Double Casting

> Double Casting Impression Marks

Establishing an identity from impression marks

> Method

First, cast the interesting surface areas of the tool with SP Impgregum.

Once the Impregum has solidified, apply MIKROSIL on top of it.

Use the same colour MIKROSIL as used at the crime scene. The double casting can now be compared with the crime scene casting under a microscope.

> Use the right type of silicone

When using Impregum for the first casting, the second casting can only be made with a Type C silicone, like MIKROSIL.

Type A silicones (silicones applied using mixing pistols) do not settle on top of Impregum, as shown in the image on the right.

In the image on the right, we show two different double casting results. The left hand result is from using Impregum with MIKROSIL brown and the right hand result is Impregum with a brown Type A silicone.

> What’s the smallest toolmark MIKROSIL can identify?

In this case, a circular saw with a drill at the center was examined to see whether it had been used to pry open a metal security box.

The marks on the recovered piece of metal showed details from being bent up by this type of tool.

Kjell used MIKROSIL to cast the center hole that had been created during the forced entry of the security box. The casting showed a small impression mark at the point where significant force had been applied to bend open the box. The size of this mark was roughly 0.3 x 0.5 mm.

A double casting was made to compare the drill’s surface with the impression marks on the security box. The super imposition technique applied during the microscope comparison showed many matching details between the double casting of the drill and the casting of the drill hole.

> Homemade Calthrops

In a nighttime robbery, a team of burglars used homemade calthrops to prevent police from intervening. These calthrops appeared on different parts of the road network at once.

Many of the recovered calthrops showed clear indication of having been put in a vice. The vise was probably used to bend the rebars.
Unusual nighttime cell phone activity led investigators to a large workshop which was normally closed outside business hours.
To begin, the examiners made a double casting of each of the three vices in the building. They applied the standard double casting method described above (SP Impregum followed by MIKROSIL) and to be sure made two double castings of each vice.
Next they made castings of calthrops that had visible vice impressions. The grey images show comparisons of impressions found on different calthrops and the contact surface of the vice.