Part 27 (2/2)

Powder brushed lightly over a latent-bearing surface will cling to grease or moisture in the ridges of a latent print, making it visible against the background. Obviously, a powder should be used which will contrast with the color of the surface. Photographic contrasts should also be considered.

A gray powder and a black powder are adequate for latent print work.

Many fingerprint powders of various colors and compositions are available from fingerprint supply houses but none are superior to the gray and black.

A very small amount of powder is placed on the brush for application to the surface. Once the contour of a print is visible, the brush strokes should conform to the direction of the ridges. All excess powder should be brushed from between the ridges. Too much powder and too little brus.h.i.+ng are the chief faults of beginners.

Gray powder is used on dark-colored surfaces. It is also used on mirrors and metal surfaces which have been polished to a mirrorlike finish, since these surfaces will photograph black with the fingerprint camera.

Black powder should be applied to white or light-colored surfaces.

Aluminum powder affords the same contrast as the gray. Gold and red bronze powders, although of a glittering appearance, will photograph dark and should consequently be used on light-colored surfaces.

Dragon's blood powder is a photographically neutral powder and may be dusted on either a light or dark surface.

On clear transparent gla.s.s, either gray or black powder may be used, it being necessary only to use a contrasting black or white background when photographing.

Prints should be lifted after photographing. Both rubber and transparent tape are available for this purpose. Rubber lifting tape is procurable in black or white 4” x 9” sheets and has the adhesive surface protected with a celluloid cover. A black powder print should obviously be lifted on white tape and a gray powder print on black tape.

Gold bronze and red bronze powders should be lifted on white tape, aluminum on black. Dragon's blood may be lifted on either black or white.

After cutting a piece of tape sufficiently large to cover the entire latent print, the celluloid covering is removed and the adhesive side applied to the latent. The tape should be pressed evenly and firmly to the surface, taking care not to s.h.i.+ft its position. It is then peeled gently from the surface and the piece of celluloid placed over the print to protect it. The operator should handle the lift in such a manner that he will leave no prints of his own on the adhesive surface. A small paper identification tag bearing the initials of the operator, date, and object from which lifted should be placed under one corner of the celluloid, or this information may be written on the back of the lift itself if it can be done in a permanent, legible manner.

If an excessive amount of powder adheres to the latent print, a more legible print may sometimes be obtained by lifting a second time (on a new piece of tape, of course).

It should be noted that a print lifted on rubber tape is in a reverse position. Consequently, in preparing a photograph of a print on such a lift, it will be necessary to print the negative from the reverse side in order for the print to appear in its correct position for comparison. Preparation of such photographs should not be attempted by persons of inadequate knowledge and experience.

Transparent tape with a durable adhesive surface is available in 1” to 2” widths for fingerprint work. The common variety of transparent tape is not suitable due to the deterioration (drying) of the adhesive surface. The print on a piece of transparent tape is in correct position. Transparent lifts should be affixed to a smooth, grainless, opaque background of a black or white color contrasting with the powder used. Every effort should be made to avoid air bubbles under such lifts. In no instance should a transparent lift ever be folded back on itself or stuck to another piece of such tape as a backing, since it is generally not possible to determine the correct position of such a print.

Groups of latent impressions, such as those of adjacent fingers or fingers and palms which appear to have been made simultaneously, should be lifted as units, that is, on a single piece of tape, as this may facilitate the task of making comparisons.

CHAPTER XIV

_Chemical Development of Latent Impressions_

Chemical treatment in the development of latent finger impressions on paper, cardboard, and newly finished or unpainted wood may involve a slightly more complicated technique than that in which powders are utilized, but the results justify the additional effort.

It is very strongly recommended that powders not be applied to articles of the above types. This recommendation is made for several reasons. First, powders cannot be removed from paper and possibly may interfere with some types of doc.u.ment examinations. In this connection, they are likely to prevent restoration of the specimen to a legible condition. Powders will not develop as many latent impressions as chemicals on paper or cardboard. In some cases they will obscure latent impressions subsequently developed chemically.

Neither scientific training nor complete knowledge of the chemical processes involved is necessary for one to become proficient in the use of chemical developers, two of which will be discussed more fully.

These two, iodine and silver nitrate, are the most commonly used, inasmuch as they are relatively inexpensive, readily procurable, effective, and easy to apply.

All specimens which are treated should be handled with tweezers or gloves.

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