The way I understand the formation of the PO image in pt/pd, cyanotype, and even carbon (though you can't see it for the pigment -- but can be seen if you leave the pigment out...info from Sandy King), is that the printing out image does not form instantly, but takes awhile to form (info from Terry King). Thus taking a longer time to reach a certain level of exposure allows for more of the PO image to form during the exposure. A brief intense exposure, but at the same level of exposure, there will be less PO image formed during the exposure (but I am guessing the same strength of PO image will form, but mostly after the exposure is over...as the PO image formation has a little lag time).
The PO image holds back the shadow exposure a little, and also holds back the mid-tones, but not as much as the shadows -- and the highlights not at all. This can give a smoother gradation between the shadows and the highlights.
To improve the tonality of some small PT/PD prints that only required a 6 minute exposure, I have given the print two to three minutes exposure, removed the print from the light to allow the PO image to form, then returned the print to the light to finish the rest of the exposure. It seemed to help...but this is a subjective test, not quantitative by any means.
I have read that for smoother tonality in salt prints to start the exposure in the sun (perhaps to kick-start the PO image and build up good shadow density), then finish the exposure in open shade.
Could this be part of a Becquerel effect?
This also reflects of what Sullivan has shown me and said. Interesting about carbon...didn't know that.
Originally Posted by Vaughn
If there is a lag time between exposure and PO image formation, that would be very interesting.
However, mask formation inherently leads to less contrast, not more... right? If you think about these two mechanisms, reciprocity failure and self-masking, they're actually working against each other.
Furthermore, if indeed there is some lag time between exposure and PO image formation, by slowing down the printing time the effect would again be less contrast, not more. This lag could give us more contrast only if we get in a lot of exposure before the PO mask begins forming, which would only occur with a faster printing time (assuming we're beating the lag to some degree). So slower printing time should equal a more direct relationship of exposure to PO image formation (less effect of lag), and thus stronger masking, i.e. less contrast. However, the quandary here is that we're getting more contrast with longer exposure times.
With masking, the shadows will build up density first, and this corresponding density will reduce the rate at which more density is formed there. The upper mid-tones will print behind the shadows with regards to time, and the masking effect less extreme here (that is, exposure will be a truer indicator of density, since less masking takes place). The result: darks are held back, upper tones print darker faster = less contrast.
With reciprocity failure, the shadows build density in a relationship that's nearest to Light X Time = Exposure. Under the stronger densities (mid-tones, high-lights) in the negative, we get further & further from this reciprocal relationship because our light value is effectively diminishing. The result is a breakdown of this expression and more light (or time) is required to get the same exposure. The result with regards to contrast is that darks print "normal", and high-lights print proportionally slower = more contrast.
For some reason I thought you were talking about Timothy O'Sullivan... He would have known about this effect.
Originally Posted by Klainmeister
That's what I kept sketching as I tried to figure this out. Self masking, as image appears slowly, would be more complete at longer times. So the impact in the shadows if it is due to time lag of image formation must be trivial...
Originally Posted by holmburgers
I am betting on reciprocity law failure in the highlights, suppose for sake of example that the reciprocity law failure makes it necessary to give 400% exposure to reach the threshold highlight...
In daylight without reciprocity failure and a normal contrast negative:
10 Light x 10 Time = Exposure = 100%
Under yellow filter causes reciprocity failure and a flat negative compensated adequately for the highlights to match:
4 Light x 100 Time = Exposure = 400%
The shadow in the 400% exposure would have less reciprocity failure, so it would be as if exposed 4 times as much, and it will be blacker. Higher contrast at lower intensity...
Sorry, Richard Sullivan from Bostick and Sullivan. He's been teaching me these processes and their controls for the last year or so...
Originally Posted by Bill Burk
The affect that I (think) see is a more even tonality, not a contrast change. But then, one adjusts the contrast as needed for whatever one's methods are.
I tend to keep the printing process/method consistent and then select a scene, expose and develop a camera negative to match the process/method. I make pt/pd prints with no contrast agents, so my negs have a lot of contrast with healthy shadow areas, which means long exposure times. Whatever the longer exposure times has done to the contrast is taken care of automatically by the rest of the process.
The images that used only a 6 minute exposure were much thinner 120 roll film negs not processed originally for alt processes -- using pd and Na2.
Thinking about it, with no experimental evidence, it seems like both reciprocity and masking are at work here. Reciprocity gets worse with less exposure. There may be a crossover point somewhere that the slowly forming mask reduces exposure enough for reciprocity to increase beyond the mask's contrast reducing effects.
What about picking a light source, paper and process...and going for it? :) Make new negatives for the new process, rather than use old images processed for other processes. Print until one is happy with the prints, give oneself at least a year or two or three to reach this point, then print more. The differences in tonality between short and long exposures are subtle. One has to print a lot and study the results before seeing many of these subtle differences caused by this and other techniques!
Enjoy the Ride!