To the photographer, fingerprints may be either a nuisance or they may actually form his subject, e.g: in crime detection and police records. In the first place they are to be avoided, in the second, the aim is to get as clean a photographic image of them as possible.
When the fingers press on the surface of a sensitized material they may leave behind prints that become visible later. There are three kinds of contamination which may be responsible: grease, perspiration and processing chemicals.
1) The natural oil on the skin may leave a greasy, water-repellent imprint. This has no effect on a finished negative or print, but if it is left on the surface of the material before it is processed, it prevents the processing solution from acting on the emulsion. If made on an undeveloped negative the result is a light imprint after processing. A greasy fingerprint on the developed negative will hold back the fixer and allow the emulsion to get blacker, so the imprint will show as a black mark on the half-tones.
A greasy finger-mark on a print before development will hold back the developer and later show as a white imprint on the half-tones and shadows.
A greasy imprint made on the surface of a print after development and before fixing will blacken over the half-tones where it has been exposed to light but it will remain clear over the highlights. Later, however, if the print is exposed to daylight for any length of time, such imprints will gradually darken and become visible. The remedy here is to wash hands in soap and hot water immediately before a processing session, and then refrain as far as possible from touching the sensitive surfaces. Forceps should always be used for handling prints.
2) Imprints of another kind are caused by perspiration on the fingers. The moisture is usually slightly acid and, although it is immediately dissolved and rendered harmless by the aqueous processing solutions, it can set up a chemical action with visible consequences if transferred to the surface of a dry negative or print. This type of imprint may appear either white on half-tones and shadows or dark or discolored on half-tones and highlights.
People whose hands tend to perspire unduly should rinse them in water frequently during processing to dilute the dissolved matter in the perspiration. They should avoid handling sensitized materials before processing and use a self-loading spiral tank for developing films.
3) The fingers can become contaminated by processing chemicals when manipulating plates and prints in.the various solutions. The ideal prevention is to use forceps or, if that is impossible, rinse the fingers in clear water before as well as after putting them in the solution.
Contamination can also occur by indirect means, i.e: from touching objects that have been handled on a previous occasion with contaminated fingers, e.g: taps, bottles and stoppers, door-knobs, and particularly towels. This is a further reason for rinsing the fingers immediately after they have been in contact with a solution. For the same reason it is always wise to wash off any dribbles of solution that run down the side of a liquid chemical bottle.
Color materials also will finger-mark from any of the above causes and the results show equally over all colors, shadows and highlights. The imprint may be black, clear or colored depending upon the type of material and the stage at which the fingers touch the surface.
The grease or perspiration that is normally present on the fingers leaves an invisible print on everything it touches. It is possible to render these imprints visible by dusting with a suitable powder. Photographic records of them can then be made. Fingerprint photography forms an important branch of criminal investigation.
Fingerprints taken for police records may later have to be photographed so that they can be projected in enlarged form for comparison with prints found elsewhere. The photographic technique here is simply a matter of making a normal black-and-white record of a clear black on white impression.