There are many developers available, but, with certain exceptions, they all work the same way. Most use two developing chemicals, known as "agents", usually Metol or Phenidone combined with Hydroquinone. These are combined with alkalis which increase their activity and other chemicals which help to prevent them reacting with unexposed halide crystals and with the air.

You will remember that the difference between the exposed and unexposed halide crystals in a film is that the light has caused small specks of silver to be created in the exposed crystals. The developer basically dissolves away the crystal around each of these "development centres" and uses the silver to grow the speck to a visible size. The more light a crystal has received, the more specks it will have, so the faster it will grow. This is why we get a negative, where the brightest parts of the subject appear as the darkest parts of the film. As the image develops, the developer breaks-down and chemical by-products form in the emulsion which slow down the process. These have to be removed by agitation or they will build-up to a point where the image develops unevenly. We could agitate continuously to make sure that no build-up is possible. However, if we arrange things properly, we can make this build-up work to our advantage. By agitating for long enough to bring fresh developer into the emulsion, then leaving the film to stand for a little while, the developer will begin to be used up in areas of high exposure (high density), while in areas of low exposure (low density) the developer will be relatively fresh. On the borders of these high and low density areas, the two solutions will mix causing a little extra development just inside the high density area and a little less just inside the low density area. This has the effect of "drawing" a fine line around the edges of the areas, making the edges appear sharper, and is known as "accutance". A second effect, known as "compensation" can be caused by allowing the developer to get used-up even more before agitation. This "holds back" the high exposure areas, while allowing the areas that have had less exposure to "catch-up".

When the development time is up, to prevent uneven development, we need to stop the action of the developer quickly and evenly. This can be done by a short wash to dilute the developer or by using a weak acid solution, usually acetic acid, called a stop bath to neutralise the alkali in the developer. At this point, we have a developed image on our film, but we also have all the unexposed and undeveloped halides left. These have to be removed or they will break-down under the action of light and eventually stain the negative. Known as fixing, this step uses a chemical which changes the insoluble halides to soluble salts. For many years sodium thiosulphate was used, usually called "Hypo" from its old name of sodium hyposulphate. Now, most fixers are made using ammonium thiosulphate in a weak acid solution, which is rather more rapid. All this chemistry needs to be removed from the film, otherwise damage will occur in time, so a good wash is needed after fixing.

In the next article I shall explain how development influences how we expose the film in the first place.

© Barry Leighton FRPS