Image dyes and stability

Discussion in 'Color: Film, Paper, and Chemistry' started by Photo Engineer, Nov 25, 2007.

  1. Photo Engineer

    Photo Engineer Subscriber

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    There have been a lot of comments on image stability here and elsewhere regarding color materials. I have made one post here with a rather lengthy summary of the subject, but here goes again with more information.

    There are two classes of dyes used in color photographic materials. One type is the Azo dye, and the other type is the Azomethine dye. The Azo dyes are made and then either coated, or imbibed into a coating as in Ciba/Ilfochrome and Dye Transfer respectively.

    In the Ciba/Ilfochrome material, the dye is incorporated by immobilization in one sheet of material, a negative image is formed, and the dye is bleached along with silver leaving a positive dye image. In dye transfer, a positive silver and gelatin image is formed by using a hardening developer. This hardened gelatin is in relief. Dye is imbibed into the film from solution and is transferred imagewise to a mordanted paper. Both of these processes can use almost the same dyestuffs and can achieve the same level of dye stability.

    A further improvement in dye stability can be achieved from Azo dyes by complexing them with a metal. This is somewhat different than just mordanting the dye in place. It is an actual electronic bond with they dye which causes a huge improvement in dye stability. These so called "metallized dyes" were under development at EK when the entire diffusion transfer and dye transfer projects were cancelled.

    These products, in general, have the highest stability to light, heat, air, pollutants and humidity.

    The next class of dyes are the Azomethine dyes, or so called chromogenic dyes used in all films and most color papers today. A coupler is incorporated into the coating and the dye is formed by the color developer.

    These dyes are more subject to fade than Azo dyes, from heat, light, air, pollutants and humidity. Much research has been done on preserving these in film and paper. The results have been gradual improvement over the years of the stability of films and papers.

    Methods to improve Azomethine dyes include forming micro crystals of dye in the coating (Kodachrome uses this and it blocks fading well). Antioxidants such as vitamin E derivatives can be used very effectively added to the emulsion or even as a sub-group on the dye itself. In addition, use of UV absorbers have been used for years to block unwanted radiation from films.

    Recently, it has been discovered that certain chemical distortion can be achieved in the dye and this prevents, or blocks dyes from being attacked by outside influences.

    Kodak and Fuji have both published reams of articles and patents on this type of work, and have pretty much reached a parity in dye stability which is just a tad below the achievable level of the Azo dyes. It may even be on a par with the Azo dyes.

    But now comes the problem.

    How do you define the test for image stability in terms of light intenstiy, spectrum, and also heat, humidity and pollutants. Beyond that, how do you determine whether a print is unusable or undesirable due to some sort of deterioration.

    These definitions have led to huge differences of opinion in the industry as to who is right about their products. The bottom line is that no one answer exists, and no one company has a monopoly on being right or wrong.

    It may be counter intuitive, but the average home gets more light in higher latitudes than on the equator but it is true, due to the position of the sun and the surrounding levels of foliage during the different seasons. In fact, in winter in the norther hemisphere, when all the leaves are fallen, an average home gets more light than in the summer. So the true cycle is hot, humid and dim to cool, dry and bright when in the north and at the equator it is hot, humid and medium bright all the time. This cycle, when not tested, is a problem when reporting data.

    Among other things, accelerated tests do not project easily back to slow normal aging due to reciprocity. Reciprocity rears its ugly head even in dye fade by any means.

    Fade in homes in the Andes have been found to be influenced by higher UV, lower Oxygen, and lower pollutants except in big cities where fade increases due to pollutants.

    Kodak has run tests world wide to accumulate data on these conditions and I know as one of the individuals reporting on this data internally that they have tried to make it as honest and straightforward as possible. I'm sure Fuji has done the same. The differences though are in the details.

    Now as for evaluation, how do we say a print is unusable. Is it unusable after 5% dye fade or 10%, or is it unusable if only one dye fades even if that fade is 1%. Well, it turns out that even this is a difficult call. A 10% fade that is even in all dyes sometimes cannot be seen unless the viewer has a reference print for comparison. A fade of cyan dye in a portrait may not be seen easily, but in a forest scene will stand out right off. Fade of yellow or magenta will stand out in portraits but might not be seen easily in a forest scene.

    Therefore, due to all of this, I would have to say that no test can be exact or definitive, but merely used as guidelines. Even there they may be way off due to the nature of the tests and the judgments made in defining these guidelines.

    The ANSI tests allow for enough leeway to provide no real answer due to the fact that accelerated tests cannot be projected easily back to real keeping.

    I think that this, coupled with my last post on the subject may be more than you ever wanted to hear on this.

    PE
     
  2. Bob Carnie

    Bob Carnie Subscriber

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    Ron,
    an interesting read, never enough good info on this subject matter so keep it coming.
    Bob
     
  3. Photo Engineer

    Photo Engineer Subscriber

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    Bob;

    I had a thought in the meantime that relates to my work.

    Imagine someone using 50,000 FC to fade a print and they find that the print is bad in one year.

    They then divide that by 100 to come up with the figure of 500 FC for 200 years assuming 50% daylight per day. So, they say that a print will endure for 200 years.

    Here are the problems:

    1. The real day / night cycle changes this value. It isn't 50:50 and it involves sunset/sunrise.
    2. Lower intensity changes the oxygen saturation and airborne diffusion values.
    3. Humidity and temperature changes this.
    4. The projection back down to ambient is not simply a division in any case. It is a complex and unknown (and sometimes unknowable) polynomial.

    Then, the bottom line, who decides when the print is bad, and what are the criteria.

    So this reduces all of the above to a simpler statement.

    PE
     
  4. Bob Carnie

    Bob Carnie Subscriber

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    Ron
    I have most of the output devices in house, to make cibas, RA4, inkjet ,
    What would be a real world test to see the effect of image fade??
    I can face mount each print to plexi and mount to a wall outside and or inside that recieves nothing but North Light or just mount them to card and put them into the north facing window.* I did this about 6 years ago and the cibas I mounted are now completely faded, I did not put any media up against this.
    Do you think putting them outside face to plexi with a wrap to stop any wetness from behind, is a good test .

    I am dying to see the cannon inks vs the epson, ra4 colour against ciba chrome.
    I have a cibachrome in my lab that looks as fresh as the day it was made, about 14years ago and I do not want to wait that long to see the results of hanging in just normal room light.
     
  5. Photo Engineer

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    Bob;

    Try 100 fc and 500 fc for about 5000 hours and 1000 hours respectively backed up by 5000 hours north window exposure. These work out to the same in FCH but the last one factors in day/night cycles.

    For heat and humidity use about 140 deg F and 70% rh and 120 / 10 for these. If you want to get fancy you can raise the pressure, pump in ozone, sulfur dioxide and etc, or you can try hydrogen sulfide. There are lots of fancy things. There are many many more humidity/temperature variations, but this will suffice.

    What you want to look at are neutrals and separation exposures, as they behave differently. Neutrals protect dyes more than individual pure colors do.

    You see what a huge experiement we are talking about?

    PE
     
  6. roteague

    roteague Member

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    Thanks for the clarity on this Ron. Personally, I've always distruted Wilhelm, and this shows that longevity of color images is a complex equation.
     
  7. Photo Engineer

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    Bob Carnie;

    With inkjet, at high temps and humidity, image spread (sharpness change) is very important. So a comparison of definition charts with Ciba, RA4 and Inkjet are warranted at 140/70.

    PE
     
  8. DarkroomExperimente

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    When I get a bottle of pills from a pharmacy...there is often a little capsule inside to help protect the medicine..I assume it absorbs moisture...

    When I buy beef jerky there's a small foil envelope "to preserve freshness" that apparently absorbs oxygen

    would it make sense to have something similar for print storage/framing to help protect against atmospheric pollutants?
     
  9. Photo Engineer

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    The little packet works until you open the main package. How would you suggest it work on an open frame or prints being stored? Any ideas? How would it protect against light?

    PE
     
  10. pentaxuser

    pentaxuser Subscriber

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    PE Not more than I ever wanted to hear. I picked up more in this concise one page thread than I previously would have thought possible for a layman. Many thanks

    pentaxuser
     
  11. Michel Hardy-Vallée

    Michel Hardy-Vallée Membership Council Council

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    But the big question remains: Why did Kodak name a B&W paper "Azo" if it does not have Azo dyes in it? :wink:
     
  12. DarkroomExperimente

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    I'm thinking more of a sealed storage environment...not for something out on display

    what do museums do with prints in storage?

    Is there something like the "getter" that used to be in vacuum tubes or the sacrificial zinc on a boat that could help?
     
  13. Photo Engineer

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    Museums often use a nitrogen atmosphere which is dried before use. They use less than normal lighting levels.

    PE
     
  14. Photo Engineer

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    I have no idea. I've mentioned this before.

    It may be that it used Sodium Azide as a preservative as some of the early emulsions did.

    IDK.

    PE
     
  15. PhotoSci

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    Predicting Image Stability

    Photo Engineer has presented a truly excellent summary of the dye technology used in modern photographic films and papers as well as some of the difficulties and pitfalls in trying to test their stability under accelerated conditions and then map those on to real world keeping.

    I’d like to add two further observations. Photo Engineer points out the counter-intuitive issues in estimating the amount of light a typical displayed image will receive (most pictures—over 90 per cent—are kept in the dark where other factors like thermal keep, humidity, and polluting gases dominate). This is why real world measurements over extended periods of time are critical.

    For example, one might expect that a photo hanging on a living room wall will receive more light in the summer months than in the winter. Not necessarily. In a typical California home where extended seasonal measurements were made, the opposite was true. Why? In the summer, trees leaf out, shading the windows; in the summer, residents close drapes or blinds more often to keep the room cooler; in the winter, the angle of the sun relative to most windows is smaller, admitting more light. Etc.

    These are, however, after the fact explanations (though true). What is important is what you measure, if you bother to do full seasonal measurements. To the best of my knowledge only one company—Eastman Kodak--has done that and published their data in studies that now span 20 years, collecting that data in countries around the world.

    One oft-quoted book on image permanence cites the author’s measurements of light levels—but these are only one-time spot measurements, which can vary by well over a factor of five in a single day, much less over the seasons. What’s important to the life of an image is the total integrated amount of light it sees over its lifetime (that’s the reciprocity principle which is true far more often than it is not), not what you happen to measure at one time you measure it.

    Getting this right in estimating projected lifetimes is important because while your print is seeing light, it’s also exposed to heat, humidity, and polluting gases. If you use too high a light level in your keeping model, you could well ignore in your predictions the damaging effects of those factors which will have more time to act during the print’s real lifetime. To put it simply: all your factors must be reasonable approximations of your real world environment.

    By the way, not just how much light, but what kind is extremely important. In the home, the light having the most impact on prints is window-filtered daylight, which must include not just the effect of window glass, but the absorption and reflection of light in the room by walls, carpets, etc as well as artificial illumination. How do you know what this is? You gotta measure it. (One reason why I’m wary of the popular “window test.”)

    Photo Engineer also brings up the important issue of “endpoints”, that is, when do you decide a picture is unsatisfactory. That is extremely subjective and could be very different for, say, a consumer, an artist, or a curator. For consumers there is only one set of endpoints that were derived from real observations by consumers looking at thousands of faded scenes of different types. This study was published by David Olfield and his co-workers in the refereed “Journal of Imaging Science”. All other published endpoint sets, no matter how they’re advertised, are just guesses, and most when actually tested against real observers fall within a “Just Noticeable Difference”, an amount that is far exceeded by other factors like process variability. There’s much more to say on this complex subject, but I’ve already gone on too long.

    Thanks again to Photo Engineer for his very lucid and important post which prompted me to add these thoughts.