Transcript Tool Marks and Impressions

Tool Marks and Impressions
Tool Mark
• Any impression, cut, gouge, or abrasion
caused by a tool coming into contact with
another object.
• Usually found at burglary scenes or
scenes of forced entry
Comparing Tool Marks
• Indented impression is left on the frame of
a door or window as a result of prying with
a screwdriver or crowbar
• Close examination can reveal class
characteristics or the size and shape of
the tool
• Usually can’t connect the marks to one
tool unless there are random nicks or
breaks that the tool has acquired
• The edges of a pry bar, chisel, screwdriver,
knife or cutting tool display microscopic
irregularities that look like ridges and
valleys
• Left as a result of the machining process
that is used to cut and finish tools
• Damage and wear of the tool can also
make imperfections
• The variety of patterns among these tools
makes it highly unlikely that any two will be
identical
• We can use these imperfections to identify
individual tools
• When a tool is scraped against a softer
surface it may cut a series of striated lines
that reflect the pattern of the tool’s edge
• Compared using a comparison
microscope with test tool marks made by
the suspected weapon
Problems with Tool Mark
Comparisons
• Difficult to duplicate the tool mark left at
the crime scene in the laboratory
• Must make test tool marks at various
angles and at different pressures
Collecting Tool Mark Evidence
• Then entire object or part of the object bearing
the tool mark should be submitted to the
laboratory as evidence
• When you can’t remove the tool mark from a
crime scene you resort to taking a picture of it
and make a cast of the mark
• Liquid silicone is the best material to use for
casting
• A lot of detail can be lost when you are forced to
use a photograph and a cast
• Never attempt to fit the suspect tool into the
tool mark this could alter the mark and
question the integrity of the evidence
• Suspect tool and mark are packaged in
separate containers to keep from
destroying their individual characteristics
• Can be trace evidence like paint chips on
the tool or impression
• Must be careful when handling and
packaging the evidence so you don’t lose
any potential trace evidence
Impressions
• Can be a shoe, tire, or fabric impression
• Preserving impressions
– Impression is photographed first from the top and at
various angles and in relation to the crime scene
• Lifting impressions
– When an impression is found on a surface that
cannot be submitted to the laboratory we can lift the
impression
Lifting Impressions
• Place a lifting material over entire
impression and use a fingerprint roller to
eliminate any air pockets before lifting the
impressions
• Can use a portable electrostatic lifting
device
Portable Electrostatic Lifting Device
Continued . . .
• Place a sheet of mylar film on top of
impression and film is pressed against it with
a roller
• High voltage electrode of the electrostatic unit
is placed in contact with the film and the unit’s
earth electrodes are placed against a metal
plate
• Charge difference develops between the
mylar film and the surface below the
impression so the impression is attached to
the lifting film
Electrostatic Lifting Film
Electrostatic Lifting Device
Casting Impressions
• Shoe and tire marks impressed into the
earth are best preserved by photographs
or casting
• Class I dental stone is the best material to
use for casting
• Cast has to air dry for 24 to 48 hours
before it is shipped to the forensics lab
Casting Footwear Impression Evidence
(Fig. 7)
(Fig. 8)
Pouring Procedure
Place a form around the impression, if needed (Fig. 8)
Pour the mixture onto the large spoon or stirring stick while
holding the spoon or stick to the side of the impression.
(Fig. 9,10) Allow the impression to dry a minimum of 30
minutes. Pertinent information, case number, date, time,
location, etc. can be written on the cast after it is dry. (Fig.
16)
Carefully remove the form from the cast. (Fig. 11) The cast must be carefully lifted. (Fig. 12) Place a knife one inch from the
form and stick it directly into the ground under the cast. Gently pry upwards. This should allow the cast to break free cleanly
from the surface.
Carefully remove the form from the cast.
(Fig. 11) The cast must be carefully lifted.
(Fig. 12) Place a knife one inch from the
form and stick it directly into the ground
(Fig. 11) cast. Gently pry upwards.
(Fig. 12)
under the
This
should allow the cast to break free cleanly
from the surface.
Do not clean the cast until has been allowed to dry 24-48 hours. (Fig. 13)
Package the cast in a box or other porous packaging material. Package it
securely to ensure that it is not broken during transport. Do not package
a cast in plastic.
Pertinent information, case number,
date, time, location, etc. can be
written on the cast after it is dry.
Comparing Impressions
• Only possible when an item suspected of making
that impression is recovered
• Test impressions may be made for comparison
• Look for class characteristics: Size, shape and
design
– Tell you if the impression was made by a particular
shoe, tire or garment
• Then look for individual characteristics: wears,
cuts, gouges, or other damage
– Can narrow down the impression to one source
SICAR
• Shoeprint Image Capture and Retrieval
• Shoeprint database
• Searches known and unknown footwear
files for comparison against footwear
specimens
• Can tell you the type of shoe
• Also compares shoe print to suspect shoe
prints from other crimes to see if they are
a match
Bite marks
• Human bite mark impressions on foodstuff
and skin is another type of impression that
can be used for comparison