[alt-photo] Re: 3 color gum and PDN calibration?

[Johnny Brian]

With overexposure the steps on the wedge should merge, allowing measurement of the degree of overexposure. Even with the long exposure, the steps do not merge with the yellow pigment. With all of the other pigments I've used, I could make the steps merge with exposures between 4 and 8 minutes. I'm puzzled why the steps do not merge with the yellow pigment. 

Thanks,
Johnny Brian

On Sep 26, 2012, at 3:53 PM, Mary wrote:

> Hi Johnny,
> 
> I use the same pigments for 3-color gum and found that Ni Azo Yellow requires the least exposure of any of the others. Try an exposure time shorter than your 5 min 40 sec. time, rather than longer. My exposures are in the 6 min range, obviously with a different setup, but perhaps you are over-exposing already, and that's why you don't see much difference between that and 11 min.
> Hope this helps.
> 
> Mary
> 
> On 9/26/2012 11:23 AM, Johnny Brian wrote:
>> Recently I've started the calibration process for 3 color gum printing with precision digital negatives, and encountered an issue I need advice about. I have been using PDN to make monochrome gum prints for a year or so, and had established 5 min 40 sec as a printing time with an Arista UV box. For the 3 color, I've used M Graham Azo Yellow, Phthalocyanine Blue, and Quinacridone Rose pigments 1/4 gm with 3ml gum solution and 3ml 10% potassium dichromate printed on Arches Platine sized with gluteradehyde hardened gelatin. I printed some small 21 step wedges and noticed that it would seem that the printing times should be different for each color. This didn't surprise me as I would assume that the pigment itself absorbs some of the UV light, reducing the amount of UV to react with the dichromate. To see the wedges more clearly, I decided to print 4x5 step wedges. For the PDN "standard" printing time, rose pigment was 4 min, cyan was 8 min but the yellow does not have merged s
> te
>>  ps outside of the pictorico film even with a 11 m 20 sec exposure. I printed with yellow at 5m 40sec, 8 min and 11m 20sec, and they actually look about the same - the longer print times are a bit darker, but the steps 1-2-3-etc inside and outside of the film are visually different - and different when measured with an densitometer. Maybe I need to increase the exposure more, but 11m 20sec seemed like a long exposure time. SO -
>> 1. Is it expected to have such wide exposure times with the 3 colors?
>> 2. Should I try and increase the yellow exposure time even more? It seems like the steps outside the film should have merged already and I'm not sure increasing the exposure will make much difference.
>> 3. Unrelated to the above, but I've noticed that the blue (cyan) really tends to stain, even with the sizing (not really usable with out sizing the paper). The sizing also significantly changes the color of the blue. Anyone have a better suggestion for a blue pigment that doesn't stain as much?
>> 
>> Maybe this question is better for the PDN list, but I thought I would start here.
>> 
>> Thanks,
>> Johnny Brian
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