Casting in criminal investigations
Casting in criminal investigations refers to the process of creating three-dimensional models of impressions left behind by footwear, tires, or tools at crime scenes. This technique is significant in forensic science as it enables investigators to develop permanent records of marks, which can be compared with items found in the possession of suspects. Perpetrators often leave identifiable traces, such as tire tracks or footprints, in various environments like dust, mud, or snow.
The casting process typically involves using materials like dental stone, which allows for detailed capture of the impression without damaging it. In cases involving snow, specific techniques and materials, such as Snow Print Wax and prill sulfur, are utilized to stabilize and capture fragile impressions. Tool marks, which can be more complex to analyze due to the potential changes in their appearance over time, can also be cast using silicone materials. Ultimately, casts serve as crucial evidence in court, helping to establish connections between suspects and crime scenes.
Subject Terms
Casting in criminal investigations
DEFINITION: Production of three-dimensional models of impressions left by footwear, tires, or tools at crime scenes.
SIGNIFICANCE: Forensic scientists use casts, permanent physical records of marks left at crime scenes, to compare with the vehicle tires, footwear, or tools found in the possession of possible suspects.
Perpetrators often leave traces of their presence at crime scenes in the form of tire impressions or footprints in dust, soil, mud, or snow. The tools used by perpetrators also leave distinctive impressions, such as the marks left on a door by a crowbar. is a way of making permanent three-dimensional records of these impressions. Because tires, footwear, and tools do not wear evenly, they develop unique use patterns. Using a cast made from an impression found at a crime scene, a forensic scientist can compare the unique wear pattern shown by that impression with the wear patterns on the possessions of any suspects. Casts often show marks with a degree of accuracy that either confirms or eliminates the presence of some persons at the crime scene. In court, casts are used as physical to show a link between the accused and the crime.
Casting in Soil
Castings in mud, dirt, and sand are made with a product known as dental stone. Dental stone creates a crisper, more detailed cast than does plaster of paris or other plasters and is less likely to be damaged during cleaning. Dental stone comes in powder form, and water is added at the to make a runny, batter-like mixture that is used to fill the impression. Photographs are taken to locate the impression within the scene, and a plastic frame is placed around the impression before the cast is made. In loose soil or sand, the impression may be sprayed with a chemical that hardens the surface before casting.
To prevent distortion of the impression, the technician fills it with dental stone gently. After the impression is filled, the dental stone is allowed to harden for at least half an hour in warm weather and longer in cold or very humid conditions. Before hardening is complete, the technician inscribes the cast with identifying information. Once the cast has solidified, it is removed from the soil and is allowed to air-dry for several days, after which it is cleaned of clinging soil and examined.
In 2021, a study published in the journal International Journal of Research Publication and Reviews proposed using a mixture of melted, shaved soap pieces to make casts from imprints in wet mud.
Casting in Snow
Casting in snow creates special problems because of the fragility of the impression. This is especially true when the impression has been made in very dry, nonpacking, or windblown snow, or in snow that has begun to melt. Two techniques can be used to cast in snow. In one, before the dental stone cast is made, several layers of a product called Snow Print Wax are sprayed over the snow to stabilize the impression. The impression is then cast in the same way as an impression in soil. Cold conditions substantially slow the hardening of the cast, and it may be several hours before the cast can be removed from the snow.
Another way of casting in snow involves the use of Snow Print Wax and prill sulfur, a pellet form of sulfur that is a by-product of natural gas refining. The pellets are melted and then cooled so that the sulfur will not melt the snow. The impression is first sprayed with Snow Print Wax and then filled with the melted sulfur. Pouring the sulfur when it is at the correct temperature—not hot enough to melt the snow yet not so cold that it starts to form crystals before it is poured—is a critical step in creating a good prill sulfur cast. After about twenty minutes, the cast is hard enough to remove. Because prill sulfur casts are brittle and easily broken, they are often embedded in a protective layer of dental stone. The Royal Canadian Mounted Police tested both methods of casting in snow and found that prill sulfur casting produced sharper, more detailed casts than did Snow Print Wax casting alone.
Casting Tool Marks
Tools that are used to pry, scrape, cut, or drill hard surfaces leave marks that can be cast. For example, a hammer hitting a nail leaves a reproducible and distinctive mark. When tool marks found at a crime scene are on objects too large to move to the laboratory (such as on a safe or a door frame), photographs are taken to locate the marks within the scene and casts are then made with sprayable silicone rubber or a similar silicone resin. This material dries quickly and accurately reproduces the unique indentations and ridges made by tools. Tool marks are more difficult to match than are tire or footwear marks, and the marks made by a tool may change over time if the tool is heavily used.
Bibliography
Bodziak, William J. Footwear Impression Evidence: Detection, Recovery, and Examination. 3d ed. Boca Raton, Fla.: CRC Press, 2013.
Du Pasquier, E., J. Hebrard, P. Margot, and M. Ineichen. “Evaluation and Comparison of Casting Materials in Forensic Sciences: Applications to Tool Marks And Foot/Shoe Impressions.” Forensic Science International 82, no. 1 (1996): 33–43.
Hilderbrand, Dwane S. Footwear, the Missed Evidence: A Field Guide to the Collection and Preservation of Forensic Footwear Impression Evidence. 2d ed. Wildomar, Calif.: Staggs, 2005.
"How It’s Done." Forensic Science Simplified, www.forensicsciencesimplified.org/fwtt/how.html. Accessed 13 Aug. 2024.
James, Stuart H., and Jon J. Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 4th ed. Boca Raton, Fla.: CRC Press, 2014.
Malviyaa, Akshita, Anamika Sharmab, and Pinky Nishadc. "New Technique for Casting Three-Dimensional Shoeprint in Wet Mud." International Journal of Research Publication and Review, vol. 2, no. 9, 2021, pp. 349-352, ijrpr.com/uploads/V2ISSUE9/IJRPR1258.pdf. Accessed 13 Aug. 2024.
McDonald, Peter. Tire Imprint Evidence. Boca Raton, Fla.: CRC Press, 1993.
Perin, Michelle. “Casting in Snow.” Law Enforcement Technology, vol. 41, no. 11, 2014, pp. 22–28. Associates Programs Source Plus, search.ebscohost.com/login.aspx?direct=true&db=p6h&AN=99493058&site=eds-live. Accessed 3 Jan. 2017.