Developing Prints on Adhesive Surfaces

This article is the seventh in a comprehensive series of articles on latent print development techniques written by Pat A. Wertheim, Director of Training for Forensic Identification Training Seminars, Ltd. Watch for future articles in each issue of Minutiæ and save these issues in a binder to compile a manual of fingerprint development techniques.

Processing adhesive surfaces for latent prints may be intimidating for a technician not often faced with surfaces such as tape, adhesive labels, or bumper stickers. But the sticky side of tape is one of the easiest and most productive surfaces to print ever encountered.

A method first reported in 1981 is dye staining with gentian violet, also known as crystal violet. See Identification News, February, 1981 for an article by Takashi Arima.

Gentian violet is generally purchased as a powder and dissolved in water prior to use. A solution of 0.1% (1 gram powder per 1000 ml water) is recommended. Although many technicians simply dissolve the powder in tap water and report fair results, the staining process works more dependably if distilled water is used and the solution is adjusted to a pH of 7 or 8 by the addition of a small amount (sometimes just a drop or two) of ammonia.

The tape is soaked in the solution for several minutes, then rinsed under running tap water. The dark purple dye stains sebaceous material and skin cells which have sloughed off of the person's fingers and palms and which have stuck to the adhesive side where the tape was touched. To the naked eye, the stained fingerprint appears to have purple ridges, but under a microscope, each ridge may be seen to be composed of tens of thousands of tiny stained skin cells.

The gentian violet process runs into problems on dark tape, however. A gentian violet print, even though unseen, may be transferred to fixed photographic paper by first wetting the paper, then placing the adhesive side of the tape on the paper, and lightly ironing the tape. However, many technicians prefer the small particle reagent method discussed below to dye stain transfer for dark tape.

A more recent method than gentian violet is the use of powder in a suspension of detergent and water. The first powder widely used was "Sticky-Side Powder," which is still the favorite of many examiners. This powder is purchased dry. One part powder, one part tap water, and one part Photo-Flo are mixed together to form a thick suspension solution, iridescent black in color and the consistency of buttermilk. This suspension may be applied to the adhesive surface by any of several methods.

One excellent method is to paint it on with a camelhair brush. Another method is to drag the tape through the suspension in a very shallow dish. Another method is to shake the suspension to a foamy state and move the tape through the foam. However done, the tape should be rinsed under running tap water within a minute or so of the application.

Many people report better results if the tape is rinsed in clean water before application of the Sticky-Side suspension. Prior rinsing lightens the background noise without significantly affecting the latent print itself.

For processing on dark tape, such as black electrical tape, substitute Small Particle Reagent powder for Sticky-Side Powder in the above formula. Sticky-Side Powder is too dark to be seen clearly on the black tape, but Small Particle Reagent has a lighter component which is visible.

Once developed, whether by gentian violet or by a powder suspension, prints on adhesive surfaces should be dried and photographed. Following that, the tape may be applied to a clear piece of plastic, such as a piece of a zip-lock bag, to protect the print.

If more than one process is desired, gentian violet should be used first. Any prints developed that way should be photographed, then the Sticky-Side Powder suspension applied. In many cases, however, technicians are choosing to forego gentian violet and use only the Sticky-Side Powder. Whatever the choice, these methods are both easy and productive. Mix some of each and spend a few minutes practicing on some adhesive surfaces around the office, and you will look forward to evidence containing tape, adhesive labels, bumper stickers, and other sticky surfaces.

In the next issue of Minutiæ, Pat will write about Small Particle Reagent.