Bullet Hole Testing Kit - BTK3

The identification of bullet holes and of marks produced by projectiles may be very important in the investigation of criminal cases involving the use of firearms. Such bullet-holes are usually characterized by the presence of metals, originating from the bullet's outer surfaces, in the margins of these holes. Numerous articles studied this phenomenon, using laboratory analytical methods, like micro-beam X-ray fluorescence (µ-XRF), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) or atomic absorption spectrophotometery (AAS). Chemical spot tests and color reactions, like the sodium rhodizonate test (SRT), were also applied for the analysis of metals in bullet-holes margins, but again – mainly in laboratory use and not in scenes of crime.

In the early 1980's, a novel (at that time) field kit for the identification of bullet-holes was developed in the DIFS [1]. The kit was based on two chemical color reactions, the SRT for the presence of lead and the rubeanic acid test (RAT) for copper. This bullet-hole testing kit (BTK) has been used in numerous shooting cases, by DIFS mobile laboratories and field technicians, for the past 25 years or so.

The original version of the BTK was composed of four components: a solvent for lead (acetic acid), a color reagent for lead (aqueous solution of sodium rhodizonate), a solvent for copper (ammonium hydroxide solution) and a color reagent for copper (rubeanic acid solution in alcohol). The color reaction took place on filter paper (Whatman Benchkote paper), wet with the solvent and pressed manually for about a minute against the questioned hole. The appropriate reagent was then applied to the filter paper. The developed color ring, purple for lead and dark green for copper, is indicative to bullet-holes. Since the sodium rhodizonate is unstable in aqueous solution, and the stability of the rubeanic acid solution was not clear, both these reagents were kept as dry powders, separated from their solvents, in disposable plastic tubes, while the solvents (water for the SRT and alcohol for the RAT) were sealed in glass ampoules inside the tubes. The solvents for lead and copper (acetic acid and ammonia) were sealed in small drip bottles.

The test paper is pressed against the hole for about a minute, then the reagent for copper, rubeanic acid, is applied. The plastic tube is marked marked green-black.

A green-black colored ring at the hole’s margins is indicative to bullet-holes (usually of brass FMJ bullets).

This original version of the BTK suffered from various deficiencies. The drip bottles containing the solvents tended to leak, and the reagents plastic tubes were relatively expensive to manufacture. Based on the experience accumulated with the use of the BTK, an improved version ("BTK2") was used in the DIFS since the 1990's, with both solvents and the RAT reagent packed in spray cans. Due to the unstable nature of the SRT reagent, its disposable plastic tube was kept intact. Nevertheless, technical difficulties were encountered even with this version of the kit – the spray cans tended to rust and leak.

Recently, a new version of the BTK has been developed in conjunction with the DIFS, the BTK3. This kit is based on the same chemical reactions for lead (SRT) and copper (RAT), but in a much more reliable and "user friendly" form. It is based on disposable plastic micro-tubes (Porex Porous Products Group), containing crushable glass ampoules, for all four chemical reagents. The acetic acid, ammonia and rubeanic acid solutions are sealed in the glass ampoules inside the micro-tubes, while the sodium rhodizonate is kept as a dry powder, separated from the water. The tubes are secured in a durable plastic carrying case, lined with polyurethane foam. Each case contains 10 disposable kits (4x10 tubes), in addition to test papers, detailed operation instructions, Zip-Loc plastic bags and reporting forms.

To the best of our knowledge, the BTK, and its new version BTK3, are the only operational field kits, used by forensic science professionals, for the identification of bullet-holes. The kit has been also applied for the reconstruction of shooting incidents [2] as well as for the estimation of bullets' diameter [3]. The BTK3 was designed for long-lasting heavy-duty field use, bearing in mind the conditions developing in mobile laboratory vehicles during the Israeli summers and winters.