jimtrue.com : school : CJT1110 : CH09: Impression Evidence
Posted by Jim True on February 17, 2003 6:00 AM. Last Updated October 22, 2006 9:23 PM
Disclaimer for all material noted here is at the bottom of this web page.
Three types of Shoe Impressions you can find at a crime scene:
Latent - Invisible
Patent - Visible (Electrostatic Dust Lifter)
Plastic - Impressed/Impression
Photograph at 90 degrees, perpendicular
When we photograph will have a scale (scale measurement; L Square (90 degree). Reason for the circles is to enforce exact reproduction.
Will cast for 3-dimensional representation, and exactly to scale. Side characteristics are visible in a cast that are not visible in the photograph.
Will also pick up microscopic items that you will not collect from the photograph: hair, fibers, glass, soil.
Electrostatic Dust Lifter; electrical charge on flat surface that will collect dust prints.
Freeform and Form Cast.
Freeform is just poured within the shoe impression. Formed cast keeps the cast material around the impression.
Partial form (a mistake),
what you'll put in the cast as it is setting: your initials, the date, case # and an arrow pointing to north. (you can also use a magic marker).
used to use Plaster of Paris, but does not have tensile stress. Needed a screen or sticks cut into it to keep it from breaking.
Primarily use Dental Stone, Lab rock.
Hairspray to support the impression
Few particles of powder first to fill in the impression slightly.
Ziploc Bags best for mixing in; inexpensive, easier to mix.
Leave the leaves and twigs and water inside the impression.
Use Bio-Foam to impress shoes without taking the shoes.
TOOLMARKS
Pried, jimmied, stabbed, etc.
*Mikrosil (Micro-Sill), remember Silicon, rubber based. Sticks to mix with; hardening agent, and filler. In Florida, most departments use Mikrosil.
In the Northern States, use Dip Pack (shaped like a pound of butter, cut like a knife). Like a candle, more silicon wax. Cannot be used in Florida because it melts.
In Northern States, Mikrosil freezes solid.
First part: the Mikrosil, and then the hardener, lengthwise (shiny fingerprint card so it will peel off, white.
Don't ever take the tool and put it back into the toolmark.
Pitches are key to comparison. Sipes (makes the groove in the tire).
Disclaimer: These are MY notes taken from classroom lectures while I'm in the classroom. While I'm perfectly happy to share my notes with my classmates and I know I take very good notes, you should still make every effort to attend the class and TAKE YOUR OWN NOTES. I will not transcribe everything the instructor says in the classroom, and I will NEVER post pre-exam reviews. My notes will not replace the value of actually attending class and taking your own class notes.I also cannot attest to their accuracy, other than they are what was provided in the lecture; you should not reference my notes as "expert opionion" by any means, and if you notice an error or omission, please do me the favor of e-mailing me with the correction and I will re-post my notes. End of Disclaimer.