Part 28 (1/2)

When iodine crystals are subjected to a slight amount of heat they vaporize rapidly, producing violet fumes. These fumes are absorbed by fatty or oily matter with which they come in contact. If the specimen treated bears latent impressions which contain oil or fat, the print is developed or made visible by the absorption of the iodine fumes and the ridges of the print appear yellowish-brown against the background.

Iodine prints are not permanent and begin to fade once the fuming is stopped. It is necessary, therefore, for the operator to have a camera ready to photograph the prints immediately.

Control of the fumes is achieved by using the crystals in an iodine gun or fuming cabinet. The iodine gun may be a.s.sembled by the individual examiner, by a druggist, or it may be purchased through a fingerprint supply house.

Material for making the iodine gun, as well as iodine crystals, may be procured from a chemical supply house or through a druggist. The gun itself consists essentially of two parts. One tube (the end of the gun through which the breath is blown) contains a drying agent such as calcium chloride, to remove moisture from the breath. Without this, the moisture from the breath and saliva would condense at the end of the gun, drip onto the specimen and cause stains which might prove indelible. The second tube contains a small amount of iodine crystals which are vaporized by the heat of the breath, augmented by the warmth of the hand cupped around the tube containing the iodine. This vapor is blown onto the specimen (fig. 420). Gla.s.s wool serves to hold the calcium chloride and iodine in place.

[Ill.u.s.tration: 420. Iodine fuming gun in use.]

Due to the amount of physical exertion involved, the gun is generally limited to the examination of a few small specimens. Where a large number of specimens are to be treated, the fuming cabinet, a box-shaped wooden receptacle with a gla.s.s front and top permitting the operator to control the amount of fumes in the cabinet and observe the development of the latent impressions, is used (fig. 421). The fumes are generated by placing a small alcohol burner under an evaporating dish containing the iodine crystals. This is set in a hole cut in the bottom of the cabinet. As soon as the fumes begin to appear in sufficient amounts, the burner is removed. The specimens may be hung in the cabinet by wooden clothes pins fastened to a removable stick which is supported by wooden strips affixed near the top edges of the cabinet. The top of the cabinet is removable to permit access.

Diagrams for the construction of the iodine gun or fuming cabinet will be furnished on request to members of the law enforcement profession.

Many specimens bear small, greasy areas which, in addition to any latent impressions of a greasy nature, will also appear yellowish-brown after exposure to iodine fumes. All these stains will eventually disappear if the specimen is placed in a current of air from a fan or vent. All latent impressions on an object will not be developed by the iodine process but only those containing fat or oil.

Due to this fact and the fact that iodine evaporates from the surface, it is used prior to (it cannot be used afterward), and in conjunction with, the silver nitrate process.

[Ill.u.s.tration: 421. Iodine fuming cabinet in use.]

No ill effects have been noted from contact with small amounts of iodine vapor but prolonged or excessive contact will produce irritation of the skin and respiratory pa.s.sages. To prevent gradual loss of the chemical through evaporation and the corrosion of surrounding metal surfaces, iodine crystals should be kept in an airtight container when not being used.

The development of latent impressions with silver nitrate is dependent on the fact that the sodium chloride (the same substance as common table salt) present in the perspiration which forms the ridges in most latent impressions reacts with the silver 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 a.s.sembled, 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 quant.i.ties 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 r.e.t.a.r.d 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 gla.s.s 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.

[Ill.u.s.tration: 422. Developing silver nitrate prints using 1,000-watt bulb reflector.]

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.