Darkroom tips: Agitation during black & white film development. A source of trouble often overlooked is improper agitation of the tank while the black and white film is being developed. As the examples show, too little agitation will result in an underdeveloped negative; too much agitation causes overdevelopment. These problems often occur because many black and white photographers misunderstand or ignore the reason for agitation. Its sole purpose is to move the solution inside the tank so that a steady supply of fresh developer reaches the emulsion all during the development period. If this is not done, the used-up chemicals, after doing their share of work on the emulsion will form a stagnant pool where development activity slows down and eventually will completely stop. Agitation must be done at regular intervals so the developer will be moved along and replaced well before it begins to be exhausted. In most cases, agitation for 15 seconds once each minute is adequate. Some fast-acting developers, though, require more frequent agitation periods. As important as enough agitation is the kind of agitation, which depends on the motion used - the way the tank is tilted (or with some models the way the film reel is twirled). The motion must be random. The tank should be tilted in two or more different directions during each agitation period. This avoids setting up a repetitive pattern of flow across the face of the black and white film, as might happen if the solution were set in motion in the same direction, and with approximately the same degree of force, every time. The black and white film might be marked by the movement of the solution in the same way that steady waves make patterns on a sandy beach.