The Other Divisions a Laboratory Holds

In one line

The remaining comparisons all ask the same question and all answer it at the level of a class, except where a physical fit is possible.

In the wording a student can write in an exam: the remaining work of a forensic science laboratory consists of comparisons of marks and materials: tool marks, which are impressions or striations left by an implement and are compared on the comparison microscope; tyre and footwear impressions, which carry class characteristics from manufacture and individual characteristics from wear and damage; glass, paint and soil, which are compared by their physical and chemical properties; and specialised work including wildlife forensics; each of these establishes a class unless a physical fit or an individual characteristic is present, and each requires a control sample taken at the time.

Tool marks

What they are. Impression marks, made by a tool pressed into a softer surface, which reproduce its shape, as when a screwdriver is forced into a door jamb. Striated marks, made by a tool moving across a surface, which reproduce the microscopic irregularities of its edge, as when a wire is cut or a padlock hasp is sawn.

How they are examined. Test marks are made with the suspect tool in a material of similar hardness, at varying angles, and the questioned and test marks are compared on the comparison microscope in exactly the way fired bullets are.

What they establish. The class of tool from an impression mark: a flat blade of a stated width, a cutting edge of a stated form. The individual tool from a striated mark, where the edge carries enough accidental damage, and this is a genuine individualising result.

Their limitation. A new tool has few individual characteristics, and a tool changes with every use, so a comparison is meaningful only where the tool is seized soon after the offence.

Tyre and footwear impressions

Recovery. Photographed with oblique lighting and a scale, and then cast where the impression is three dimensional, or lifted where it is two dimensional on a surface. Photograph first, because a cast may fail.

Class characteristics. The tread pattern, the size, and the manufacturer's design, which identify a make and model and give a shoe size or a tyre specification.

Individual characteristics. Wear, which is peculiar to the user's gait or the vehicle's alignment; cuts, nicks and holes; and objects held in the tread.

What they establish. The class from the pattern; the individual shoe or tyre where enough accidental features correspond, which again is a genuine individualising result.

And a second use. A track of impressions gives the direction of travel, the stride length and something about the speed and manner of movement.