From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 11/02/01-10:49:18 PM Z
Nick Makris wrote:
> ...
> The test image I am working with is a 4"X6" 21 step photoshop image
> and the current sensitiser mix consists of:
>
> 7 drops of PD (Palladium)
> 7 drops of FO (Ferric Oxalate)
> 1 drop of PC (Potassium Chlorate 4%)
> 1 drop of PT (Potassium Chloroplatinate)
As Carl has pointed out, the Chlorate is likely too strong. For that
amount of solution, try for 0% to 1/8% as normal; 1/4% to 1% for
increased contrast; stronger may produce graininess or image degradation
for the Crane paper. This may require a better negative.
If your solutions are properly mixed, the metal salt and sensitizer
should be used in equal parts. Additional metal will be wasted. But
the solution strengths must be proper to at least meet the threshold of
what is necessary. For your example: if 1 drop of K2PtCl4 is used, then
6 drops of palladium solution should be used. All solutions must be
"matched". See my guide for detailed information.
But let's assume that your coating is OK, since you have stated that the
boarder has a good black.
> I may be expecting too much here, but let me continue. While the
> results do appear quite qood, I am trying to zero in on the problem
> of definition in the 1-5% and 95-99% areas of the scale.
The biggest problem is likely that the digital negative does not have
enough steps for the desired dynamic range. This can also be thought of
as part of the posterization problem. Because not enough individual
steps are available, discrimination is lost as you have indicated.
> Consider that I have been able to
> produce a negative that shows an even gradation in those areas,
> but that gradation gets lost in the print.
> My last test was 5min 15sec and the results are as follows:
> Highly desirable black in the areas that were not affected by the
> negative (ie, the sloppy part)
> Very nearly desirable black in the area of base plus fog (ie, a
> distinguishable difference from the above desireable black - less
> than 1/2 stop lighter and perhaps somewhat colder).
> The dark and light areas are exactly the opposite - no density
> gradation in the last 5% areas. Either all black or all white.
>
> The question is where do you go from here. The subtleties are
> escaping me. It seems that if I add time (which I am about to do)
> I will lose definition in the 90-100 instead of 95 to 100.
> If I don't add time, I'll not be able to get the blackest black.
This relates to another problem. About a third of the 256 steps (8-bit
data) are needed to get enough density in the negative to print a proper
black that can have some discrimination from Zone I. It would seem that
this might be remedied by applying a curve adjustment, however in
applying the adjustment the third of the steps are lost anyway.
What needs to be done:
The printing time must be established for the blank substrate. It does
not matter if the substrate partially blocks UV as this will be
accounted for with more exposure. However, it does mater if the ink or
inks do not block UV (another likely problem, some inks are useless
because of this).
The ink density must be enough to enable a pure white.
The data must contain enough steps for the Pt/Pd process which is
estimated to be at least 16-bit (assuming the least 2 bits to contain
noise and error leaving 14 solid bits of data.) This bit depth must be
achieved not only in the software but with the printer. (This is what I
have estimated to be necessary to avoid posterization limitations.)
The ink or multiple inks must be able to convey the bit depth of the
data. This may require the use of quad inks or such.
You are correct in your analysis of simply adding exposure time. This
will not correct your problem. I have not yet found any digital
negative that can produce a full dynamic, full range, fully
discriminated Pt/Pd print. As a solution, a reduced range may be
selected. However, this will lack range or discrimination or the light
or dark ends. What may be better would be to use a printing process
requiring less range or discrimination until a proper digital negative
can be produced.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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