Research and Development of Impression Evidence.

摘要

The collection and preservation of tool mark and impression evidence is an important part of the field of criminalistics. The ability to discern class and individual characteristics and use these to either identify or exclude an item as a possible match is a powerful tool in a criminalist's arsenal. The ability, then, of a casting agent to resolve the fine scale details of a tool mark or impression becomes of the utmost importance. An innovative approach to this problem is utilizing Magneto-Rheological (MR) fluids as an agent to capture the impression in situ. These materials are fluid under most conditions, but form a solid when a magnetic field is applied to them and can be used in lieu of Dental Stone or Mikrosil for collecting impression evidence. The goals of this project were to establish an optimal formula composition of an MR casting fluid, determine the limitations and benefits of using MR solutions as a casting fluid, and finally, if successful in the prior goals, develop a method of creating 3D images for permanent storage of the casting impressions. The purpose of this project is to provide a new method for preserving evidence that provides higher resolution casts, which does not need to be prepared at a crime scene, and which has a lower cost than conventional techniques. The trial MR fluid compositions were developed through trial and error by adjusting the concentration of the components in the fluid. Of the trial solutions prepared for this project, the optimal MR fluid was comprised of 25.0 g 325 mesh iron, 0.75 g cellulose, 0.75 g sodium nitrite, 0.15 g sodium chloride, 0.5 g Silicon Dioxide, and 10.0 mL-distilled water. This solution created long lasting, durable, and high resolution casts, which enabled the visualization and analysis of small details not discernible on the original object. With every MR solution, the casting substrate needed to be non-porous so that the solution wasn't absorbed by the substrate, and non-magnetic so the solution was not affected by the magnetic field of the substrate.