Dichromated gelatin is a light-sensitive material made from gelatin (including ordinary food gelatin such as Knox) and a small amount of either ammonia dichromate or potassium dichromate.
The basic formula for dichromated gelatin is water plus gelatin plus either ammonium or potassium dichromate. The amounts of each ingredient influence the characteristics of the result. Exposure energy requirements, color shift, emulsion thickness, etc., are all impacted by the formulation.
It is convenient, then, to have a standard for formula reference. Richard Rallison promoted using a system of three numbers to describe a formula—grams of dichromate, grams of gelatin, and grams (milliliters) of water. For example, 8-30-250 would be the notation for a recipe consisting of 8 grams ammonium or potassium dichromate, 30 grams of gelatin, and 250 grams of water.
To make comparisons among formulae, the gelatin number is always 30 in Rallison’s notation. The three numbers can be scaled equally up or down for producing different quantities of emulsion. (Personally, I usually scale the numbers to 7.1 grams of gelatin, 7.1 grams being the mass of gelatin in quarter-ounce packet of Knox brand gelatin.)
Thickness and Bandwidth
The ratio of gelatin to water affects the viscosity of the emulsion, and that in turn affects the typical thickness of emulsion on the glass plate. The thickness influences the bandwidth of the final hologram. Rallison reported the following results for emulsions applied by 80 RPM spin coating method:
|xx-30-350||5 – 6 µm||50 – 150 nm|
|xx-30-250||8 –9 µm||10 – 50 nm|
|xx-30-200||10 – 12 µm||10 – 50 nm|
|xx-30-150||20 – 24 µm||~8 nm|
Replay Color Shift
The ratio of dichromate to gelatin influences the color shift. The following table has typical values for exposures taken at 514 nm:
My personal guess at typical exposure requirements for the basic recipe 8-30-300 emulsions.
|405 nm||5 mJ/cm2|
|442 nm||15 mJ/cm2|
|475 nm||40 mJ/cm2|
|488 nm||60 mJ/cm2|
|514 nm||125 mJ/cm2|
|532 nm||200 mJ/cm2|
A great many factors may have a dramatic effect on sensitivity, notably humidity and temperature, so the above table is only a point of reference. Sensitivity also varies inversely with the dichromate concentration—halving the amount of dichromate would double the exposure requirement, for example.
Weird DCG Recipes
3.6–30–467, 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, 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 ml 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. [In Rallison’s Thick DCG paper, he associated ammonium nitrate with hardening.]
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).
- 100 g water
- 6 g gelatin
- 0.9 g ammonium dichromate
- 2 ml ammonia 35% (added last)
Household ammonia is typically 5-10%. [Presumably, the ammonia inhibits the dark reaction.]