I shall start with Black and White, since it's simpler than colour and more people process and print their own B&W.
A B&W print normally starts when the film is exposed. So how does the film work? Modern films are incredibly complicated wonders of chemistry and manufacturing techniques, but simply, all B&W film is made up of a flexible plastic base coated with a layer called the emulsion. This is a mixture of crystals of one or more Silver Halides trapped within gelatin. The Halogens are a group of elements; Bromine, Iodine, Chlorine and Fluorine, which like to react with other elements to produce Halides. When they react with silver, they produce substances which are sensitive to blue and Ultra-Violet light. From a photographic point of view only three are useful; silver bromide which is the most sensitive, followed by silver chloride, then silver iodide. The reason for only three is that we need to be able to process the film using chemicals dissolved in water and, unlike the other three, silver fluoride is soluble in water and so would wash out of the emulsion.
Some of you may be puzzled how we can have films which are sensitive to all colours, such as panchromatic B&W and colour films, when I have said that silver halides respond only to blue and UV. Well fortunately, there are dyes which can be added to the emulsion which will sensitize the halides to light of other colours and can also increase their sensitivity.
When the silver halide crystals in the emulsion are exposed to enough light, known as the Threshold, it causes some of the halide to be knocked off, proportional to the amount of light, leaving microscopic specks of silver behind. These specks form the latent image, hidden until we develop it.
In the next article I shall explain how developers work and how they influence how you expose the film in the first place.
© Barry Leighton FRPS