by Din » Wed Jan 15, 2025 1:16 pm
Joe Farina wrote: ↑Wed Jan 15, 2025 9:01 am
If the objective is to harden the surface of the gelatin to limit differential swelling later on, I would think that the amount of time the gelatin is in the fixer would be quite important.
Actually, thinking further, I might not have expressed it properly. Bearing in mind that the hardener stiffens the gelatin, while not reducing the dichromate, the idea is not to just harden the surface, but to harden the entire volume of the gelatin, with the upper parts hardening more of the volume. In other words, the hardening should be a function of the penetration of the hardener, decreasing as you move into the depth of the gelatin. In still other words, the surface has to harden more than the body, but the body itself has to harden to some extent. The idea is that as the water enters the emulsion, the upper parts expand less than they would normally have done, while the lower parts have not yet begun to swell. So, the water-generated expansion in the water bath is inversely proportional to the depth. This should mitigate the differential swelling.
But, as you say, the hardener is mostly water, so you have to balance the swelling of the emulsion during the hardening stage against the hardening itself - a delicate balance, as I mentioned all of DCG holography is.
Joe Farina wrote: ↑Wed Jan 15, 2025 9:01 am
As for the aluminum sulfate, I assume it's just a hardener of gelatin, and not a reducer for the dichromate.
Yes, I think so. I've seen aluminium sulphate as a hardener in some photographic sites.
Joe Farina wrote: ↑Wed Jan 15, 2025 9:01 am
Lin, in his 1968 paper in Applied Optics, suggested the use of a 2% sodium bisulfate solution as an alternative to Kodak fixer. I assume that these sulfites/sulfates cause hardening by reducing dichromate. I haven't tried the sodium bisulfate yet.
I have never tried to harden by chemically reducing the dichromate. But, had I known of the Lin paper, I would not have tried it. The reason is that the chemical reduction of the dichromate has to be uniform throughout the emulsion. However, the reduction of the dichromate depends on the local density of the dichromate. If the density of the dichromate is not uniform, then some parts of the emulsion will harden more than other parts, leading to "greenies".
Joe Farina wrote: ↑Wed Jan 15, 2025 9:01 am
Perhaps it will be feasible to use a solvent with hardener in a fix which doesn't easily penetrate gelatin, but which still hardens the surface of the gelatin.
In 1986, I was researching into "laser protection goggles". These are goggles which are highly transparent - there's a figure of merit for transparency known as the photopic efficiency, and these goggles had to have a very high photopic efficiency - in normal light, but have a high rejection for laser wavelengths. The problem was that when you examined the Bragg plane structure under a spectrometer, you saw the main peak, the planes that are tuned to the laser rejection wavelength, with subsidiary minor peaks on either side of the main peak; these subsidiary peaks were killing the photopic efficiency. This was a problem that needed solving, but all the researchers before me had not solved. It occurred to me that the cause of the subsidiary peaks were due to variations in the Bragg plane spacing, caused by differential swelling, which the fixer had not not completely removed. So, it occurred to me that if I zapped the entire emulsion with uv, there would be partial hardening, since dichromate has a high absorption for uv. Of course, too much uv would make the entire emulsion resistant to water swelling and give a low efficiency to laser wavelengths, while too little would still give subsidiary peaks, but smaller and less pronounced. Anyway, after much trial-and-error, I found the right combination of uv wavelength and exposure energy to keep the main peak unaffected but get rid of the subsidiary peaks. I finally got a very high photopic efficiency with a very high rejection of laser wavelengths.
The point of all this is in 2012, I wondered if I could use the same technique - pre-exposure of the emulsion by zapping with uv - instead of fixer. I had some limited success, but I didn't have the right uv source; the company where I researched the laser protection goggles, National Technical Systems had a much higher budget for research than I did.
[quote="Joe Farina" post_id=73412 time=1736949697 user_id=2184]
If the objective is to harden the surface of the gelatin to limit differential swelling later on, I would think that the amount of time the gelatin is in the fixer would be quite important.
[/quote]
Actually, thinking further, I might not have expressed it properly. Bearing in mind that the hardener stiffens the gelatin, while not reducing the dichromate, the idea is not to just harden the surface, but to harden the entire volume of the gelatin, with the upper parts hardening more of the volume. In other words, the hardening should be a function of the penetration of the hardener, decreasing as you move into the depth of the gelatin. In still other words, the surface has to harden more than the body, but the body itself has to harden to some extent. The idea is that as the water enters the emulsion, the upper parts expand less than they would normally have done, while the lower parts have not yet begun to swell. So, the water-generated expansion in the water bath is inversely proportional to the depth. This should mitigate the differential swelling.
But, as you say, the hardener is mostly water, so you have to balance the swelling of the emulsion during the hardening stage against the hardening itself - a delicate balance, as I mentioned all of DCG holography is.
[quote="Joe Farina" post_id=73412 time=1736949697 user_id=2184]
As for the aluminum sulfate, I assume it's just a hardener of gelatin, and not a reducer for the dichromate.
[/quote]
Yes, I think so. I've seen aluminium sulphate as a hardener in some photographic sites.
[quote="Joe Farina" post_id=73412 time=1736949697 user_id=2184]
Lin, in his 1968 paper in Applied Optics, suggested the use of a 2% sodium bisulfate solution as an alternative to Kodak fixer. I assume that these sulfites/sulfates cause hardening by reducing dichromate. I haven't tried the sodium bisulfate yet.
[/quote]
I have never tried to harden by chemically reducing the dichromate. But, had I known of the Lin paper, I would not have tried it. The reason is that the chemical reduction of the dichromate has to be uniform throughout the emulsion. However, the reduction of the dichromate depends on the local density of the dichromate. If the density of the dichromate is not uniform, then some parts of the emulsion will harden more than other parts, leading to "greenies".
[quote="Joe Farina" post_id=73412 time=1736949697 user_id=2184]
Perhaps it will be feasible to use a solvent with hardener in a fix which doesn't easily penetrate gelatin, but which still hardens the surface of the gelatin.
[/quote]
In 1986, I was researching into "laser protection goggles". These are goggles which are highly transparent - there's a figure of merit for transparency known as the photopic efficiency, and these goggles had to have a very high photopic efficiency - in normal light, but have a high rejection for laser wavelengths. The problem was that when you examined the Bragg plane structure under a spectrometer, you saw the main peak, the planes that are tuned to the laser rejection wavelength, with subsidiary minor peaks on either side of the main peak; these subsidiary peaks were killing the photopic efficiency. This was a problem that needed solving, but all the researchers before me had not solved. It occurred to me that the cause of the subsidiary peaks were due to variations in the Bragg plane spacing, caused by differential swelling, which the fixer had not not completely removed. So, it occurred to me that if I zapped the entire emulsion with uv, there would be partial hardening, since dichromate has a high absorption for uv. Of course, too much uv would make the entire emulsion resistant to water swelling and give a low efficiency to laser wavelengths, while too little would still give subsidiary peaks, but smaller and less pronounced. Anyway, after much trial-and-error, I found the right combination of uv wavelength and exposure energy to keep the main peak unaffected but get rid of the subsidiary peaks. I finally got a very high photopic efficiency with a very high rejection of laser wavelengths.
The point of all this is in 2012, I wondered if I could use the same technique - pre-exposure of the emulsion by zapping with uv - instead of fixer. I had some limited success, but I didn't have the right uv source; the company where I researched the laser protection goggles, National Technical Systems had a much higher budget for research than I did.