Latent fingerprints usually are a mixture of secretions from the various glands found in the skin. When fresh, they contain a large amount of water (98.5-99.5%), together with small amounts of fatty acids, urea, amino acids, and/or salts. Chemical methods to visualize fingerprints make use of the presence of these components. Ninhydrin, for example, reacts with the amino acids to produce purple compounds. Silver nitrate on the other hand reacts with the small amounts of chloride ion present. Upon reaction the light sensitive silver chloride is formed, which on illumination decomposes to silver, thereby producing a silver image (just as in photography).
 
Powders do not work by chemical methods but simply adhere to the fingerprint. With fresh finger prints, the water still present in the print contributes significantly to the adhesion of powders. With older fingerprints, powders adhere in principal to the fatty deposits from the sebaceous glands secretions.
 
The prints of the friction ridges may be transferred by substances other than skin secretion, such as dust or oil. In such cases, the fingerprint often is immediately visible, but for proper examination further development will usually be necessary.
 
The friction ridge pattern also appears in finger impressions in soft matter or in dust covered surfaces. There exists a basic difference between such an imprinted trace and a normal fingerprint formed by transfer. A print formed by transfer gives a positive image of the friction ridge pattern while a finger impression in soft matter gives a negative image.
Show less