Collecting Impressions as Evidence

Impressions can be made by: Shoes and footprints Tires and tire tracks Tools Teeth

Collecting impressions Priority on preserving the impression and/or its reproduction. Photograph first, with scale, to show all observable detail Shoot various angles Use side-lighting Show position in relation to scene

2 types of impressions: Negative impressions – made by an object leaving a void in the substrate, i.e. stepping in a pool of blood and leaving a footprint in it Positive impressions – made by an object leaving residue on a surface, i.e. leaving behind blood or dirt as one walks

Collecting impressions Preferable to collect the original impression (i.e. on glass, paper, floor tile) If the impression cannot be taken to the lab, a reproduction must be made. Lifting tape Casting dental stone Electrostatic dust lifter Snow-Print-Wax Chemical developers

Lifting tape – use as with fingerprints Use lifting material large enough to cover whole print Good for lifting light dust or dirt Dental stone – gypsum mix, like plaster, that hardens into a solid 3-D model

Electrostatic dust lifter – Uses electrical charge and mylar sheets to create a static electrical charge that will lift dust/dirt marks Snow-Print-Wax – used for casting prints in snow – creates a wax layer to protect the print from the plaster

Chemical developers – As with fingerprints, ninhydrin and amido black can be used to enhance the appearance of prints.