Ask the average policeman who comes to your house what is left behind in latent fingerprints and he will probably say "oils." That could happen if the burglar were eating fried chicken, just put on Vitalis, or was rubbing his oily face... but, oil and fatty substance secreting glands on the human body (sebaceous glands and apocrine glands) are NOT present on surfaces lacking hair... that is, NOT on finger, palm or barefoot friction ridge skin. A slight amount of trace oil can sometimes be present because of "flow" on the skin surface from the back of the hand to the front, but such oil is only a slight trace compared with the oil you get on your fingers from rubbing your nose or forehead. Natural secretions from the sweat secreting glands (called eccrine glands) on fingers and palms (and bare feet) secrete NO OILS. They secrete about 98 or 99 per cent water, and the solids dissolved in the water include amino acids, proteins, polypeptides, and salts.

If you want to duplicate the basic effect of fingerprints on evidence, clean off a rubber name stamp so it leaves no ink traces behind. Next dip it into a solution of chicken broth. Shake off the excess liquid and stamp the impression on a drinking glass. Within a few minutes the liquid will dry and the impression will be almost invisible. Make another test impression on a piece of paper and let it dry. Unless you happen to have oily chicken broth you have just roughly (very roughly) duplicated the natural secretions present in latent fingerprints at crime scenes. Some researchers have compared dried eccrine gland secretions to a very faint coating of dried "shellac."

Latent fingerprints on glass, paper, or similar nonporous surfaces should be chemically processed to develop the best quality (greatest contrast) ridge detail. Picking up the glass with a handkerchief around your hand would destroy the test print... so would putting it in a plastic bag or envelope to carry it to a crime lab. Doing the same with the paper would not harm the test print there because it has soaked down into the paper fibers. Dusting the glass with fingerprint powder will probably destroy the latent print (there's usually no oil for it to stick to) and will almost never give the excellent results possible with chemical processing. Without chemical processing, the best you could probably do would be to "rejuvenate" the impression on the glass by breathing on it and then dusting with powder immediately after you see the "fogged" area on the glass evaporate. The moisture from your breath will leave the ridge detail slightly moist for a short while and increase the chances of fingerprint powder sticking to the non-oily print.

Many police will remark that a certain surface cannot be processed for fingerprints. Any surface that is about as smooth as the miniature corrugated cardboard type ridges on your fingers can potentially bear identifiable latent fingerprints... and the flexibility of the finger skin can often also conform to relatively rough surfaces such as imitation leather dashboards. Fingerprints from crime scenes have been identified on papers, cigarettes, fruit, crumpled aluminum cans, plastic garbage bags, bed sheets, dead bodies (prints on bodies are usually contaminated prints involving body fluids, lipstick or some other substance transferred via the suspect's fingers), and thousands of other surfaces. Fingerprints on paper, cardboard and unfinished wood can last for up to forty years (per actual casework histories) unless exposed to water (and contaminate transfer prints can even then sometimes persist). Fingerprints on non-porous surfaces such as plastic, metal and glass can last for years if not exposed to water and if left undisturbed.