The Science of Fingerprints, United States. Federal Bureau of Investigation [good books to read in english TXT] 📗

nitrate solution to form silver chloride. Silver chloride is white but is unstable on exposure to light and breaks down into its components, silver and chlorine. The ridges of the fingerprints developed in this manner appear reddish-brown against the background. Immersion in the silver nitrate solution will wash traces of fat and oil from the paper; consequently, it is necessary to fume the specimen for latents of such a nature prior to treatment with silver nitrate.

Once the requisite equipment is assembled, the steps in the process are these:

Dip the specimen in the solution, blot and dry it, expose to light, and photograph latents when contrast is good.

Chemically standardized solutions are not required for the successful application of this process. It has been determined through long practice that a 3-percent solution of silver nitrate is adequate for the purpose, although concentrations up to 10 percent are sometimes used. A solution of approximately 3 percent may be prepared by dissolving 4 ounces of silver nitrate in 1 gallon of distilled water. Smaller quantities of 3-percent solution are made by using the components in the same proportion. For instance, one quart of water will require 1 ounce of the crystals. For a 10-percent solution, use 13-1/3 ounces of crystals per gallon.

An alcohol solution may be preferred. This is prepared by mixing 4 ounces of silver nitrate crystals, 4 ounces of distilled water, and 1 gallon of grain alcohol, 190 proof. The alcoholic solution dries faster, and when treating paper bearing writing in ink, it is less likely to cause the ink to run. On the other hand, the alcoholic solution is much more expensive and there is some loss by evaporation while in use.

The solutions may be used several times before losing their strength and when not in use should be kept in brown bottles in cupboards to retard deterioration. If the strength of the solution is doubtful, the operator should attempt to develop test latent impressions before proceeding on evidence.

Silver nitrate crystals and distilled water in small amounts are obtainable from druggists or in large amounts from chemical supply houses. Dealers in distilled water are located in many communities.

Tap water should not be used in the preparation of the solution because it generally contains chemicals which will partially neutralize the silver nitrate.

It is suggested that the solution be placed in a glass or enamelware tray approximately 18 by 12 by 5 inches for use, a size used in photographic development. Treatment with this solution is called "silvering." The specimen is immersed in the solution so that the surfaces are completely moistened, then taken out, placed between blotters to remove the excess solution, and dried. The drying is readily accomplished with an electric hair dryer. Blotters may be dried and used several times before discarding. It is not necessary to work in a dark room. Work in an illuminated room but not in direct sunlight. Soaking the specimen in the solution does not aid development and is actually undesirable as it requires a longer drying time. The specimen should be reasonably dry before exposing to the light, otherwise the latent prints may be developed while the paper is still wet, thus necessitating drying in subdued light to prevent darkening.

Development of the latent impressions occurs rapidly when the specimen is exposed to a blue or violet light source. A 1,000-watt blue or daylight photographer's lamp, a mercury arc (most ultraviolet lamps are of this type), or carbon arc is excellent for the purpose (fig. 422). If a weaker light is used, a stronger mixture of the solution should be prepared. For instance, if a 300-watt bulb is used, the 10-percent solution would be preferable. Direct sunlight will cause the latent impressions to appear very rapidly and if several specimens are exposed at once it is not possible for a single operator to properly control the development. Sunlight coming through a window pane will serve for development. Where fingerprints containing sodium chloride (normally exuded from the sweat pores in the ridges) have been deposited, the silver chloride formed will darken against the background.

 

Fig. 422

[Fig. 422]

 

As soon as the ridge detail of the prints is clearly visible, the paper should be removed from the light. Continued exposure will darken the paper and the contrast will be lost.

Paper so treated should be kept in darkness; that is, in a heavy envelope or drawer until ready to photograph.

Immediate photographing, as in the case of iodine prints, is not always essential, since the prints are permanent and become illegible only through eventual clouding of the background. Prompt photographing is recommended, however, as, in exceptional instances, silver-nitrate prints have become illegible in a matter of hours. Darkening ordinarily will occur slowly if the paper is preserved in absolute darkness, and silver-nitrate prints so preserved more than 10 years have been observed to be quite clear.

Items such as cardboard cartons, newspapers, road maps, large pieces of wrapping paper, or smooth, unpainted wood surfaces, too large for dipping, may be treated by brushing the solution over the surface with a paint brush (fig. 423). Brushing does not damage or destroy latent impressions on surfaces of this type. Cardboard boxes may be slit down the edges and flattened out to permit easy placement under the light.

 

Fig. 423

[Fig. 423]

 

Wet paper should be handled with extreme care to prevent tearing. In treating very thin types of paper the solution is best applied with a cotton swab or brush.

Photographs, Photostats, and blueprints of any value should not be treated with silver nitrate, since the developed prints or stains cannot be removed without destroying them.

In working with silver nitrate, wear rubber gloves or handle all specimens with tweezers; avoid spilling it on clothing. It will cause dark brown stains on clothing, skin, and fingernails. Such stains are not easily removed. Areas of the skin subjected to prolonged contact are deadened, will turn black and peel.

If removal of silver nitrate prints (called "de-silvering") is desired, this may be accomplished by placing the specimen in a 2-percent solution of mercuric nitrate in a tray similar to that used for the silver nitrate.

To prepare a small amount of this solution, dissolve two-thirds of an ounce of mercuric nitrate crystals in 1 quart of distilled water and add one-third of a fluid ounce of nitric acid. Shake well. This solution, too, may be used several times before losing its strength and is not necessarily discarded after each use. It is not necessary to keep it in a dark bottle.

The specimen bearing silver nitrate prints is immersed in this solution until all traces of the prints disappear. It should then be rinsed thoroughly in water to remove all mercuric nitrate. If this is not done the paper deteriorates, becoming brittle and crumbly. A tray of distilled water may be used for rinsing or a tray of ordinary tap water changed several times during the rinsing. The specimen is then laid out flat to dry.

Wrinkles, such as are left in paper after ordinary drying, may be prevented by ironing with a moderately hot iron. An electric iron with a temperature control is desirable. If kept too hot it will scorch or wrinkle the paper somewhat. The bottom of the iron should be clean so that unremovable smudges will not be left on the paper.

No ill effects have been noted from working in the 2-percent mercuric nitrate solution with bare hands for very short periods, but it is a caustic solution and it is suggested that the specimens be handled with tweezers or that rubber gloves be worn if contact is prolonged.

CHAPTER XV The Use of the Fingerprint Camera

 

If a fingerprint is visible, an effort should be made to photograph it before any attempt is made to develop it. In every case a print developed with powder should be photographed before lifting. It sometimes happens that the print does not lift properly although it may be quite clear after development.

The camera which is especially adapted to the purpose and which is easiest to handle and operate is the fingerprint camera, one type of which is shown in figure 424. This camera has several advantages in photographing fingerprints: