ninhydrin and DCG

[Mihai]

I understand that it is possible to chemically "seal" a DCG hologram with ninhydrin. There are only two posts here that mention ninhydrin. I haven't found anything useful searching google either. So, how is this done? I assume that in the final IPA bath should be added some ninhydrin, but how much? will this work?

[Joe Farina]

I understand that it is possible to chemically "seal" a DCG hologram with ninhydrin....will this work?

I don't want to discourage anyone from trying it, but I would personally just stick with the "traditional" method of sealing DCG with a cover plate. But who knows, maybe the ninhydrin will be useful.

Apparently the idea first appeared in Applied Optics, Vol. 37 No. 2, January 10 1998: "Methylene blue dichromated-gelatin holograms: antihumidity methodfor taking off strongly adsorbing humidity groups" by Wang, Zhu, et al.

For the procedure quoted below, I'm assuming they have already produced a finished DCG hologram (exposed and processed) because they call it a "grating" when describing the rest of the process. But I'm not sure.

From page 327:

"Experiments on the modification of the MBDCG holograms were carried out with the following procedure: (A) A MBDCG grating was soaked at 80C for 40 minutes in 5% ninhydrin/ethanol, which produced a dark purple color. (B) The plates were washed in running tap water for 10 minutes until the purple color disappeared. (C) The plates were then dehydrated in 90% isopropyl alcohol for 3 minutes. (D) Next, they were dehydrated in 100% isopropyl alcohol for 3 minutes. (E) Finally, the holograms were dried with hot flowing air."

[Dinesh]

From page 327:

"Experiments on the modification of the MBDCG holograms were carried out with the following procedure: (A) A MBDCG grating was soaked at 80C for 40 minutes in 5% ninhydrin/ethanol, which produced a dark purple color. (B) The plates were washed in running tap water for 10 minutes until the purple color disappeared. (C) The plates were then dehydrated in 90% isopropyl alcohol for 3 minutes. (D) Next, they were dehydrated in 100% isopropyl alcohol for 3 minutes. (E) Finally, the holograms were dried with hot flowing air."

If this is all there is, then it sounds unlikely that ninhydrin was used as a sealant. If the plates were processed in the usual manner (water and IPA) after the ninhydrin stage, then the ninhydrin could not seal the hologram. If it did, then the hologram could not be further processed.

For the procedure quoted below, I'm assuming they have already produced a finished DCG hologram (exposed and processed) because they call it a "grating" when describing the rest of the process. But I'm not sure.

The reason they call it a "grating" is that the research workers generally don't make a display hologram since there are too many unconrolled variables. They start off by making a grating so that all the variables can be controlled.

[Joe Farina]

I noticed the word "grating" and assumed it meant a finished DCG hologram (i.e., a holographic grating or a hologram of an object). I do know that researchers often make gratings for testing purposes, since they are easy. When reading their description, I wasn't sure exactly what they were putting into the ninhydrin (an exposed and/or processed DCG layer?) But after a closer look at the paper, it seems they were indeed using a finished grating to begin with, before the ninhydrin.

They claim it modifies the gelatin molecules. In the abstract it says that the ninhydrin-treated gratings are stable in high humidity conditions, which they define as 80 to 85% relative humidity. That doesn't sound very impressive to me. It implies that anything above 85% is still unstable, which is definitely not good enough.

[Dinesh]

Oh

I noticed the word "grating" and assumed it meant a finished DCG hologram (i.e., a holographic grating or a hologram of an object). I do know that researchers often make gratings for testing purposes, since they are easy. When reading their description, I wasn't sure exactly what they were putting into the ninhydrin (an exposed and/or processed DCG layer?) But after a closer look at the paper, it seems they were indeed using a finished grating to begin with, before the ninhydrin.

Oh, I see. On reading your post and the quote from the paper, I see that the inference is that they've already recorded the hologram before insertion into the ninhydrin. From your quote alone, it's unclear whether they recorded and processed the hologram to completion before this treatment with ninhydrin/PVA, ie was the MBDCG grating that was inserted in ninhydrin a latent image or did they develop it first.

They claim it modifies the gelatin molecules. In the abstract it says that the ninhydrin-treated gratings are stable in high humidity conditions, which they define as 80 to 85% relative humidity. That doesn't sound very impressive to me. It implies that anything above 85% is still unstable, which is definitely not good enough.

I suppose it depends on where you are and what you intend to do with them. In our lab in San Diego, the humidity rarely rises above 50%, usually it's in the high 30's. In this situation, we rarely seal the plates while we're testing them. Once we have the appropriate test results, we do seal them and test them again to ensure that nothing has changed, before we ship them out. The humidity around here is low enough that unsealed holograms survive for quite a long time, even a year or more. Having said that, when we took sealed holograms to India where humidity is upwards of 80 to 90 and temperatures are in the high 80's F (30's C), they began to disappear at the edges. That problem has been solved by new sealing methods, but unsealed holograms in such conditions would not last more than minutes.

Without the paper, I can't say, but how do they justify this claim of 80 to 85% for stability? One reason I ask is that I've done a series of tests for the emulsion that Martin talked about, the FAO. I notice that despite rising humidity (we've had a lot of rain!), there seems to be no degradation of the image, while corresponding dcg holograms made at roughly the same time seem to be disappearing . This got me to wonder why dcg holograms decay so fast in humidity. The reason often quoted is that water seeps into the gelatin which then swells, being softer than commercial AgX gelatin. However, much is made of the fact that Slavich emulsions are soft and yet they survive without any change in lambda. You'd think that if the "soft emulsion" theory is correct, then there should be at least a little shift in wavelength proportional to the softness of the gelatin and local humidity conditions. If however, it's an alteration of some protein, as the workers with ninydrin claim, then the structure of the chromic-gelatin bond may be more hydrophyllic. Martin's FAO may be less hydrophyllic by virtue of not having any chromic-gelatin bond. In other words, it could be that dcg is more prone to disappear in humid conditions because of the polarisation of the chromic-gelatin bond. Just speculation, of course.

[Joe Farina]

Dinesh, if you would like a copy of the paper, please send me a PM with an email address, and I will send a pdf copy.

[Mihai]

Thank you for your answers. For now, I'll stick to the traditional way of sealing DCG. With ninhydrin, it seems to be too much of a hassle, and i think, more expensive.