Blyth Colour Tuning
The Blyth Colour Tuning method originated from an observation Jeff posted to the forum and its testing was taken up by a number of people. The results were very promising. The original thread is here:
- 15g Citric Acid
- 100ml Water
Dilute as needed. Soak the hologram to be swelled for 30 minutes. Then dry. Amazingly squeegee technique is not very important here!
I cut a 15 x 20 cm film hologram in 4 pieces and applied Jeff's Citric Acid solution method.
- (upper left): untreated (514 nm)
- (upper right) 11.75 % Citric Acid (=15% x 0.75)
- (down left) 7.5 % Citric Acid (=15% x 0.5)
- (down right) 15 % Citric Acid
The photo is not the best one (the hologram is free of any noise), but the effect of the various concentrations is obvious. In comparison to sorbitol-treated holograms, the overall quality is better.
- Exposure at 514 nm
- Holographic film: Finegrained HF-53 from ORWO
- This emulsion is much harder than Slavich material.
From Jeff Blyth
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 occurred 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 hologram 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 a long 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 observation about squeegeeing technique being less critical with citric acid solution compared to sorbitol or glycerol solution.
So as we all know, the 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 accommodating lots of water molecules which take on partial induced charges opposite to the ions they surround. So the amino positive ions get surrounded by a cloud of partially negatively charged water molecules and vice versa around the negatively charged acid groups . This allows the original 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, (this 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 a 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 in 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 (more 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. These 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 molecule is forced to be accommodated into the gelatin structure as 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 satisfied 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 one 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 unequal 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 somehow 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 an 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 was 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 could 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 not 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”.