Porous Evidence

Paper, checks, currency, unfinished wood, and cardboard are all examples of porous evidence. At a crime scene this type of item might be a tissue box, pieces of mail, business cards, pictures, books, notes, and walls with porous types of paint. There are a few different processes available to lift a print from a porous item; it really depends on the condition of the item. Investigators often use chemical methods such as iodine fuming, silver nitrate, or ninhydrin to locate latent prints on porous materials. When one of these chemicals comes into contact with the chemicals present in the fingerprint residue the print become visible.

.Iodine fuming takes place in a fuming chamber. The process works by heating up solid crystal iodine which creates vapors that adhere to the oily residue of print, producing a brown colored print. One of the drawbacks of using iodine fuming is that the print fades quickly after the fuming takes place and therefore must be photographed quickly. Alternatively, if the print is sprayed with a starch and water solution, it can be preserved for several weeks (Thompson, 2010). Silver nitrate, when exposed to latent prints, reacts with the chloride of the salt molecules found in print residue, forming silver chloride. When exposed to ultraviolet light, silver chloride turns black or brown, making the print visible. This method works particularly well on impressions left in cardboard and paper-like surfaces.
The object on which the print is located can be dipped in or sprayed with a ninhydrin solution, which reacts with the oils in the print’s residue to create a bluish print. One of the drawbacks of using ninhydrin is that the reaction is very slow, often taking several hours for the print to become visible. To accelerate the reaction, the object containing the print can be heated to 80 to 100 degrees Fahrenheit (Thompson,