Magic and the chemistry of photography

Film is a process of taking evenly spaced grains of silver that have been crystallized into halides by potassium derivatives and making them dance when exposed to precise amounts of light. During manufacture this photosensitive substance is pressed into thin strips of film and attached to a corresponding strip of plastic. Due to the paucity of our knowledge of the laws of physics and also to trade secrets of the film companies, no one really knows how it all works. It is a mystery, such as the art can be referred to a kind of magic being played with light. When Edison and others first began debuting the earliest photographs, it must have seemed magical to their first viewers. For the first time in history, a moment could be captured with perfect accuracy and clarity, albeit if only in black and white in the early days.

As light passes through the film, the silver halides react by converting to a spectrum of clear to gray to black. They create a latent image to be developed. The luminescence of modern film has a ratio of these “colors” of 1 to 4 to 6, respectively. Early films were only sensitive to blue light, until dyes were invented, primarily for blue and green (known as “orthochromatic”), later red was invented (“panchromatic”). It is even possible to record infrared light waves. Because early films were only blue-sensitive, they could be developed under a red light. Now we need completely dark room.

What happens in the dark room is simple chemistry. Development of the the film and the print is a chemical process incorporating up to a dozen different chemicals. At a set amount of time, temperature and agitation, various reducing agents convert the activated halides into metallic silver. These are only active in alkaline environments, so an accelerator like borax, sodium bicarbonate or lye are added. Its concentration determines how quickly the film develops. Sodium sulfite then acts as a preservative to prevent these chemicals from breaking down over time. Additionally, restrainers can be included to add clarity. The stop bath halts the development, Some photographers add acetic acid to it. The fixer, typically a thiosulfate, among other chemicals, dissolves the undeveloped halides, which are then washed off in a final bath. Prints can be toned with such chemicals as sulfides, selenium or gold in order to attain certain effects.

Source: Schafer, John P. The Ansel Adams Guide to Basic Techniques of Photography