By: Don Penven
Recovering toolmark impression evidence can be a valuable contribution toward successful prosecutions—if it is done properly.
Toolmarks are frequently apparent at points of entry occurring during a burglary, but this form of impression evidence covers a number of other 3-dimensional impressions uncovered at crime scenes—most notably—bites marks.
When forced entry into a structure is evident, tool marks may be present. Of course some types of forced entry rely on brute force, i.e. a shoulder or foot against a door, a rock through a window or a motor vehicle rammed into a storefront. We’ll deal with toolmarks and the impressions they leave. Bite-marks are recovered in the same manner as toolmarks.
Toolmarks are recorded/recovered by three different means:
- Photography
- Casting
- Physically removing the entire toolmark from the structure by cutting it out or submitting the door or window to the crime lab.
Photography
Regardless of which of the other means of recovery are used, all such impression evidence must be photographed prior to any other recovery method.
- Take a few overall photos of the impression from several feet away in order to show its relationship to other nearby 0bjects.
- Place a photographic scale adjacent to the impression, but not obscuring any part of it.
- Take several close-up shots using different angles of light. (A flash attachment with a remote cable is a must.)
Casting the Impression
Due to the very nature of tool and bite marks, any sort of plaster or dental stone is not the casting medium of choice. These impressions require a casting medium with a degree of flexibility. Silicone, in its many forms, has proven to be the best possible medium. Most of the commercially available silicone formulas use Polydimethylsiloxane as their basis. Silicone casting materials are available in liquid, paste and putty formulations. Each of these formulas requires a catalyst to activate the hardening or set-up reaction. These three different silicone formulations are covered below.
- Liquid Silicone Rubber: This casting compound is a mixture of quite a number of ingredients: Get the MSDS Here: This formulation is white in color and may be used on everything from toolmarks to footprints in soil or snow. The catalyst for liquid silicone is clear in color. This formulation should be mixed in a bowl or similar container. The liquid is best used on flat, horizontal surfaces.
- 2. Silicone Paste
A. Mikrosil is a product imported from Sweden. It is a paste and is therefore provided in toothpaste-like tubes. Its catalyst is blue in color. Mikrosil is available in white, brown , gray and black. Mikrosil is mixed on any flat surface. The distributor of this product supplies vinyl plastic strips as mixing surfaces. You May Download the MSDS Here.
B. Polyvinylsiloxane is also a paste, but it is applied using a device similar to a caulking gun. It uses a paste for the base compound and the catalyst is also in paste form. The extruder gun employs special tips that mix equal parts of catalyst and base as the material is applied to horizontal or vertical surfaces. Download the MSDS Here:
This formulation is ideal for use on irregular or vertical surfaces
- Silicone Putty: Durocast is offered in putty-form and thus, it is effective on virtually any surface. This compound is mixed in the palm of the hand. Even though it is a putty, it produces amazing detail. It has been used to lift the embossed printing from U.S. currency. Download the MSDS Here: http://www.sirchie.com/Assets/msds/doc1001.pdf
Technical manuals for these formulations may be found at the links below:
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