chemistry of photography

Chemistry of photography

Silver Halide, our friend
Thanks to silver halide, the light sensitive material, now we enjoy the joy of pressing that shutter button and immortalize the moments. More on chemistry side; halide group members posses single negative charge. The halides are: fluoride, chloride, bromide, iodide, astatine, lutetium and lawrencium. In modern films, normally silver bromide or silver bromoiodide is used. For infrared photography, silver chloride is used instead. For silver bromoiodide, iodide ions are incorporated to silver bromide crystals during growth.

No, it’s not only silver halide on your magical film slide! Silver halide crystals are mixed with other substances such as gelatin (peptides of collagen). Gelatin provides support and protection for silver halide and other light sensitive materials. Fortunately, it also increases the sensitivity of silver halide.

Heat treatment during film manufacturing helps to increase sensitivity towards light (speed or ISO). Usually, other chemicals such as tiny amount of sulfur and even gold may be added during the heat treatment.

Magic Begins!
Okay, now that you know what silver halide is, let’s start the actual process. The magic begins when lights hits the silver halide. Silver halide will be decomposed to silver. However, this change is indistinguishable from the rest of the film. Too see the image; you have to develop it first.

Firstly developer is added. The developer reduces the silver ions to black colloidal ions (hence white area appears as black area in the negative). Bear in mind, the unexposed silver halide will remain as silver halide, hence not affected. Time of development is crucial on the images; hence it must be controlled strictly.

Next, is to stop the chemical reaction between the developer and the exposed silver halide. Since developer is alkali (pH higher than 7), we need something acidic (pH less than 7) to neutralize it and halt any further chemical reactions. Modern developers usually have hydroquinone (p-dihydroxybenzene) as the major constituent. Stop bath is used for this purpose. Usually ethanoic acid is the major constituent of the stop bath.

Done with the exposed silver halide, now let’s deal with the unexposed ones. As mentioned before, silver halide our friend is unstable in presence of light. Leaving half developed film strip under light will spoil the image, since the remaining silver halide will continuously converts to silver. To tackle this, we need fixer which usually consists of sodium thiosulfate and ammonium thiosulfate (ammonium thiosulfate usually gives faster reaction). These active ingredients will react with silver halide and produces water soluble complex. This enables the unexposed silver halide to be washed away. Voila! Magic completed. Now you have your developed film! You can dry it up after rinsing the excess chemical properly with water. Fixer usually contains hardening agents which hardens the emulsion to prevent scratches on the negatives.

Additional step will be adding wetting agent. It is actually made of soap. The wetting agent helps the film to dry more evenly. Surely you don’t want to have uneven dried films don’t you?

Colors films
For color films, there are 3 different light sensitive emulsions. Each layer of emulsion is sensitive to different visible wavelength (light): blue, red and green. Mixture of these colors will give out other colors.

Slides or positive film is developed in similar fashion. However, it must be developed in two steps: first black and white image is developed, and then the color dyes are developed. Lastly, silver is washed out with fixer as in black and white developing.

Making Images
You have your negative, now you want your positive images. Here, negative is projected to silver halide impregnated paper; it will be developed to get the positive image. Want the old classic sepia colored photographs? Let’s do toning on the photograph then. With slight alteration during picture handling, tones of the picture can be altered. For example reaction between silver and thiosulfate in acid condition will produce sulfur. Sulfur then reacts with silver images and result in nice brown sepia tone (Ag2S).