Ultimate U08 / Bleach : reversal or rehalogenating ?

Silverhalide Emulsions / Chemistry.
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thycore
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Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by thycore »

Hey folks,

In my quest of understanding emulsion shrinkage (ie. when it does happen in my process).
I came to an interesting reading here :

https://ocw.mit.edu/courses/media-arts- ... mistry.pdf
bleach.jpg
bleach.jpg (282.81 KiB) Viewed 13301 times
Question, anyone know what kind of bleaching process is made on Ultimate Holography ?

There is no "fixer" stage in the process, but maybe the developer also acts as fixer with some tanning agents or whatsoever.

So.... Any clue of the kind of chemicals i'm playing with ?
Martin
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by Martin »

For a better understanding I'd look for Bjelkhagen's book (https://link.springer.com/book/10.1007/ ... 40-70756-1). It covers at length your questions. Not to forget also the search function of our forum.

Due to the loss of material (metal silver that is) involving reversal bleaches, you've to expect emulsion shrinking. As far as I know there's only one exception to this: Through "DCG processing" (IPA baths/hot air drying) you might end up with the original layer thickness. But this makes sense only for larger grain emulsions, e.g. the old Agfa materials. Reversal processing won't lead to grain growth - image modulation stems from the original (undeveloped) AgX grains. Since less AgX remains in the layer, you usually get less index modulation though.
Question, anyone know what kind of bleaching process is made on Ultimate Holography ? There is no "fixer" stage in the process, but maybe the developer also acts as fixer with some tanning agents or whatsoever.]
I assume the bleach is a rehalogenating one. In the earlier days of ULTIMATE Gentet mentioned a formula based on ferric sulfate + potassium bromide.
And yes, there's no fixer after the bleach.

As for your former question regarding colloidal development: at the end of process you've a mixed layer of both phase AND amplitude nature. That's why you can achieve bright holograms like those made by Valery. I guess the degree of the amplitude thing essentially depends on the grain size and shape.
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thycore
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by thycore »

Thank you Martin for your time & explanations, I purchased that book right now, I'll sure give it a read !

I'm in the step of the learning process what I need to understand better what happens in my materials to fine-tune my set-up and results :)

So... how is this possible to have a Rehalogenating bleach witout a Fixer prior to it to remove unexposed AgBr ?
(steps in ultimate process only involves a developer and a bleach, no intermediate bath except for fresh water)

Could it be... ultimate bleach is in fact acting like reversal bleach ?

As for Valery's holograms what amazed me that he was able to have good diffraction efficiency without even bleaching the hologram... if the hologram isn't bleached then only the surface of it would responds to light as amplitude hologram and "first-contact" phase ? If I wanted to do some dark backgrounded holograms it would mean I have to use a fixer bath instead of a bleach bath, correct ?
Ed Wesly
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by Ed Wesly »

Here is a recipe for the "Brilland bleach" that is what is said to be the recipe that Gentet uses:

30g Ferric sulfate (should be Fe III, which is what's usually sold)

30g Citric acid (readily available in drugstores)

30g Potassium Bromide

1000 cc water

It is from this site: https://holowiki.org/forum/viewtopic.php?t=5906 However, I am suspicious of this formula, as other similar bleaches have some sort of EDTA involved: http://edweslystudio.com/Formulae/Bleac ... eEDTA.html, http://edweslystudio.com/Formulae/Bleac ... yHaze.html, http://edweslystudio.com/Formulae/Bleac ... dEDTA.html. I like the fact that citric acid is used in this formula instead of sulfuric.

It was an interesting discovery that one can develop, then bleach in a rehalogenating bleach and still get modulation. The mechanism is that the developed silver plates itself on the virgin grains as it is rehalogenated, swimming, in effect from the bright fringe areas to the dim ones. This leaves all the original material in the emulsion, just redistributes the silver halide crystals into a map of the fringe pattern, so there is no shrinkage or color shift. For more details while you are waiting for Hans's book, see: http://edweslystudio.com/Formulae/Developers/CWC2g.pdf

I always wonder if the concept of developing and then using a rehalogenating bleach was pre-meditated, or simply a happy accident, like someone forgot to fix, or if someone just said let's see what happens if we don't fix. It seems that Nick Phillips was the first to write it up, as far as I know, http://edweslystudio.com/Publications/T ... illips.PDF. Holographic quality emulsions were in use in the latter part of the 19th century, but no one reported on this novel processing scheme until 1980.

I have not tried the reversal bleach on Ultimate plates, it should work if you want to control color via shrinkage. Give it a try and let us know!

NB: I have discovered that a lot of the holo-wiki's formulae are corrupted! For instance, under PBQ bleaches, the mercuric chloride formula is given! Best to refer to this: http://edweslystudio.com/Formulae/Devel ... meset.html
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Martin
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by Martin »

Ed Wesly wrote: Mon Oct 01, 2018 3:35 pm Here is a recipe for the "Brilland bleach" that is what is said to be the recipe that Gentet uses:

30g Ferric sulfate (should be Fe III, which is what's usually sold)

30g Citric acid (readily available in drugstores)

30g Potassium Bromide

1000 cc water

Yes, that's exactly the bleach I had in mind - thanks, Ed. I seem to remember it worked pretty well. But those were the Agfa days. With these ultra-fine grained emulsions it may make sense tinkering around the amount of KBr though.
I always wonder if the concept of developing and then using a rehalogenating bleach was pre-meditated, or simply a happy accident, like someone forgot to fix, or if someone just said let's see what happens if we don't fix.
My guess would be that it simply emerged from the photographic concept of re-development. In the context of Lippmann photography e.g. Neuhauss explicitly mentions a copper sulfate (+ KBr) bleach he used prior to re-develop (for amplification) some of his plates.
Martin
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by Martin »

thycore wrote: Mon Oct 01, 2018 9:36 am As for Valery's holograms what amazed me that he was able to have good diffraction efficiency without even bleaching the hologram... if the hologram isn't bleached then only the surface of it would responds to light as amplitude hologram and "first-contact" phase ?
it's way more complicated than that.

Referring to a paper by Crespo, Bjelkhagen points out:
They have studied the influence of the silver-ion concentration in the emulsion on the diffraction efficiency of colloidal silver-halide holograms. The most common procedure used for these types of holograms is based on rather diluted emulsions processed in semi-physical developers in such a way that silver particles of the order of 20 nm are obtained. A semi-physical developer contains not only a reducing agent but also a silver-halide dissolver (...). In principle, during the development process Ag+ ions are reduced to metallic silver in the latent image centers. The number of these centers grows with increased exposure, whereas the number of silver ions in a given emulsion is constant, i.e. it is dependent solely on the concentration of silver in the emulsion and is independent of the exposure time. If the exposure time is short, few centers will be formed and silver ions will produce silver particles in the same way as when a conventional developer is used. The densitiy of the emulsion will increase with exposure. However, above a certain exposure value (the number of latent image centers is high), due to the action of the semi-physical developer, the number of silver ions transferred to every center diminishes considerably, which results in silver particles of a smaller size than for conventional processing, and which, in turn, causes a lower density of the emulsion, i.e. low absorption (colloidal silver). (Bjelkhagen, Silver-halide recording materials, p.46)
So after development these layers turn red to yellow (depending on the size and shape of the silver particles. The smaller the grains the more image modulation is of the phase type. Maybe someone with better insights into Raleigh and Mie scattering theory could elucidate on this. Anyway, since these layers usually strongly absorb/filter blue light, the reconstruction of the blue image of a reflection hologram becomes very dim.
Din
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by Din »

Martin wrote: Wed Oct 03, 2018 5:24 am The smaller the grains the more image modulation is of the phase type. Maybe someone with better insights into Raleigh and Mie scattering theory could elucidate on this.
Well, roughly, Rayleigh scattering is predominant for small particle size, roughly 10% of λ. Rayleigh scattering is direction independent, and scatters both in the forward (the direction of the incident) light and the backward light. Also, because Rayleigh scattering is independent of the direction of incident light, scattering occurs for both transmission and reflection holograms in random directions within the emulsion. It's also heavily dependent on the wavelength of light, being proportional to the fourth power of the frequency, and so to the fourth power of the inverse wavelength. That is R ~ (1/λ)⁴. So, particles of about, less than, 50nm will strongly Rayleigh scatter, and the scattering is more intense for lower, blue shifted, wavelengths. So, as the colour changes from red to yellow, scattering increases. The scattering ratio, ie the ratio of intensities of scattered light, is:

R(y)/R(r) = [λ(r)/λ(y)]⁴ = [630/590]⁴ ~ 1.4

That is, the yellow will scatter about one and a half times more.

Mie scattering dominates as the wavelength increases to about λ. Mie scattering is not λ dependent, but it is directionally dependent, being stronger in the forward direction (in the direction of the incoming light). It's also much stronger than Rayleigh scattering, which is why, by the way, you can see a small white cloud against a large expanse of blue sky - the water molecules in the cloud are far larger than the nitrogen molecules in the air. In terms of the direction dependency, Mie scattering will scatter towards the viewer in transmission holography, and away from the viewer in reflection holography. However, given the range of particle sizes in terms of these fine grain materials, i'd say Rayleigh scattering is probably more influential in terms of noise. Remeber also that the efficiency of a hologram is not equal to the brightness of a hologram. There are scotopic issues. Roughly, if V(λ) is the efficiency of light reception at λ, then, for blue against yellow:

V(450)/V(590) ~ 0.038/0.757 ~ 0.05

That is, yellow is about 20 times brighter than blue for the same luminosity (efficiency of hologram)
Martin wrote: Wed Oct 03, 2018 5:24 am The smaller the grains the more image modulation is of the phase type.
One thing I have to point out: the image modulation is not the same as the index modulation. The index modulation is determined by the density of developed grains, since it's the index difference between developed and undeveloped grains, that is, by the ratio of the density of the light parts to the dark parts of the planes in the emulsion. As I've mentioned elsewhere, all emulsions in the last 40 years have been volume holograms due to the fact that emulsions are in the range ~ 8 - 10 microns, giving a Q factor ~ 100 (a Q factor <~ 10 will give an amplitude hologram). Therefore index modulation must be considered as a volume effect, which is dependent on the slant of the Bragg planes.

Image modulation, ie the MTF of the image, is most probably determined by the the noise in the reconstruction in a display hologram and the scattering from the model itself during recording. The early model for efficiency in holography, pre-Kogelnik, was based on the MTF of the hologram, but, I think, they did not consider holographic noise in using MTF as a measure of efficiency. Today, I think MTF will be based o the amount of scatter within the emulsion.
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thycore
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by thycore »

My mind just blown up. :shock:

Thank you so much for these explanations :-) I’ll need some dozen readings to fully compute all this info !
Martin
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Re: Ultimate U08 / Bleach : reversal or rehalogenating ?

Post by Martin »

Great reply - thank you for shedding light on scattering, Dinesh!
Din wrote: Wed Oct 03, 2018 8:22 am So, particles of about, less than, 50nm will strongly Rayleigh scatter, and the scattering is more intense for lower, blue shifted, wavelengths. So, as the colour changes from red to yellow, scattering increases.


You're referring to the (laser) recording wavelength, right?

As for colloidal developed layers, there obviously exists a correlation between the silver particle size and the color of the layer (transmitted light): with the PFG-03 emulsion I got red, orange or yellow layers (depending on the composition, dilution of the developer, processing time/temperature). A yellow layer indicated the smallest grains I could get from that emulsion.
I wonder how that would look with todays' even finer grained materials.
I would have guessed the layers tended to become more (and more) transparent - the smaller the Ag particles.
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