Ph and warm water

[Martin]

Martin, what do you mean with the 'assumption of a "polymerization of gelatin"', is it the terminology you don't like?

I've no problem with the term "polymerizing" gelatin. I simply think it's already "polymerized" when we use it for DCG, AgX etc. With DCG what we're doing by exposing a DCG layer to our lasers mainly is crosslinking gelatin (or at least parts of it).

I think that the loss of material is mainly due to the limited solubility of the gelatin before it's "melting point" is reached.

What I had in mind was the possibility of the dichromate having both a hardening (crosslinking) and softening ("de-crosslinking") effect.

Having had a quick look into my collection of DCG/SHSG papers, actually there's an interesting paper - Phillips, Rallison, Barnett, Schicker, Coleman, Dichromated gelatin - some heretical comments, SPIE 1914.
"RR", Richard Rallison, seems to have made the following observation:

When D.C.G. layers are processed, then a gradual build-up of deposited gelatin is observed in the processing dishes or vessels.
This observation been confiremd by one of us (RR) over many years of work in the field of D.C.G. processing. One has to ask why does this situation arise - is it relevant to the underlying mechanism in the formation of the D.C.G. image?

[Dinesh]

Actually, the phrase "polymerising of the gelatin" is a little bit of fuzzy thinking, the way I use the word "polymerisation". I think of the process of cross linking as a sort of polymerisation, even though it's strictly not polymerising. (See, I told you Chemistry was not my strong point!)

What I had in mind was the possibility of the dichromate having ... softening ("de-crosslinking") effect.

Martin, can you elaborate. I've never heard of the Cr "softening" the gelatin. The production of the dcg emulsion must crosslink some of the gelatin. Are you saying that some of this cross linked gelatin is uncrosslinked by exposure?

We average about 10 dcg plates a week or some 45 plates/month for both display and technical optical functions. I've never seen any deposit of gelatin in the processing trays. The problem with that "heresy" paper is that there don't seem to be any objective testing of whatever they found in the processing trays and no real description of the amount and kind of gelatin used for the emulsion. Thus, I get the idea that they may have found some sort of slimy deposit and assumed it was gelatin. It could also be that their gelatin layer is thicker than ours or of a lower bloom strength. Also, I use running tap water to process the gelatin but if you use still water for protracted periods of time, then it's possible that some of the surface gelatin would end up in the water. I suppose true falsifiability would require that two plates, one dcg and on AgX were both immersed in water in two separate trays at the same temp and time.Then the whatever ended up in the either or both these trays could be tested. If both trays have a slimy deposit, then the effect is independent of any specific dcg connection.

Maybe that would be "Control of the reconstruction wavelength of Lippmann holograms recorded in dichromated gelatin," Toshihiro Kubota, Applied Optics, Vol. 28, No. 10 (15 May 1989), pages 1845-9. But it seems to deal more with shifting towards blue than controlling bandwidth.

Thanks for the reference. You may well be right that it was more for blue shifting than bandwidth control. I haven't seen that paper for twenty odd years, but at the time it did make an impression on me because I was trying to figure out the physical mechanism behind the effect. Actually Kubota was sitting next to me at an SPIE social function. I told him I admired his work and he asked for particulars. Like an idiot, I mentioned his AgX/dcg colour Japanese doll, at which he rolled his eyes. Obviously he's always reminded of that doll and he's tired of it! Afterwords I kicked myself at not remembering the "control" paper and asking about that!

[holorefugee]

Hello Tony,

The gelatin molecule is a so called zwitterion. Around its isoelectric point (type A: pH 7-9 / type B:4.7-5.4) the net electrical charge is zero and the swellling is minimal.

With increasing deviation from the gelatins isoelectric point the swelling as well as its solubility/washing out are increased.

So, if you want to use high temperatures, adjust the baths pH to be close to the used gelatins isoelectric point.

Ahmet

Thank you Ahemt, That helps.

[Martin]

Martin, can you elaborate. I've never heard of the Cr "softening" the gelatin. The production of the dcg emulsion must crosslink some of the gelatin. Are you saying that some of this cross linked gelatin is uncrosslinked by exposure?

I can't say much about it. I sometimes noticed bits of gelatin in the dried trays I had used for DCG processing. I've no idea whether this relates to a specific type of gelatin (in my case it's been with culinary, acid treated gelatin). I was simply wondering if anyone else ever noticed this. Then, luckily, I found that paper with a reference to Rallison. I concluded that chances were high that the observation I had made was probably not just mere fantasy.
However, I'd never venture to say the softening (hydrolizing) of dichromate was overriding its crosslinking effect.

I don't know if there is any ongoing research on DCG - havent' seen/heard much from the Canadian group lately. Hopefully they haven't become a victim of the overall austerity frenzy.

We average about 10 dcg plates a week or some 45 plates/month for both display and technical optical functions. I've never seen any deposit of gelatin in the processing trays.

Interesting.

The problem with that "heresy" paper is that there don't seem to be any objective testing of whatever they found in the processing trays and no real description of the amount and kind of gelatin used for the emulsion.

Yes, that's true.

Thus, I get the idea that they may have found some sort of slimy deposit and assumed it was gelatin. It could also be that their gelatin layer is thicker than ours or of a lower bloom strength.

The way I read that paragraph was that at least for Rallison gelatin deposits in the trays wasn't something extraordinary - it seemed to occur regularly.

Also, I use running tap water to process the gelatin but if you use still water for protracted periods of time, then it's possible that some of the surface gelatin would end up in the water. I suppose true falsifiability would require that two plates, one dcg and on AgX were both immersed in water in two separate trays at the same temp and time.Then the whatever ended up in the either or both these trays could be tested. If both trays have a slimy deposit, then the effect is independent of any specific dcg connection.

Yes. On the other hand, I'm not sure if an AgX layer would be a good example. A gelatin layer ("armored") with zillions of AgX grains may actually behave very differently from a DCG layer.

[Tony]

This has been a fascinating thread and really learned a lot.

But I am still a little unsure how this is used in a practical sense.
As a weekend warrior what how does pH effect me?

Should as a matter of coarse always strive to match my Ph with my gelatin?
If I am looking at narrowband vs. broadband outcomes should I look at increasing or decreasing the Ph of my bath?
Is there a finished hologram that looks dim, shifted in color or some other effect in which one would say, "this is due to a Ph issue"?

My gelatin has a ISOELECTRIC POINT between 4.7-5.1. DI water has a pH of around 7 and my tap water is around 6.7. Should I lower it?

The other night I tried a simple test. Running two holograms with the same parameters except one with warm tap water the other cool (2 minutes each no agitation). I really could not see much in the final result.

As a side note, I do get a small amount of slime in my water bath after 4-5 plates.

[a_k]

If it is DCG you are talking about it is quite surprising for me that swelling in cold or warm water should have no effect on the outcome. You must be using some very hard plates.

With a water bath having a pH of 5 i would expect a more narrow band result with your gelatin than with a pH of 9.

[Dinesh]

Is there a finished hologram that looks dim, shifted in color or some other effect in which one would say, "this is due to a Ph issue"?

Not as far as I know. I can't think of something in the image which I could definitely say, "This was caused by pH".

On the other hand, it's a fairly complex issue. Any effect arising from pH would also involve the water temperature, the bloom strength of your gelatin, the temp difference between the water and the alcohols, the type and insertion time of your alcohols etc etc etc. The final image is a mashing together of all of these variables and it's almost impossible to distinguish one particular parameter and say THAT, caused THIS. It's impossible to see, for example, an orange final image and be able to say, "Had the pH been different, this image would have been green". The above discussions can give you an idea of what changes to expect as you change the pH, but it's very difficult to isolate a small change in the face of large changes, especially since the small change also effects the big change. For instance, if you were to change the pH of your first water bath, then, unless you absolutely dried the plate inside and out, when you immerse the plate in the first (usually dilute) alcohol, you carry the pH of the water into the alcohol bath. This may affect the ability of the alcohol to dehydrate and the change in water pH has now a secondary, knock-on effect.

If the image has significant faults, such as greenies, or a white haze across it, then I can usually figure out what went wrong.

Generally, as a weekend warrior, I'd suggest just carry on doing what you're doing if it produces results. If you're not happy with the image for some specific reason (besides "it should be brighter"!), then change one thing at a time and notice the result. I've always maintained that dcg holography is like juggling about 10 magnetic balls at once, each ball being a different size and magnetically attracting the other balls as they fly around you.

[Dinesh]

Regarding the gelatin in the tray effect , I've just spoken to Don Broadbent, who some of you may know. Don is the Jedi Master of dcg holography, as far as 'm concerned! Anyway, he says that he does see gelatin at the bottom of the water tank when he's cleaning it out. But, since I use running water, we figured that I don't see it because my water tray is being constantly "flushed".

[holorefugee]

If it is DCG you are talking about it is quite surprising for me that swelling in cold or warm water should have no effect on the outcome. You must be using some very hard plates.

With a water bath having a pH of 5 i would expect a more narrow band result with your gelatin than with a pH of 9.

I am confused, what buffers would shift the water pH?

[a_k]

An acid would be used to decrease the pH, a base to increase it.