From: Linas Kudzma (lkudzma@earthlink.net)
Date: 01/17/02-07:35:34 PM Z
Whito,
I understand the proposed mechanism of Na2PtCl6 contrast boosting as
follows. In highlight areas (areas of low UV exposure/low ferrous oxalate)
the two step reduction of Pt(IV) to Pt(II) and then to Pt(0) metal consumes
half of the ferrous oxalate in a non image forming manner (the first
reduction from IV to II). Therefore, the highlight areas exhaust available
ferrous oxalate rapidly, forming a faint or no image. In shadow areas
(areas of high UV exposure/high ferrous oxalate) there is an EXCESS of
ferrous to fully reduce all the Pt(IV) to Pt(0), along with all the
Palladium. In this way, the effect is more apparent in the highlights. I do
not get weak Dmax when using Na2PtCl6.
When I responded last night I did it from memory. I had not gone back to
look at the actual paper I did my test on. Later, I found the test paper
with notes. The contrast boost with Na2PtCl6 is even more pronounced than I
stated. Details are as follows:
Paper: Cranes Cover 90 Natural
UV: 20 Minutes in my BL tube box, printing a Stouffer 21 step tablet
Developer: Potassium Oxalate 100F
Clearing: Citric Acid
Two separate sections were coated at opposite ends of the sheet:
2 drops 27% Ferric Ox, 1 drop Pd solution, 1 drop K2PtCl4 solution
("normal" Pt)
Printed Stouffer scale shows 13 distinct steps from step 7 to 19
2 drops 27% Ferric Ox, 1 drop Pd solution, 1 drop Na2PtCl6 solution ("NA2"
Pt)
Stouffer scale shows 8 distinct steps from step 6 to 13
Both strips were exposed at the same time on the same piece of paper.
Therefore, drying, humidity and exposure were identical for both. Maximum
black was very nice and dense on both.
I really like using Na2PtCl6 for contrast. My negatives never need more
that about 20% of the metal as Na2PtCl6 to get the look I like. Maybe at
this lower amount the contrast boost is subtle and looks similar to that
from the normal K2PtCl4? I don't know. Certainly at 50% the results
summarized above are dramatically different, even if the exact chemical
mechanism is unclear.
Regards,
Linas
> [Original Message]
> From: Witho Worms en Jorien van Santen <verzet13@xs4all.nl>
> To: <alt-photo-process-l@sask.usask.ca>
> Date: 1/17/2002 8:01:44 AM
> Subject: Na2PtCl6
>
> Linas,
>
> The way you have formulated the workings of 'Na2' I suspect that the
overall
> density is reduced. I don't see why 'Na2' only works in the lighter parts
of
> the print. It should behave the same in the shadows thus giving a overall
> weaker print ?
>
> Witho,
> I have not done extensive tests, but I have done some, and can say with
> absolute certainty that the so called NA2 platinum (Na2PtCl6) available
> from B&S raises contrast considerably more that "normal" platinum. In a
> test with 1:1 ratios of Pd/Pt, with each type of platinum, the so called
> "NA2" complex gave a 3-4 step compresion of the scale when printing a
> stouffer step tablet.
>
> The chemical explanation offered in the technical paper at the B&S site
> makes perfect sense to me. It takes twice the amount of ferrous oxalate to
> reduce the Pt(IV) of the "NA2" complex to metalic platinum Pt(0). The
> Pt(IV) complex uses up the available ferrous oxalate rapidly in areas of
> lower exposure, and this compreses the scale.
>
> Linas
>
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