Failing art

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holorefugee

Failing art

Post by holorefugee »

Joe,

I thought you would be interested in this story:

http://www.bbc.co.uk/news/science-environment-12453610
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Failing art

Post by Joe Farina »

Thanks Colin. Actually the impermanence of Chrome Yellow is well-known. A quote from Mayer (The Artist's Handbook of Materials and Techniques, 5th Edition, page 42):

CHROME ORANGE. CHROME RED. CHROME YELLOW. Lead Chromates. A large variety of shades, from a pale primrose yellow to a deep orange-scarlet, are produced by variations of the process of manufacture. They are opaque, work well with oil, and are used in large quantities in cheap paints. Even the best grades are not permanent, turning dark or greenish. They may also react with some of the other colors. Replaced perfectly by cadmiums for artist's use. Introduced in 1797.
holorefugee

Failing art

Post by holorefugee »

It is the mechanism that is new. I seem to remember there is some Cromium-3 and 6 in the DCG processes. I wonder if there is a chance of somethinkg like print out in DCG holograms that are not washed all the way clear of Chromates?
Dinesh

Failing art

Post by Dinesh »

holorefugee wrote:I wonder if there is a chance of something like print out in DCG holograms that are not washed all the way clear of Chromates?
No, there's no "print out" in dcg, although I think "print out" may not be the best term to use in dcg. Print out refers to the darkening in the film caused by unexposed the reduction of silver halide to silver in unexposed grains. There's no darkening involved in dcg. If there is "unwashed chromates" in the dcg, they'll cross link under the action of light and create local hardness, which will, in turn, cause a change in refractive index (Kramers Kronig). However, whether this alteration of index will manifest itself in anything visible is an open question. As far as I can see, depending on when this occurs, it'll have to be extremely localised for any visible manifestation. If the surrounding material is also hard, then any effect will be mitigated by the surrounding (hard) material. Of course, if the film is relatively newly developed, then the film may still be soft, and if there is a large alteration of index due to a large concentration of the unexposed chromate, then you have a large index change near a soft film and you may see a local blue-shift.

In practice, the Cr6++ is actinically (is there such a word?) reduced to Cr3 which is then cross linked to neighbouring gelatin molecules. So there's no "free" chromate - it should have been washed out. If, after processing, the plate is still yellow, then the Cr6++ could cross link, but the sensitivity of dcg is so low and there is so little blue-green light in most environments that I doubt there's going to be any serious printout.

This is a dcg hologram I made ca 1987. As you can see, the whites are still white so no printout in 25 years!
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holorefugee

Failing art

Post by holorefugee »

Thank you for helping me with the chemistry. I know I have some of those papers saved. By "Print out" I mean any thing that would effect the archival storage of a DCG hologram. But since we are going from 6 to 3 and that is the failure in the pant perhaps we are safe.
Dinesh

Failing art

Post by Dinesh »

Actually, one reason I don't like the term "print out" for dcg is that the opposite problem is more likely to occur. It's not that there's unexposed "stuff" in the emulsion, it's that local environments degrade the emulsion that have little to do with light levels. One of the big problems with dcg is water absorption and/or heat. The weather in San Diego is mild enough that 10 year old unlaminted holograms still look OK. But, holograms that were laminated exactly the same way disappeared within weeks in India, where local temperatures were around 100 or so and humidity was around 80%. Recently, we had a deluge here in Southern California and our lab humidity went up to 80% (it's normally between 35% and 55%). At this level, merely taking the exposed hologram to the laminating station two rooms away was enough to make the hologram disappear. Our experience in India changed our laminating habits so now we can take a hologram to such extreme climates and the holograms will survive (we've tested this for about a year and so far they're good). Generally, for display holograms, this is not always that big of a deal, the image is still visible, if a little green shifted. However, for technical optics, where the customer wants a fairly tight spec - say an efficiency of above 80% at some peak wavelength with a 20nm bandwidth - if the hologram swells just a little my measurements are invalid and the spec is no longer obeyed. Also, we have a set technique for getting these specs - such-and-such an efficiency at such-and-such a peak lambda with such-and-such a bandwidth requires us to shoot it with these parameters at these angles. However, as the film dries in the coating station, it also swells at these high humidities. The film was thicker than our standard thickness and all our strictly derived methods went out the window!

Also, silver holograms backed by some laminating material also went dark, not due to print out, but humidity. My theory, based on an actual experience some years back, is that after time the silver emulsion sticks to the backing. Under high humidity, the backing separates from the glass and takes the emulsion with it. We had one of Edwina's Bog Man holograms on film that's now disappeared. I'm trying to coax it back into existence with a little success. There's got to be something funny in that. A hologram of a 5000 year old mummy that was shot to help preserve the actual mummy is now in need of preservation itself (the hologram was made because the mummy itself was too delicate to be taken to the various universities for study and preservation)!
holorefugee

Failing art

Post by holorefugee »

DCG printout is a bad word but failure of the chemistry because of Cr6 to Cr3 conversion is not something we have studied. I hope all the Cr is in the rinse water but who knows?
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