CHEMISTRY of PHOTOGRAPHY

@CSU Stanislaus

CHEM 2502 Chemistry of Photography Laboratory

Section 5 -- A Project

The purpose of this final experiment is to apply some of the principles we have talked about in lecture and laboratory to create an experiment of your own design. We have listed below several possible systems which can be used for designing experiments. You may use any of these or propose alternates. Each group must discuss their project with the instructor well before starting in order to carefully define your experiment. This will also allow time to get any necessary materials. The purpose, procedure, results and conclusion should be recorded in your notebook just as you did in your previous experiments. In your conclusion discuss the relevant chemistry you have explored and answer the question posed in the purpose. Turn in any additional materials from your experiment such as prints or pinhole cameras, etc., along with your notebooks.
If you do not know what you want to try or if you want more information about any of the suggested projects, discuss it with your instructor. Remember, before starting work on the project, the team members must have their project approved by their lab instructor.

Possible Topics for Projects

• Preparation of color prints using a color kit.
• Study of the effect of wavelength on sensitivity of slow and fast emulsions; sensitization of emulsions.
• Preparation of prints using historical methods, e.g., calotypes or talbotypes.
• Preparation and testing of other or mixed developers.
• Using a pinhole camera to study the effects of pinhole shape or diameter.
• Preparation of a colored image using Maxwell's process.
• Study of infrared sensitive film.
• Preparation of a standard or a high contrast emulsion. (Procedure follows.)
• Kurlian photography.
• Intensification.
• Reduction of density using a slow bleaching solution.

For some good ideas and background information on possible projects please see:

• Creative Darkroom Techniques, by Kodak;
• 101 Experiments in Photography, by R.D. Zakia and H.N. Todd (Morgan & Morgan, 1969);
• Alternative Photographic Processes, by K.E. Wade (Morgan & Morgan, 1978).

Preparation of a Photographic Emulsion

Procedure

With the lights on, dissolve 1.2 g of potassium chloride and 0.60 g of potassium bromide in 50.0 ml of distilled water. Place this solution on a stirrer hot plate and bring the temperature to 35ºC. Add 10.0 g gelatin. Set this aside to thicken. In another beaker dissolve 3.0 g of silver nitrate in 30.0 ml of distilled water. Assemble the constant temperature apparatus shown below. Run the hot water for a while to ensure temperature is about 50ºC. Heat the potassium chloride-gelatin mixture in the beaker to about 50ºC.

The remainder of this experiment must be done with a safety light as the photographic emulsion is extremely light sensitive once it is prepared. Add the silver nitrate solution slowly, with continued stirring, to the gelatin-halide mixture. The rate of addition should be no greater than 1 mL per 3 seconds. Maintain the temperature at 45ºC, with stirring for 2 hours. At the end of this digestion period, add another 5.0 g of gelatin. Once this has dissolved, you can chill the emulsion with an ice bath. If necessary, the experiment can be stopped at this point. Wrap the emulsion beaker with foil, label and store in the refrigerator.

If the emulsion is shredded and washed, then cold tones will be observed. If it is left unwashed, warm tones will be obtained. The emulsion should now be melted and with continuous stirring, held at approximately 45ºC for 1½ hours. At the end of this second digestion period. 0.125 g of chrome alum
(KCr(SO₄)₂·12H₂O), (dissolved in a minimum of H₂O) can then be added.

The emulsion can now be coated onto any desired object. This should be done as quickly as possible because the emulsion can get very thick when the chrome alum is added. Pour the warm emulsion into several Petri dishes. Add sufficient emulsion to just cover the surface of the glass. A thickness of less than 1 mm is desired. Try to make all of the Petri dishes about the same thickness. The Petri dishes can now be allowed to cool, wrapped in foil and stored in the refrigerator until needed.

Expose a Petri dish by setting the negative directly on the emulsion. Start with the same exposure that was used for the photographic paper. Develop the emulsion by pouring the developer, stop bath and fixer directly into the dish. Use the times you used for the photographic paper. Experiment with your emulsion to obtain the best possible print. Can you think of any other surfaces you could coat? What is the purpose of each of the chemicals in the emulsion? What would be the result if the AgCl were to be replaced by AgBr? AgI?