Redshifting / Postswelling DCG

[a_k]

Hello everyone,

What techniques are you using to massively redshift DCG's? I know, with the proper processing it is possible to tune the reconstruction colour but what i mean is a huge colour shift from almost UV/violet/blue to deep red or at least 200nm. There was a discussion about the topic and also an article in the holowiki about postswelling with citric/succinic acid called "Blyth colour tuning" but they were discussing tuning of AgX material.

I've tried postswelling with citric acid but had no success. The main proublem was that at the high concentrations of the acid it was not possible to have the holograms in the bath for the required extended periods of time. The gelatin layer gets attacked and separates from the glass substrate.

Ahmet

[Joe Farina]

In 2007 there was a large amount of activity on this subject on the old forum. It was a good example a number of people working together to get some spectacular results, and to me it was the most interesting work done through the old forum. I'm a bit pressed for time at the moment, but I will begin to get my hard copies together. With a subject as important as this, I was very careful to make a complete set of hard copies. Tony did a careful series of tests which were successful in red-shifting (and color-tuning) DCG. This was based on an idea by Jeff Blyth, with contributions from a number of forum members. I will let you know when I get this together, I need to refresh my memory also.

[Tony]

I would be happy to help out as well.
Yes using CA does peel the gel off the plate. There is some trick in getting the right % of CA and water and getting a shift.
I think it might require a higher bloom strength gel as well.
That is a huge range (200nm) but should be possible.

Best of luck

Tony

[a_k]

Thanks Joe, Tony. I remember the thread you mention. It indeed is a good example of a succesful cooperation with many people joining the discusion.

I did not try succinic acid, citric acid was much easier to obtain. With low concentrations the red shift was only weak and at higher concentrations the stability of the coating suffered too much. Paulos' test hologram on holowiki looks very impressive, with a large colour shift across the whole visible spectrum, but i was not able to get any results even remotely comparable to it with DCG on glass plates.

[Martin]

With low concentrations the red shift was only weak and at higher concentrations the stability of the coating suffered too much. Paulos' test hologram on holowiki looks very impressive, with a large colour shift across the whole visible spectrum, but i was not able to get any results even remotely comparable to it with DCG on glass plates.

The way I remember that discussion was that Jeff pointed out that citric acid or succinic acid did have much more effect on formaldehyde (or alike) hardened gelatin layers than those crosslinked with Cr(III).

[Joe Farina]

Just going from memory, succinic acid worked quite well for Tony on DCG. But for big wavelength shifts, I think the problem (Tony please correct me if I'm wrong) was that if the concentration of succinic acid went up (to get a bigger shift), the gelatin had more of a tendency to lift off the plate. I asked Tony to try a particular type of silane to help promote adhesion, but that didn't work. In hindsight, I think I suggested the wrong type of silane to use for this purpose (it was a silane intended for use with Hxtal epoxy). I think it would be better to try the type of silane that Jeff Blyth recommended for his TMG DCG system. I still have some of this silane, and regret this wasn't the type I sent to Tony earlier.

[Martin]

I meanwhile dug out Jeff Blyth's post on Jul 01, 2007 (hopefully I won't infringe any copyrights :

I have been doing a bit more on this since I have receiving an appreciative email from Rob Taylor (Forth Dimension Holographics) about the newly found virtues of the citric acid post swell. system. In it he mentions how forgiving it is to the squeegeeing technique. I have noticed this too and have just been wiping off the excess citric acid solution casually with tissues and have not seen smeary streaks of darker red which would have occured with sorbitol solution. Now this convenient fact indicates something about what is happening down at the molecular level.

Also I just might possibly have discovered something of interest for DCGers to investigate as a means of changing those finished too-blue colored holograms into red ones - a trick which I think John Pecora has discussed more than once on this forum over recent years. However with only 2 days of observation I am being rather optimistic to think I have along term answer to that old perennial DCG problem but I hope that DcG'ers will now try some experiments with old "Bluies and Greenies" as John puts it before just recycling the glass I will go into a bit of DCG detail at the end of this post.

First though I need to hypothesize what is happening at the molecular level to try to understand the observations about seqegeeing technique being less critical with citric acid solution compared to sorbitol or glycerol solution.

So as we all know, building bricks of gelatin are amino acids. In neutral pH conditions these make themselves into internal acid-base structures with the negative - positive ions neutralising each other.
The swelling in water is caused by both the positive and negative ions choosing to open themselves up to accomodating lots of water molecules which take on partial induced charges opposite to the ions they surround. So the amino positive ions get surrounded by e cloud of partially negatively charged water molecules and vice verse around the negatively charged acid groups. This allows the orginal electrostatic attraction between the oppositely charged components of the amino acid to slacken and the components to move apart by a factor of 2 or 3 times their unswollen distance.

An accepted way of keeping gelatin based holograms swollen with water has been to try to replace a lot of the water with non-volatile very hydrophilic "polyalcohols" such as glycerol or sorbitol.

These alcohols get involved in the cloud of water molecules surrounding the charged amino acid groups. The size of this cloud of water molecules around the oppositely charged amino acids is very imprecise, variable and dynamic, (his description will be important), it instantly can change with temperature and humidity changes so it is difficult to control color changes of gelatin based reflection holograms. (They act as superb humidity change sensors - a fact I am personally gaining from in the development of "Smart" holograms to test for water in aviation fuel)... Just breathing on them can make a wavelength change of tens of nanometers as we all know.

However in the case of hologram treated with citric acid and then blow dried at room temperature we are left with a swollen gelatin which is different from the case of one swollen with water plus sorbitol or glycerol. In citric acid we have in effect the line of 3 carbons ins glycerol now with their alcohol groups (-OH) replaced by carboxylic acid groups (-COOH) except for the central carbon which has the (-COOH) added in place of H leaving one alcohol OH still there (mor on this later). These -COOH groups introduce a different effect to cause the swelling of the gelatin. This time the citric acid (-COOH) groups can partially displace the original internal (-COOH) groups from their attraction to the amino groups. Thse displaced (-COOH) groups are still firmly attached to the gelatin biopolymer of course and are not free to wander off in solution so the rest of the citric acid molecules is forced to be accomodated into the gelatin structureas most of the surrounding water is now evaporated off thus leaving the gelatin in a swollen state when it is left to equilibrate with ambient humidity.-- Fortunately it is a chemically weak arrangement easily completely reversed by plenty of fresh water so that it becomes energetically more favourable for all the ions involved to go back to surrounding themselves with water-molecule clouds again. The upshot of this is that if you are not satisified with the color of your treated hologram you can go back to square one without any difficulty. I have not found the slightest trace of the effect of citric acid after rewashing in water. --An important feature for any precious holograms whose color you are trying to tweak.

In the above model on can sense why the removal of excess surface liquid on a hologram treated with citric acid solution is more forgiving (in the final result) from an uneqal treatment with a squeegee blade compared to the same situation with an excess of sorbitol solution. In the case of excess sorbitol that final sheath or cloud of water/sorbitol molecules which I described above as "very imprecise, variable and dynamic", as they surround the amino acid ions they can be far too sensitive to small variations in residual water causing corresponding local variations in reddening of the final replay color as the clouds expand or contract. Whereas in the case of the citric acid, the reddening is caused mainly by a specific alteration of the internal molecular structures of the amino acids and perhaps not much by a variable cloud around the ion..

Now experienced DCGers have long since found that you cannot change the color of a finished too-blue DCG by playing around with sorbitol treatment. Anything that attracts water is anathema to DCG holograms. So the question naturally arises can one somwhow do it with this different citric acid mechanism?

Well I took a blue green finished DCG, left it in 10% citric acid for 10 mins, (I cut the time down from my previous 30 min recommendation because I noticed the gelatin was starting to come off the glass after 5 mins) I then briefly wiped it with a tissue and plunged it into a stirred beaker of ~100% ipa at room temperature. (I needed to avoid using ipa/water solution as it was likely to loose citric acid. The acid fortunately seemed to prefer to keep its weak attraction to the gelatin rather than dissolve in ipa. only). The good news is that the resulting hologram after a long cool blow was a deep red hologram instead of a blue green one. But the bad news is that that only 3 hours later it had vanished.. But ...don't go away yet...

I tried a variant.... The problem of course likely to be too much water attracted in still--the water cloud around ions was probably still there to some extent which caused the air-void fringe structure to be unstable and disappear. So an improvement be got by using an alternative organic acid without a residual alcohol -OH group still present?

So I tried succinic acid instead. This is a non-poisonous but quite strong organic acid (a "natural" product too) with the 2 alcohol groups in ethylene glycol replaced by -COOH groups.
I found that it saturated at room temperature at around the 6% level but treating green silver halide holos with it did make useful color shifts to yellow (in the case of BB plates but in the case of the harder Fuji film) it was though much less effective at causing the amount of color shift you can get from the same concentration of citric acid. The question is then is this less hydrophilic acid able to keep a color shift in DCG? So far my test sample is still maintaining its green to orange shift after 36 hours but I would not put any money on its permanence. So I am hoping some DCGer will pick the idea up, get in a bit of succinic acid and play around with sealing it up etc. --it could be an interesting alternative to recycling those "Bluies".
Jeff
YOU CAN'T HURRY HOLOGRAPHY!

And this one on Fri Jun 06, 2008:

(...)
I have had an after-thought to add as a postscript to this long thread..
Following the theory I put forward on page 2, then we need to consider how the gelatin was hardened in addition to just how much the gelatin was hardened prior to the final citric acid (or succinic acid) treatment.

The theory is that the relatively strong organic acids citric or succinic are both able to displace many of the acid groups in many of the amino acids that make up the gelatin film. Therefore the citric or succinic acid has to be accomodated into the structure of the gelatin at the same time as the displaced acid groups from the amino-acids. (These dsplaced acids groups are of course permanentely bonded to the gelatin matrix and are not free to wander off into solution).
Now with regard to changing the final colour of silver halide gratings Ron Olsen reported the big differences in different brands of recording material.
Now if the original hardening was all due to aldehyde hardening e.g. like formaldehyde or gluteraldehyde and also from your own degree of hardening from the developer** you used [**see footnote] it is the amino groups of the gelatin that are primarily getting crosslinked and not the accompanying acid groups. This means that the citric or succinic acid swelling is going to be more effective if the hardening had been done only via the amine groups because that would leave the carboxylic acid groups free to be displaceable. Proprietary plates from different sources can have different types of hardener, some can have both amino and carboxylic groups crosslinking.

In the case of DCG however we have predominantly the carboxylic groups involves to that means I think that the effective swelling in terms of nm of shift for a given amount of succinic acid is going to be lower than in the typical silver halide material.

**Footnote on developers.-- Hydroquinone, pyrogallol but NOT ascorbate developers have -OH or alcohol groups attached to a benzene ring which makes them want to reduce the AgBr to silver metal and get oxidized to aldehyde in the process. These aldehydes then become crosslinkers of the gelatin amino groups.
Jeff
YOU CAN'T HURRY HOLOGRAPHY

[a_k]

Thanks a lot Martin. I don't assume that Jeff would mind to be cited in this context.

The bottom line of Jeff's detailed explanations seems to be that:
- Citric acid allows a large colour shift but the effect disappears after a certain amount of time
- The colour shift achieved with succinic acid is less dramatic but remains (water solubility is only around 6%)
- With DCG a smaller amount of red shift is to be expected because the carboxylic groups are already used for crosslinking.

It looks like this method would not produce a large red shift with DCG. Maybe the succinic acid could be replaced with another organic acid, with similar properties but with a higher water solubility? Any ideas?

[Tony]

Jeff's explination is a bit wordy for me to get my arms around how CA and SA color shift.

Does it shift by loosening the gelatin matrix (the springs) and therefore expanding the fringes?
Or is it somehow an agent to allow some water into the fringes?

Glad to see this as a discussion, it is very interesting.

[Joe Farina]

My guess is that something is happening with the gelatin molecules.

I think you have the paper by Courts & Little, Tony, called "Citrate-Promoted Helix Formation in Gelatin." The first sentence states: "The modification of the shape of gelatin molecules in dilute solution brought about by citrate and certain other buffers containing aliphatic carboxylic acids is a general reaction for gelatins."