Basic Toolmark Methods

> Toolmark Methods

What are striation, impression and reference marks? This short introduction to toolmark examination covers the most basic elements

> Common Evidence

Toolmarks are very common evidence at crime scenes.

When a door is forced open by a screwdriver or a crowbar, high quality marks are often left behind and can help the investigator identify the tool that was used.

> Striation and Impression Marks

There are two types of toolmarks which are valuable when establishing an identity: striation and impression marks.

Striation marks are like scratch marks. Over time, the contact area of tools like screwdrivers, knives or crowbars is worn down by friction, creating a unique pattern on the surface. Striation marks are created by the tool as a result of these surface damages.

Impression marks are like still images of the contact surface of the tool. In the case of a crowbar forcing open a door, an impression mark forms when the crowbar digs into the door frame to apply the last bit of force before opening the door.

> Reproducing Striation Marks

To investigate whether a tool was used to make striation marks at the crime scene, we create reference marks using the suspected tool.

Reference striation marks can be made with lead plates, but our preferred alternative is to use a wax plate. Most often, wax plates reproduce microscopic details with higher quality.

The images to the right show an example of how to create a reference mark on a wax plate which is then cast by MIKROSIL and compared under a microscope with the toolmark from the crime scene.

> Double Casting Impression Marks

With impression marks, we establish a reference by using the double casting technique, read more about it here.

> Case Examples

> Sliced Tyre

We want to see whether a seized knife was used to create a hole in a tyre.

First, the tyre hole is cast with MIKROSIL, and then the knife is used to make a comparison hole in the tyre. The hole is also cast.

Once both castings are made, they are compared under a microscope.

> Pistol Hammer

It is possible to deduce whether a specific gun was used to fire a recovered bullet casing. The seized gun is fired to create a test casing for the comparison and then the percussion caps of the two casings are cast using MIKROSIL. If the same gun fired both bullets, the indentations made to the percussion cap by the hammer should be near identical.