Unusual Recipes

[jsfisher]

I've been scouring my library's holdings for journal articles regarding DCG. I had been looking for just some general rules about sensitivity vs wavelength vs thickness vs color shift vs whatever. In the process, I've come across recipes that are not quite the standard fare most talk about here.

What what its worth, the following are some of the more interesting I've found so far. The addition of ammonium nitrate is alleged to improve sensitivity, but I'm not sure what the other two are supposed to accomplish. Has anyone tried any of these? I've just begun playing with adding ammonium nitrate to KDi. Initial results are encouraging, but I have to work through many of my own DCG process issues.

3.6-30-467 plus Chromium Acetate, Ethanol
Markova, Nazarova, and Sharlandjlev, “Control of the Spectral Position of DCG Reflection Holograms,” Institute of Optical Materials and Technology.

• 64.3 g gelatin, Bloom strength of 210
• 7.71 g ammonium dichromate
• 0.64 g chromium acetate
• 65 ml C2H5OH (ethanol)
• Distilled water to make 1000 ml

Plates are coated with the solution at 50°C by doctor-blade method to 20 µm.

5-30-200 plus Ammonium Nitrate
Bahuguna, Beaulieu, and Arteaga, “Reflection display holograms on dichromated gelatin,” Applied Optics, volume 31, issue 29 (1992).

• 2.5 g of ammonium dichromate
• 1.5 g of ammonium nitrate
• 100 cc of distilled water
• Heated to 70°C
• 15 gm of USP grade Baker's gelatin (125 bloom strength) powder slowly added while stirring

Spin-coat at 100 rpm the still ~70°C emulsion for 90 seconds under hot-air gun. Dry vertically in a dark box. Plates are ready after about 6 hours.

Sensitivity was reported as 100 mJ/cm2 at 488 nm.

4.5-30-500 plus Ammonia
Coblijn, Alexander B., "Theoretical background and practical processing techniques for art and technical work in dichromated gelatin holography", SPIE Institute Series Vol. IS 8 (1990).

• 100g water
• 6g gelatin
• 0.9g ammonium dichromate
• 2ml ammonia 35% (added last)

[Note: Household ammonia is typically 5-10%.]

[a_k]

The ammonia in the third recipe serves to increase the pH of the solution. In an alkaline environment the dichromate<->chromate equilibrium is moved to the right, thus increasing the chromate:dichromate ratio. When the plates are dried after the coating, most of the ammonia evaporates and the equilibrium is shifted back towards dichromate. I would love to know the purpose of adding the ammonia. Maybe the result are plates sensitised with a mixture of chromate/dichromate.

[Dinesh]

I would love to know the purpose of adding the ammonia.

It's to inhibit dark reaction.

[Martin]

In an alkaline environment the dichromate<->chromate equilibrium is moved to the right, thus increasing the chromate:dichromate ratio. When the plates are dried after the coating, most of the ammonia evaporates and the equilibrium is shifted back towards dichromate.

I am not sure but I would have expected that once the dichromate had been converted to chromate, it remained stable - unless put in a strongly acidic environment. Some time ago we did discuss that latter aspect here on the forum.

[Martin]

The addition of ammonium nitrate is alleged to improve sensitivity, but I'm not sure what the other two are supposed to accomplish. Has anyone tried any of these?[/i]

I tried the addition of potassium nitrate. It certainly accelerated speed, though at the expense of greatly increased scatter. It obviously had a strong softening effect on the (food grade) gelatin I had used.

[Martin]

I would love to know the purpose of adding the ammonia.

It's to inhibit dark reaction.

That's certainly true since chromate is said to be much more stable than dichromate. By the way, I'm not aware about extensive research on the blue sensitivity of potassium chromate sensitized gelatin. At least for 405nm exposures the difference in speed between ammonium dichromate and potassium chromate sensitized layers didn't seem that large. I don't know how this would play out at >450nm though.

[Dinesh]

At least for 405nm exposures the difference in speed between ammonium dichromate and potassium chromate sensitized layers didn't seem that large. I don't know how this would play out at >450nm though.

Even at >405, the difference is really not that great. Since exposure to dcg is much more an "analogue" process, ie continuous rather than grain by grain as in AgX, there is no actual exposure figure for dcg. So the changes in exposure due to small changes in ratio swallows up the small differences in speed between potassium and ammonia.