But the big question remains: Why did Kodak name a B&W paper "Azo" if it does not have Azo dyes in it?
Using film since before it was hip.
"One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal
, Vol. 1 (8 Jan. 1819): 8-29. p. 11
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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?
Museums often use a nitrogen atmosphere which is dried before use. They use less than normal lighting levels.
Originally Posted by DarkroomExperimente
I have no idea. I've mentioned this before.
Originally Posted by mhv
It may be that it used Sodium Azide as a preservative as some of the early emulsions did.
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.
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