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  1. #21
    RH Designs's Avatar
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    [QUOTE=Marco B;941331
    Looking at your last post though and the further explanation, I think we basically agree about the effect and usage of a pre-flash [/QUOTE]

    Absolutely!
    Regards,
    Richard.

    RH Designs - My Photography

  2. #22

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    Another Approach

    Quote Originally Posted by polyglot View Post
    If I could get better results with my current materials,
    that would be preferable.
    Currently I'm working with the SLIMT. Effectively the thin
    areas of the negative are increased in density. That is,
    shadow areas in the print are held back. The print is
    given a post exposure short soak in an extremely
    dilute potassium ferricyanide solution.

    If interested search via Google for, latent image
    bleaching . Dan

  3. #23
    polyglot's Avatar
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    How dangerous is this ferricyanide bleaching business? Judging by the name it doesn't sound like something I want to splash around in the dark (ok so it might be post-fix except for you saying it's on the latent image but nevertheless I'll need an open tray of it on my already-full wet-side). Local photo stores don't carry it, so my only option to get it would be if it's safe enough to ship.

  4. #24
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    Quote Originally Posted by polyglot View Post
    How dangerous is this ferricyanide bleaching business? Judging by the name it doesn't sound like something I want to splash around in the dark (ok so it might be post-fix except for you saying it's on the latent image but nevertheless I'll need an open tray of it on my already-full wet-side). Local photo stores don't carry it, so my only option to get it would be if it's safe enough to ship.
    It isn't so bad. The cyanide is bound up in the salt. Keep it out of acid and you will pull through. You can order it from PF.

  5. #25
    Marco B's Avatar
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    Quote Originally Posted by polyglot View Post
    How dangerous is this ferricyanide bleaching business? Judging by the name it doesn't sound like something I want to splash around in the dark.
    Like Jason says, ferricyanide is not something to really worry about, except for not mixing it with strong acids, as it might than release toxic and potentially deadly cyanide gas. I remember one APUG member even saying "you could use it as fertilizer in your garden", as cyanide is a potential nitrogen source for plants, to illustrate how relatively harmless it is.

    To put it also in context: the blue color of cyanotypes is actually a pigment called "Prussian blue", which is made from ferricyanide too, and is complex of iron cations - the ferrous part of the ferricyanide - called ferricferrocyanide.

    And the same pigment Prussian blue is also used in artists oil and acrylic paints, and was used for the famous "bleuprint" technic of copying. Actually, Prussian blue was one of the first chemically synthetized color pigments developed and found its way into oil painting sometime in the 19th century. Before that, most color pigments used in paints were more or less "natural" pigments.

    Marco
    My website

    "The nineteenth century began by believing that what was reasonable was true, and it wound up by believing that what it saw a photograph of, was true." - William M. Ivins Jr.

    "I don't know, maybe we should disinvent color, and we could just shoot Black & White." - David Burnett in 1978

    "Analog is chemistry + physics, digital is physics + math, which ones did you like most?"

  6. #26

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    Also Via Google ----

    Quote Originally Posted by polyglot View Post
    How dangerous is this ferricyanide bleaching business?
    Those with sulfuric or other strong acids hanging around are
    warned not to mix the two. Quit safe; part of every Kodak
    Sepia kit. It's the bleach. The latent image bleach is used
    one- shot. It is used EXTREMELY dilute. Ten grams from
    P. Formulary will make about 100 liters of a stronger
    dilution or 1000 liters of the weaker dilution.

    Also via Google search for, "variable contrast from graded
    papers" . I don't recall the method being exclusively for
    graded papers. I'd think it would also work with
    VC papers. Dan

  7. #27
    polyglot's Avatar
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    thanks guys.

  8. #28
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    I finally made it into the darkroom and figured out my preflash: 1/3s f/22 full-height on the spare enlarger. 1/2s gave the tiniest bit of tone after drydown but 1/3s was clear. Also discovered the club's safelights aren't very.

    I found that preflash has a visible effect but the reverse of what I wanted! I only tried it with white/unfiltered light from an enlarger and didn't get around to preflashing magenta.

    It seems that if I preflash, I can get detail in the highlights at a higher print grade but the contrast index in the highlights is lower. In other words, it seems to roll the curve off at the highlight end, which is what you'd expect given that you're adding a bunch of non-image light. In the highlights, the image is largely made up of preflash (and without preflash they'd be blank) whereas in the shadows, the preflash has a very small contribution to the exposure. Say I have a scene I could fit into the paper's dynamic range at grade 2 (but it looks flat) or at grade 3 and lose the highlights; with preflashing I can print it at grade 3, get more CI in the shadows and still hold detail in the highlights, but the highlights will be closer to grade 1.5. So it's definitely a useful technique that I will be exploring heavily with some high-key photos. I think it should also be good for reducing grain in skies and I have an image I think it will work wonders on next week for that purpose.

    I tried printing with less-yellow light (120Y instead of 200Y) for the low-contrast part; it made no difference. I think that's a function of how my colour head works - say I wind in 100Y, the position of the dichroic filter means that blue is blocked from 1/2 of the flux from the bulb. So I seem to get exactly the same image whether I use (say) 10s of 200M and 4s of 200Y as if I used 8s of 200M and 4s of 100Y. If I was using discrete filters in a B&W head, this approach (say, print with 1 and 4 not 00 and 5) may well be valuable, I don't know.

    I also tried yellow-first split-grade but found that I got results much flatter than I wanted. Typical exposures I'm using without preflash are between 4:1 and 8:1 M:Y, so by the time I have visible tone from yellow exposure, there's way too much of it. Might be I'm doing wrong, I dunno - I've only been printing for about 6mo. I think I might just need to pay more attention to drydown. And I think I'm definitely getting shoulder effects in some of my shots, so I'll need to pay a bit more attention there - seems particularly problematic when pulling Pan-F.

    Thanks for all the suggestions, they're appreciated. If I had a flatbed scanner I'd post the results, but my only digitisers are a DSLR and film scanner.

  9. #29
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    Further experimentation tells me that it's not the printing, it's the negatives. I did a portrait session for a friend: some HP5 and some Acros. The HP5 has printed beautifully and has lovely sparkling highlights. The Acros looks dead and flat even when printed across the full paper range at a too-high grade - it manages to look flat and overly-contrasty at the same time

    My current hypothesis is that I'm getting developer exhaustion in the highlights (using Rodinal 1+100 for 18 mins, agitation as per MDC) so I'm going to try some more Acros in D76 again (as I recall, that worked well for me in the past).

    I really don't want to give up Acros because it's so very fine-grained. HP5 just gets crunchy when printed large; while it looks great from a distance, I like having big, finely-detailed prints you can get your nose lost in.

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