Chemical Processing

All chemical processes should normally be performed at a laboratory. The numerous chemicals, their mixtures, and various applications to different types of evidence are best accomplished by the laboratory technicians. Federal regulations specifically govern the handling of dangerous, toxic chemicals.

All paper products suspected of bearing latent fingerprints are best developed with chemicals. Paper acts as a blotter when touched; therefore, the latent prints will not rub off as they do on a nonporous surface.

Latent fingerprints are developed when the chemicals react with the organic and inorganic chemical substances of latent print residue.

All chemical processes are dependent upon the presence of mineral organic matter in the perspiration. Since its composition varies, not only between individuals but from time to time in the same individual, perspiration accounts for the uncertainty and characteristic "spotty" development of chemical processes.

A frequently used chemical process is the silver nitrate method. A 3-percent solution is recommended but it may be weaker or stronger. One and one-half teaspoons of silver nitrate to a pint of distilled water will make about a 3-percent solution. Tap water generally contains undesirable chemicals that will partially neutralize the silver nitrate.

An object containing fingerprints does not have to be soaked-just wet. After wetting with the nitrate solution, the object is blotted to remove excess solution. The object can be placed under ultraviolet light, sunlight, or photofloodlight. Under ultraviolet, the images appear quickly; they appear less quickly with the photoflood. The object itself will continue to darken with prolonged exposure, to the point where the print images are obscured.

If it is desirable to clear the object it may be placed in a 3-percent mercuric nitrate solution.

After bleaching and washing, a document is placed between sheets of blotting paper and ironed with a moderately hot iron. The blotting paper is turned occasionally to dissipate the steam.

Continuous observation of the object is required during the time of light exposure to ensure that the latent prints do not overdevelop. Latent prints developed with silver nitrate should be photographed immediately and then protected between two sheets of black paper. To preserve prints developed by the silver nitrate process, the object may be subjected to the action of an ordinary photographic fixing bath. The amount of contact and the amount of pressure exerted by the fingers are two variables that affect paper latents the most.

Old fingerprints on paper can best be developed by using ninhydrin. It is a patented product available in spray cans from fingerprint supply houses. It develops latent prints, which appear pink to purple with good ridge detail. The vapors of ninhydrin are toxic; care should be exercised in its use.

It may also be available for use in white powder form, which must be mixed with one of several solvents. The choice of solvents depends upon the particular need of the user. The liquid solvent has no effect on the latent print and is used only to dissolve and dilute the chemical powder. Most solvents used may cause ink to run or to smear. For this reason, it is advisable that evidence that contains hand printing, writing, or typing be processed through the document division to analyze the handwriting before it is processed with ninhydrin solution.

Some prints developed with ninhydrin may appear immediately after processing, while others may take several hours or even days to develop. Ninhydrin may be removed from documents by using a 1- to 3-percent solution of diluted ammonia.

A metal object held over a piece of burning magnesium ribbon will have deposited on its surface a film of white magnesia powder. Latents developed in this way are very persistent, and even scrubbing has failed to entirely obliterate a print developed in this manner.

The iodine method of developing latent fingerprints uses iodine vapors which are physically absorbed into the grease and oil deposits in the print. When the object is porous, such as paper or unpainted wood fuming with iodine is preferred. This is not to say that fuming painted wood and other nonporous surfaces is precluded; sometimes it is even more effective than powder. But soft porous surfaces lend themselves more readily to the iodine process.

Iodine is not only very satisfactory for fresh prints on paper, but also on glass, finely woven fabrics, shiny-skinned fruits, cartridges, silver, and "greasy" prints.

The iodine fuming gun supplied with a fingerprint kit is ideal, but one can be easily made from any glass tube of about 1/2-inch diameter or a little larger. Iodine crystals are introduced into the tube and held in place by a suitable material, such as glass wool. Fumes are produced by exhaling through the glass tube.

A drying agent such as calcium chloride is used to remove moisture from the breath before it passes over _{the iodine crystals.} The calcium chloride is also separated from the iodine crystals and held in place by _{the glass wool.} If some substance such as calcium chloride is not used to absorb the moisture in the breath, droplets of iodine-stained water may drop from the end of the tube onto the object.

The heat of the breath causes the iodine crystals to vaporize faster than they would in the air, producing a usable volume of iodine vapor which is played over the suspected area.

As soon as a print is suitably developed, it should be photographed. No attempt should be made to lift the print with lifting tape. This applies to powder-developed prints on paper as well because the tape will tear the fibers of the paper.

If it is desirable to preserve the iodine-developed print, the paper may be placed between two pieces of glass and the edges of the glass sealed with masking tape.

It is also possible to lift an iodine print using a thin ^{plate of silver.} The polished silver should be held against the developed print for a few seconds and then subjected to the light from a photoflood lamp. The ridges where the iodine was in contact with the silver will turn black and be permanent. The silver plate may be cleaned with "whiting" (a finely powdered chalk) for further use.

Ammonia fumes will clear iodine-developed prints. In addition to ammonia, a weak solution of photographic fixer, 15 grams of sodium triosulphate to a pint of water, may also be used to clear iodine-developed prints from paper. The paper should then be washed and ironed. In this procedure, as with the silver nitrate process, care must be taken to ensure that the paper will withstand wetting and washing.

The fuming gun must be well cleaned after each use. Also, you should take care to make sure that no loose iodine crystals are in the kit and that iodine crystal containers are tight. Iodine vapors are very corrosive and will attack metals and other parts of the kit. A common and recommended practice is to keep the iodine crystals separate from the kit until they are needed for immediate use.

A product called Driodine has been developed. It is porous ground glass saturated with iodine. The resulting powder permits actual iodine dusting without the problems normally associated with using pure iodine. Driodine is simply poured over the surface under examination and left from 15 to 30 seconds to produce an iodine-developed print. This method can be used in lieu of or in addition to iodine fuming.

Another method for developing latent prints on flat, porous surfaces through application of iodine is with the use of photographic film. The film is completely exposed and developed. When needed, it is moistened until the emulsion becomes tacky and is then pressed against the iodine-fumed print. The film is then dipped in a mixture of chrome alum and nitric acid. This mixture dissolves the silver background which is not protected by the silver iodine, leaving a fingerprint that may be compared directly or used for making photographic prints or enlargements.