A pdf of all known bleaches

Silverhalide Emulsions / Chemistry.
dannybee
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A pdf of all known bleaches

Post by dannybee » Sat Jul 11, 2020 1:16 pm

holography BLEACH.pdf
(97.45 KiB) Downloaded 85 times
just made it simple to print out , rather than keep going to Ed's Site , Great work Ed :D

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admin_jsfisher
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Re: A pdf of all known bleaches

Post by admin_jsfisher » Sun Jul 12, 2020 12:02 pm

I've added the PDF to the PDF collection. It is in https://holowiki.org/data/pdf/Misc_others/

The information also appeared in the wiki, but it clearly needs to be updated with new information compiled by Ed.

Grayham
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Re: A pdf of all known bleaches

Post by Grayham » Tue Sep 08, 2020 10:58 am

Thank you

Loic74
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Re: A pdf of all known bleaches

Post by Loic74 » Fri Sep 11, 2020 2:42 pm

Many thanks

Dmulligan2525
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Re: A pdf of all known bleaches

Post by Dmulligan2525 » Fri Jan 15, 2021 11:45 am

What is the purpose of using these different bleaches? Do they change the outcome of the hologram?

lobaz
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Re: A pdf of all known bleaches

Post by lobaz » Sun Jan 17, 2021 10:05 am

Dmulligan2525 wrote:
Fri Jan 15, 2021 11:45 am
What is the purpose of using these different bleaches? Do they change the outcome of the hologram?
This is a very interesting and thought-provoking question.
A quick answer would be: yes, some bleaches affect the outcome significantly.

First of all, there is a piece of history behind. At first, holograms were processed with chemistry used in photography. Many researchers and practitioners in holography tried to beat it, and indeed, some recipes worked better than other ones. However, it is not possible to pick "the best one", because each may excel in different conditions:
* a bleach that works well with one developer may perform badly with another,
* a bleach that works well with one AgX material may perform badly with another,
* some bleaches give you brighter hologram, but with more noise - and vice versa,
* some bleaches are better for transmission holograms, some are better for reflection holograms,
* some bleaches preserve the emulsion volume (and thus fringe spacing in reflection holograms), some do not
* some bleaches are faster, but aggressive processing may affect the hologram quality,
* some chemicals are expensive, difficult to get, hard to work with, poisonous, smelly, ...
* lifetime of some bleaches is very short, the others may survive for many years,
* holograms processed with some bleaches are sensitive to printout, ...
* and so on

It is always necessary to find what combination of hologram type + AgX material + developer + bleach works best for you. For example, I don't make holograms too often, so I prefer those that last for long time, do not smell badly and are not extremely poisonous.

If you are interested in the subject, definitely get "Silver-Halide Recording Materials" by Hans Bjelkhagen.

Petr

Din
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Re: A pdf of all known bleaches

Post by Din » Sun Jan 17, 2021 1:56 pm

The difference is the way in which bleaches work.

The point of the bleach is to convert an amplitude hologram to a phase hologram. The emulsion is a silver halide, which is clear. When you develop the hologram, the silver halide - a clear substance - to silver which is black, an amplitude hologram. This works by robbing some of the light used to reconstruct the hologram. So, an amplitude hologram is weaker, because less light goes through the hologram. If you simply develop (and, maybe, fix) the hologram, the hologram is dark. A phase hologram does not rob any of the light used for reconstruction, ideally. In practice, there is always some loss. A bleached (phase) hologram works because it changes the density, very locally, inside the emulsion - some parts of the emulsion are denser than other parts. The denser the part of the emulsion, the more the light is "delayed", the more some parts are 'delayed with respect to other parts, the more the light is altered from the 'pure' reconstruction beam to a structured, image forming beam. It is this loss and creation of density variations that determines which bleach to use, and the loss and density variations work with the developer. So, as Petr says, you have to match the bleach to the developer.

There are two basic differences in the way bleaches work - rehalogenating bleaches and reversal bleaches.

In a rehalogenating bleach, the developer converts the exposed silver halide grains in the emulsion to silver, which is black. Hence, the plate goes dark. In order to change the dark plate, which, remember, absorbs light, to a clear plate which does not absorb light, it's necessary to make the plate go back to it's unexposed state, when it was clear; but, you cannot take it all the way back to its 'virgin' state, because you must retain the image creating, black, parts of the emulsion. So, you have to alter the black, exposed, silver back to the original silver halide - but only where there was image forming black silver, and wash away all the unexposed parts which did not go black on development. So, you 'rehalogenate', ie convert the silver back to a silver halide.

In a reversal bleach, you dissolve away the exposed, black silver, leaving only the unexposed silver halide. This does not convert the exposed black silver into a clear silver halide, it removes the black silver. In this case, the image 'data' is not in the exposed parts of the emulsion, it's in the unexposed, 'opposite' parts of the emulsion. You reverse the image forming data into it's exact opposite.

After these two major categories, the next part of the choice rests with the type of developer you use. Some developers are very aggressive, and convert silver halide into silver very quickly. If you don't stop development at the right time, unexposed parts will also go black. In this case, you want a bleach that may act slowly, so you allow the bleach to convert the exposed silver, but stop bleaching as the bleach attacks the unexposed part which have also gone black. In other situations, depending on the developer, you may have underdeveloped. In this case you may use a bleach that corrects underdevelopment. So, you have to 'marry' the correct development scheme to the development scheme.

lobaz
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Re: A pdf of all known bleaches

Post by lobaz » Mon Jan 18, 2021 5:20 am

Thank you for the comment, Dinesh.
Are you aware of any rules of thumb when matching a developer and a bleach?

Din
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Re: A pdf of all known bleaches

Post by Din » Mon Jan 18, 2021 9:51 am

Some bleaches are faster than other bleaches. So, as a rule of thumb, I'd say that you'd need a fast bleach for a volume hologram, and you can use a slow bleach for a surface hologram. At American Banknote, we used D19 and the ferric bleach. For volume work, we generally used a high contrast developer and Van Ranesse's bleach, which was basically potassium dichromate and sulphuric acid. Back in the day, we used pyrogallol developers and Van Ranesse, but you can't use pyrogallol anymore. Today, or rather three years ago, I used CWC2 (JD-4?) and PBU.

lobaz
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Re: A pdf of all known bleaches

Post by lobaz » Tue Jan 19, 2021 5:27 am

Thanks, Dinesh. I mostly use Fe-EDTA or Ultimate. I mostly do reflection display holograms. I don't like PBUs due to their short shelf life.

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