Christine Craig is a forensic scientist for the Commonwealth of Virginia and specializes in working with impressions. Jason Byrd is an assistant professor of forensic science and biology at Virginia Commonwealth University and the chairman of the American Board of Forensic Entomology. They provide the following explanation:
Fingerprint patterns and characteristics are formed before birth. They will remain unchanged until decomposition destroys them after death, or unless the dermal layer is injured, producing a scar. Fingerprints are unique to each individual--including identical twins--and have been used for over a century for identification and crime-solving purposes.
The skin found on the fingers, palms and soles of the feet of humans (and some primates) is known as friction skin. This skin is unique because it does not have hair follicles or oil glands, and because it is composed of ridges that are believed to be adapted for increased friction to help when handling various objects and walking. These so-called friction ridges are composed of rows of sweat pores, or eccrine glands, that constantly secrete perspiration. This perspiration--along with grease and oil transferred from other parts of the body--adheres to the friction skin and is transferred from the skin to other surfaces when contact is made with objects. The transferred outline of the friction ridges is what is known as a latent print.
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Latent prints are not readily visible to the naked eye. As a result, these "hidden" prints must be "developed" in some way to increase their visibility and contrast. The most common method of developing latent prints on nonporous objects is to physically enhance them by applying fingerprint powder. Fingerprint powder is composed of many different ingredients that can vary greatly depending on the formula used. Most black fingerprint powders contain rosin, black ferric oxide and lampblack. Many also contain inorganic chemicals such as lead, mercury, cadmium, copper, silicon, titanium and bismuth. Fingerprint powder is applied by brushing it onto the surface and works by mechanically adhering to the oil and moisture components of the latent print. When the powder particles adhere to the grease or moisture forming the latent prints, it causes them to become visible. The developed latent prints are then readily observable and able to be collected, preserved and examined.