Abstract: Dried human bones can provide a valuable source of latent print evidence that should not be overlooked. Processing of both unpreserved and chemically preserved human bones demonstrated that black magnetic powder yielded the best latent print developments of the five processes tested.

Introduction

Police laboratories and forensic anthropologists are often presented with human skeletal remains of unknown provenience. Human skulls are especially common and may be the result of antiquity theft [1], grave robbing, recent foul play, or trophies of past wars [2, 3, 4]. For example, in 2001 the first author received multiple cases of skulls and long bones for which the provenience and medicolegal significance was unclear, including a calvarium found in a book bag alongside a rural road, a skull found in a pond, and a nearly complete skull found in a drug suspect’s closet. Each of these cases may be of forensic importance, but only the putative owner of the latter skull has been identified.

Following a presentation of these and other cases at a New York State Association of County Coroners and Medical Examiners (NYSACCME) conference, the question arose as to whether latent prints could be developed on bones in order to determine who has handled the remains. The authors felt that successful latent print processing of bones was conceivable, but the question was, Which process(es) would be most suitable for human bones? No specific literature was found specifically regarding the latent print processing of human bones. However, two technical reports were discovered that relate to the processing of antlers and horns [5, 6].

Latent, by definition, means hidden and not readily visible to the naked eye. As such, processing is required to develop the prints for visualization to allow documentation. With any latent print processing, the technician or investigator is faced with the problem of selecting the process that is most likely to be effective for the surface of the article that is to be processed [7]. Applications of the wrong techniques may result in the destruction of potential latent print evidence [8]. Human cortical bone has a somewhat unique and variable surface, and two surface conditions were utilized in this study – chemically preserved and unpreserved – which demonstrate very different characteristics.

Categorization of Bone Surfaces

In general, latent print examiners characterize surfaces as porous, nonporous, or semiporous [9]. Assigning bone to one of these categories first requires an understanding of skeletal anatomy. The human skeleton is composed of two types of bone – compact bone that forms the hard outer surface, and the underlying highly porous trabecular, or spongy, bone. While at first glance the surface of normal human bone appears smooth except in areas of muscle attachment, cortical bone can be highly textured and is actually quite porous due to vascularization. Thus, for the purposes of latent print processing, normal bone is best categorized as a semiporous surface. However, bone fracturing, animal gnawing, sun bleaching, burial, water exposure, and other environmental factors may erode the cortical bone and further increase its surface porosity. Many types of antemortem skeletal pathologies, such as infections, metabolic diseases, or healed fractures, can also compromise cortical bone. Thus, under such conditions the bone may be categorized as highly porous. Any hope of latent print development is effectively eliminated if the cortex is completely removed and the underlying spongy bone is exposed.

Both normal and abnormally porous bone surfaces can be altered to render them nonporous. Skeletal specimens in museums or teaching institutions are typically treated with one or more coats of a consolidant, such as Acryloid B-72 or polyvinyl acetate (PVA), to conserve poorly preserved bones or to prevent handling damage [10, 11]. In the private sector, trophy skulls are often coated with homemade or wood-based preservatives such as shellac. Trophy skulls may also be painted, dyed, or inked [2], which may also dramatically alter the normally porous nature of bone, especially if applied in multiple layers.

Materials and Methods

Dry human long bones and skulls were utilized for this study. Prior to the current analysis, approximately half of the 18 teaching skeletons in the first author’s laboratory were preserved with Acryloid B-72 (ethyl methacrylate), a thermoplastic resin produced by Röhm and Haas, in order to reduce trauma due to long-term handling. Acryloid B-72 is also known as Paraloid B-72 in the United Kingdom. This consolidant produces an artificially smooth, glossy, slightly yellowed and nonporous cortical surface. Prior to experimental print deposition, the bones were washed with dish soap and warm water to remove any previous latent print deposits.

Latent prints were deposited upon the smooth surfaces of both preserved (nonporous) and unpreserved (semiporous) femora and skull vaults. To ensure consistency, all prints were deposited by the first author via natural secretions from the hand. To test temporal effects, latent prints were applied to different sets of femora and skulls at five weeks, three weeks, and a few minutes before processing. The bones were allowed to sit, undisturbed, in the laboratory pending the processing. A total of 9 skulls (5 unpreserved, 4 preserved) and 12 femora (5 unpreserved, 7 preserved) were tested.

The various processes utilized for this study all began with a visual examination with the assistance of an alternate light source (SPEX Crimescope CS-16 utilizing wavelengths of 415 nm to 670 nm, and white light). No remarkable latent print observations were noted during the initial visual examinations. Further processing methods utilized included cyanoacrylate fuming, Ardrox with additional alternate light source examinations, black powder, black magnetic powder, and ninhydrin.

Results

A discussion of the results for each method follows. Table 1 provides a detailed summation of the results, though it does not ref lect processing sequence.

Cyanoacrylate Fuming

Cyanoacrylate fuming was accomplished by using a Print Pouch available from the Lynn Peavy Company. For fuming, the preserved and unpreserved bones were placed within a glass fuming chamber with one Print Pouch and a container of warm water for a period of approximately 30 minutes. After fuming was complete, the bones were visually re-examined with the use of the Crimescope. No latent print developments were visible after fuming alone.

Ardrox

Some of the bones that had been subjected to cyanoacrylate fuming were then subsequently treated with a methanol mixture of Lightning Ardrox P133D and rinsed. Examination of the Ardrox treated bones with the Crimescope failed to yield any visible latent print developments on either preserved or unpreserved bones. However, the light and slightly yellowish color of the preserved bones could have interfered with the visualization of any Ardrox developed latent prints. Given that the cyanoacrylate fuming and Ardox treatments did not yield positive results in the first group tested, these methods were abandoned and other processing techniques were utilized on the remaining samples.

Black Powder

Using a standard fiberglass brush, Lightning black powder was applied to the other set of bones that underwent initial cyanoacrylate fuming as well as bones that were not fumed. Some areas of print development were noted. However, the powder also enhanced the surface texture of both preserved and unpreserved bone, which interfered with the visualization of the latent prints (Figure 1).

Figure 1

Recently deposited latent prints on a section of a preserved
femur after dusting with black powder.

Magnetic Powder

Lightning black magnetic powder was applied to cyanoacrylate fumed and unfumed bones with the use of a magnetic wand. Recently deposited latent prints on both preserved and unpreserved bones were readily developed whether the bones were fumed or not (Figure 2). There appears to be an inverse relationship between identifiable latent print development and time elapsed since print deposition. A difference was also noted between the quality of latent print developments on preserved versus unpreserved bones that were deposited at the same time in that the preserved bones appeared more conducive to latent print retention and development (Figure 3).

Ninhydrin

With regard to the ninhydrin processing, only one section of an unpreserved skull vault was utilized due to the fact that preserved bone was considered to be a nonporous surface. A petroleum ether based ninhydrin (2,2-dihydroxy-1,3-indandione) solution was applied to the vault where latent prints had been deposited at nine weeks, one week, and a few minutes before processing. Once treated with ninhydrin, the skull was allowed to dry and sit at room temperature and humidity for a period of eight hours. No latent print developments were noted. The skull was then subjected to high humidity, but still no developments resulted.

Figure 2

Recently deposited latent print upon an unpreserved skull vault
after dusting with black magnetic powder.

Figure 3

Preserved (left) and unpreserved (right) skulls after having
been dusted with black magnetic powder.

Discussion and Recommendations

Dry human bone was more amenable to latent print development when coated with a preservative, presumably because it reduces the normal surface porosity of the cortex. However, many of the developed prints were still insufficient for comparison. Although the time elapsed between print deposition and development appears to be an important variable, on the whole, older latent prints on preserved bones were still more visible than recently deposited prints on unpreserved bones. We also determined that standard tape-lifting procedures were not useful because the tape also picked up the normal striations of the bone, which potentially masks friction ridge detail. We therefore recommend photography as the primary documentation technique.

Black magnetic powder easily outperformed cyanoacrylate fuming, Ardrox, black powder, and ninhydrin. Although black magnetic powder is the preferred method in this study, it is likely that other colors could produce comparable prints, because we feel that the magnetic properties of the technique are what make it useful. Unlike regular powder, magnetic powder did not develop or enhance the natural surface texture of the bone.

Our results are highly congruous with those of Otis and Downing [5], who evaluated latent print processing techniques on antlers, which is even more porous and textured than human cortical bone. These authors also concluded that magnetic powder dusting produced the highest quality prints and that the results of cyanoacrylate fuming and granular (nonmagnetic) powders on antlers were not nearly as favorable.

The forensic implication of this study is clear. If forensic scientists can process human bones (both preserved and unpreserved) prior to extensive forensic study, it is possible to obtain the print of a previous handler who may have potentially deposited the bone(s). It should be noted that latex gloves can smudge or fully erase existing latent prints, so all handling by forensic personnel should be minimized prior to print processing. We do not recommend pretreating unpreserved bone with preservatives (e.g., Acryloid B-72) prior to print processing, because this will not increase the integrity of the latent print since it was initially deposited on the porous or semiporous bone surface.

With the exception of magnetic black powder, studies of latent processing chemicals on PCR-based DNA typing suggest that there is little or no effect of the reagents applied in this study on the ability to obtain DNA profiles of bloody fingerprints deposited on a variety of substrates [12]. However, it is currently unclear what degrading effect, if any, such chemicals may have on skeletal nuclear or mitochondrial DNA that may be required for personal identification of the remains. Though the outer surface of the bone is typically removed prior to genetic analysis, coating treatments (e.g., cyanoacrylate, ninhydrin, and Ardrox) may penetrate the cortex. Therefore, we suggest removing a bone sample prior to any chemical treatment. Similarly, the effects of the latent print processing reagents on both standard and accelerated mass spectrometry (AMS) radiocarbon dating methods are unknown. Since radiocarbon dating is often the only technique available to assess the antiquity of unidentified human remains, we recommend that investigators conduct appropriate bone sampling prior to latent print processing.