yellow, Magenta, and Cyan. The adjustment curves for each color were made with a stepwedge as outlined in Ron Reeders new book on Digital Negatives, August 2010 (This is a fabulous book!)

Using a Stoeffer 21 step wedge, I determined a basic UV exposure of 10 minutes for Yellow, 10 minutes for Magenta, and 10 minutes for Cyan using a fluorescent tube lightbox purchased from Bostic and Sullivan. My paper is Lanaquarelle with bloom gelatin sizing. The chemicals are from the Bositc and Sullivan Gum Kit, including potassium dichromate 10%, and 14 degree baume gum Arabic. I know the paper is sized properly because the highlight areas clear without any difficulty.

All watercolors were purchased at Daniel Smith and used as follows in my mix:

Quinadricone Rose (PV 19) - 0.5 grams pigment, 6 ml gum, and 6 ml K Dichromate
Pthalo Blue RS (PB 15) - 0.5 grams pigment, 6 ml gum, and 6 ml K Dichromate
Hansa Yellow Medium (PY 97) - 0.5 grams pigment, 6 ml gum, and 6 ml K Dichromate.

Average humidity at each printing was 60%

Questions:

1. Since the third cyan layer seems to come off in the wash in spite of 10 minutes of UV exposure, do I increase the UV Exposure time for Cyan? How much?
2. Or should I increase the amount of cyan pigment in my mix for the third layer? How do I determine how much?
3. Is it common practice to use different concentrations of pigment for each color layer or do most gum printers use the same concentrations of pigment for each color layer?

Thanks in advance for any ideas

 

[iansand]

sounds too simple, but are you sure you added the sensitizer to the cyan? If it all washed off then that seems to be a possibility

 

[Vivaldi]

Iansand,

Thanks for your reply. Yes, I am sure I added the sensitizer to the cyan. In fact, some of the darker blue colors do show up in the print but not in adequate amounts. One one occasion, I repeated the cyan layer a second time with the same mix as initially for 10 minutes UV exposure and that gave too much blue to the entire print. On a second occasion with a different print, I repeated the cyan layer a second time with just a tiny amount of Pthalo Blue in the mix, and that just barely started to turn the leaves a slight shade of green. But that was not enough blue.

 

[R Shaffer]

I would suspect your exposure times, especially if blue is washing off. I never could figure out how a stouffer wedge would have much relevance to an OHP negative. I create an OHP negative with 3 to 5 of the 21 step wedges all stacked up. I then print this with a different exposure for each wedge. I have attached a 3 strip example. Lets assume they are exposures of 2, 4 & 6 minutes. For me 6 minutes looks a little overdone and 4 looks a little under. So I would say my exposure should be around 5 to 5 1/2 minutes or 6 with extra development. I find these exposure strips very valuable for determining how much exposure to just get shadows or how much to really get into the highlights.

Have you tried just making a 1 shot gum print with blue?

As a side note, I use a single QTR curve for cyanotype and all gum colors. And then make slight variations in exposures and dilutions to dial it in.

 

[Vivaldi]

R Shaffer,
Thanks for the info and idea about making a 1 shot gum print with the blue digital separation negative. I'll try that, because maybe it is something in the way I am generating the QTR separation negatives. When I was calibrating the blue curve independently, I was able to get a similar looking stoeffer step as you show above using my same blue chemistry with 10 minutes UV Exposure. Additionally, using the 100 step wedge provided by Ron Reeder, I was able to get pure white at zero and intense blue at step 100 (with 10 min UV Exposure) when I calibrated the blue independently using QTR.

 

[R Shaffer]

You could just be loosing too much tooth in the paper to hang on to the blue layer, seems a bit odd as usually the sizing breaks down. See how the blue looks by itself and maybe try adding the other layers on top of it if blue looks good. Sounds like your negs are all dialed in.

 

[PVia]

I've had this happen as well, and I think Rob hits it on the head. The paper is losing tooth AND the darker color takes longer to expose through, but there's a fine line here. There's a point where still-water clearing (the blue) can take much, much longer if you overexpose (hours)...so you have to find that sweet spot. All papers are different and each one has to be dialed in re this factor...

 

[Vivaldi]

I tried printing just the monochrome Pthalo Blue Layer using my cyan negative (with curve) on the Lanaquarelle with bloom gelatin sizing as before, and I got a perfect cyan print. I am new to gum bichromate printing, but it looks as though I am going to have use a different UV exposure time for blue, yellow, and possibly Magenta, as long as I use Lanaquarelle with bloom gelatin sizing. I will be out of town for a week, but when I return, I will also try printing the magenta and yellow layers (each at 10 minutes UV exposure) on top of the cyan layer to see if the same problem occurs after printing the cyan layer first.