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Thread: Fibre RA4

  1. #21
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    Ryuji;

    On FB support, the support absorbs both water and soluable chemicals from the emulsion melt. On RC, the support does not.

    For this reason, on RC support, hardener, salts and surfactants are present in the wet emulsion layer initially at a lower concentration than they are in the same thickness of emulsion coated on FB support. The water and soluable materials are effectively 'sucked' out into the FB support as the emulsion layer thins down due to moisture loss. The water and smaller ions will diffuse out more rapidly than some of the larger organic or ionic species.

    In the former case, the effective ionic strength and the pAg are different than in the latter case. We really cannot measure the effective redistribution of chemistry into the FB, we just know it is different. It is related to rates of absorption of the moisture relative to the other chemistry by the FB in the moments after coating.

    Therefore, all consitiuents in the melted emulsion, prior to coating may have to be adjusted before high quality coatings can be made on these two supports. It depends on the emulsion and the FB paper. Among the varieties of FB paper, it also depends on surface and degree of calendering as well as whether barium sulfate or barium oxide are used (pH for example may be quite different), and what level and type of gelatin, humectant and hardener were used in the barium layer. RC is more uniform in this regard, of course.

    After coating, on drydown, the emulsions have different keeping properties and sensitometric properties unless these adjustments are taken into account by other changes. This is because the water soluable chemicals are evenly distributed in the FB paper as a function of the depth of penetration of the water as coated, but on RC support, all ionic species remain in the surface layers.

    This has great impact on my 'primitive' emulsions, having tested them on a variety of FB and RC supports. It also has impact on larger scale production.

    Reequillibration of chemsitry in FB papers is what often causes FB papers to keep more poorly than RC papers. With RC papers, the exact final chemical balance can be ajusted rather precisely, whereas in FB papers it is continually changing.

    The remarks on color paper regarding optical properties that you made are, for the most part, incorrect. All negative color papers had the yellow layer on the bottom since before 1960. The reason is due to the required photographic speed of the yellow (blue sensitive) layer relative to the other layers and dye stability, not optics.

    PE

  2. #22

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    Quote Originally Posted by Photo Engineer
    The remarks on color paper regarding optical properties that you made are, for the most part, incorrect. All negative color papers had the yellow layer on the bottom since before 1960. The reason is due to the required photographic speed of the yellow (blue sensitive) layer relative to the other layers and dye stability, not optics.
    In old color paper material, red- and green-sensitive emulsions had rather high blue sensitivity, and therefore they had to be placed below blue-sensitive (cyan-forming) layer and a blue-absorbing yellow filtering layer.

    In more modern papers, residual blue sensitivity of red- and green-sensitive emulsions is low enough to remove the yellow filter and shuffle around the layers.

    I've checked these at least for the case of Fuji paper products.

  3. #23

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    Quote Originally Posted by Photo Engineer
    For this reason, on RC support, hardener, salts and surfactants are present in the wet emulsion layer initially at a lower concentration than they are in the same thickness of emulsion coated on FB support. The water and soluable materials are effectively 'sucked' out into the FB support as the emulsion layer thins down due to moisture loss. The water and smaller ions will diffuse out more rapidly than some of the larger organic or ionic species.

    In the former case, the effective ionic strength and the pAg are different than in the latter case. We really cannot measure the effective redistribution of chemistry into the FB, we just know it is different. It is related to rates of absorption of the moisture relative to the other chemistry by the FB in the moments after coating.

    Therefore, all consitiuents in the melted emulsion, prior to coating may have to be adjusted before high quality coatings can be made on these two supports. It depends on the emulsion and the FB paper. Among the varieties of FB paper, it also depends on surface and degree of calendering as well as whether barium sulfate or barium oxide are used (pH for example may be quite different), and what level and type of gelatin, humectant and hardener were used in the barium layer. RC is more uniform in this regard, of course.
    I agree with some of what you said above, but I don't agree that those factors are strongly related to coating defects you mentioned in the other thread. Plus, in the other thread you didn't discuss RC, only baryta and non-baryta fiber papers. So my previous comments apply only to baryta and non-baryta fiber papers and not RC. I coat on plastic films and glass plates, and so I know there is some difference there, but not as drastic as you described. What I do is to use a synthetic polymer blend that increases adhesion between the gelatin macromolecules and the substrate, and use different hardeners depending on the polymer blend (so that the synthetic polymer, the modified gelatin, and raw gelatin are crosslinked each other). I don't consider this to be a reformulation of the emulsion. Nucleation, ripening, growth, desalting, digestion, stabilization, and spectral sensitization are identical. Just different addenda.

  4. #24
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    I repeat, since the time that Kodak began using a color mask, NO Kodak negative color paper had the yellow layer on top. This well predates the 60s. That is why the color paper has a cyanish cast when wet, due to the cyan layer being on top. Of course, this cyanish cast is greatly reduced in the modern papers but was quite strong in the early papers.

    Agfa color paper did have the yellow layer on top until they began using masked color negatives, and the same is true of Fuji and Konishiroku. This took place for these companies sometime in the 60s. It is also the time when they converted from the water soluable Fischer couplers to the Kodak oil soluable couplers.

    PE

  5. #25
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    Quote Originally Posted by Ryuji
    I agree with some of what you said above, but I don't agree that those factors are strongly related to coating defects you mentioned in the other thread. Plus, in the other thread you didn't discuss RC, only baryta and non-baryta fiber papers. So my previous comments apply only to baryta and non-baryta fiber papers and not RC. I coat on plastic films and glass plates, and so I know there is some difference there, but not as drastic as you described. What I do is to use a synthetic polymer blend that increases adhesion between the gelatin macromolecules and the substrate, and use different hardeners depending on the polymer blend (so that the synthetic polymer, the modified gelatin, and raw gelatin are crosslinked each other). I don't consider this to be a reformulation of the emulsion. Nucleation, ripening, growth, desalting, digestion, stabilization, and spectral sensitization are identical. Just different addenda.
    I am trying to include all of the papers that I have coated on, and all of the results that I have seen.

    I include in the emulsion formulation the stabilization step as you do.

    Therefore, if a change in the level of stabilzer is needed, it is a change in the emulsion formula. In the case of RC or film, a given dry emulsion layer 'sees' more stabilzer than 'seen' by the same thickness of emulsion coated on plain paper or baryta paper and therefore the results are different. The stabilzer has equillibrated within both the emulsion and the paper stock in the case of any FB paper. It has not in the case of RC or film support.

    This usually would be seen as a slightly higher speed on a FB support with worse keeping over time everything else being equal.

    PE

  6. #26
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    To continue with baryta vs non-baryta papers, one must consider all of the above.

    Baryta papers are heavily calendered with tons of pressure applied to the surface of the barium layer. This makes the baryta layer less permeable than the fibres of plain paper and is relatively independant of the presence or absence of a gelatin subbing layer.

    You might say that in order of permeability, RC is impermeable, Baryta is more permeable, and plain paper is very permeable. Again, this is not dependant on whether there is a subbing layer present on any of these supports.

    In addition, the baryta layer contains humectants and is loaded with the baryta itself. Regular paper is just a fibre substrate. Therefore, when I coat on plain paper vs a baryta paper, I see a big difference in repellancy. I also see what appears to be a difference in equillibration of chemistry somewhat like coating on RC, in other words, the baryta is between plain paper and RC in terms of reequillibration of chemicals and diffusion of chemicals and water. Within about 10 minutes though, the back of the baryta paper is as damp as the plain paper. It is initially more repellant!

    This is all tied together with the emulsion formula. If a given emulsion is adjusted to a given pAg with salt and then surfactant is added, it may coat well on plain FB, but not on baryta FB. You may need to change either the salt level or the surfactant or both. If the salt is changed, then the emulsion is changed. The surfactant only needs to be changed if the repellancy can be cured by this method, but the salt level must be changed if salt diffusion or stabilzer diffusion become important.

    Again, nothing is simple and this is why it is an art as well as a science.

    PE

  7. #27

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    Quote Originally Posted by Photo Engineer
    You might say that in order of permeability, RC is impermeable, Baryta is more permeable, and plain paper is very permeable. Again, this is not dependant on whether there is a subbing layer present on any of these supports.
    I wouldn't say that. Your discussion is missing one huge subject: internal sizing of the paper. There are a huge rage of paper stocks available for photography, watercolor, printmaking and other applications. One very important aspect of the difference is that they are adjusted to different degree of hydrophilicity of cellulose by using suitable kind and amount of internal sizing agent.

    Selected from non-baryta variety, I find papers that are most suitable for silver-gelatin photography are internally sized with dimeric alkylketene derivatives, stabilized by starch derivatives, and most preferrably, additionally surface sized with gelatin blend containing at least 20% trimellitiaged gelatin, or at least 30% phthalated gelatin, hardened with usual hardeners like vinylsulfones, active halogens, s-triazines, epoxides, glutaraldehyde, etc. These papers are only modestly absorbent and exhibit very good adhesion as well as wet strength. They also dry fast and flat as a fiber paper. These papers are inexpensively available in variety of sizes and surface textures.

  8. #28
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    Ryuji;

    Any given paper, whatever interal characteristics, becomes less permeable when overcoated with baryta and calendered. The internal characteristics become more evident as the paper becomes wet after coating. So, the generalization stands with respect to order of permeability with the various barytas falling within the center of the range. The family of plain fiber based papers falls on the higher side of permeability as I stated previously.

    However, since the other readers don't care about all of the chemistry expounded above, I won't restort to any of that. I'll just recommend a family of plain papers and baryta papers to those who wish to duplicate my work. This includes the formulas to coat on them. This will be available, with detailed instructions and formulas.

    I have stated elsewhere that a good point to start is Strathmore Smooth right out of the package with no extra preparation. This is what was used for the 8x10 print sample that I posted in another thread. I think that naming a good paper is better than citing a bunch of chemical names. Posting the example is the next best thing. I've done both.

    I think that is enough.

    PE

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