Impression Evidence II
3D Impressions
3-D Impressions
Impressed Evidence
Typically found in exterior surfaces
 Shoe deforms surface
Sand/soil/snow – other soft
Vary widely
 Shallow or deep
 Quality varies
 Can have great detail or none
3-D Impressions
Value of 3D Impressions
 Impressions with sufficient detail can be
associated with a specific item of footwear
 Impressions in clay-based soil
Can retain great detail
 Impressions in sand & small rocks
Little detail
 Dry packed snow
Greater detail than wet, melting snow
3D Impression Evidence
 Sketching
 Give lab knowledge where occurred @ scene
 Lab can recreate as closely as possible to original
o Differences in how recreated in lab v scene can affect making
proper match
 Field notes
 Photography
 Necessary for court presentation
3D Impression Evidence
 Most impression evidence has 3D characteristics … surface
 For forensic purposes, 3D impressions have depth in addition to
length and width and commonly found outdoors in a soft or
malleable receiving surface such as soil, sand or snow.
 The quality (detail) varies with,
 The receiving surface’s malleability, texture and composition.
 The detail present in the source origin.
 The mode by which the impression was transferred to the
 The affects of weather: temperature, rain and snow.
3D Impression Evidence
 The scene investigator has no control over how the impression was generated or
its clarity.
 Responsibility: capture detail as completely and as clearly as possible.
 Two activities: 1. Photography and 2. Casting.
Casting defined:
“the filling of a three-dimensional footwear impression with a material that will
acquire and retain the characteristics that were left in that impression by the
 Each archiving technique complements the other
 Photography and casting are not an either or decision. Both are critical and both
must be done in order to properly archive the impressions.
Lifting & Preserving 3D Footwear
Impression Evidence
What to Cast
Indented (Impressed) Dry
Wet, 2D Prints on Concrete
Impressions in Snow
Impressions Covered by Water
Photography -v- Casting
Shows impression as found at the scene
Direct mold of the original impression
Condition and detail
Reproduces microscopic characteristics
Can give best reproduction from coarse
Gives reproduction of sides of outsoles
not reproduced in photographs
A backup for casting
Backup for photography
Casting Footwear
Characteristics of Forensic Quality Casting
Produce very fine detail
Flow easily into impression
Cleaned without loss of detail
Easily obtained with consistent quality &
Easily mixed – not require special equipment
Set in reasonable time
Unlimited shelf life
Quality of Casts
Receiving surface malleability,
 texture & composition
 Sand
 Clay
 Loamy soil
Detail present in the impression
 Mode by which impression transferred to the surface
 Mechanics of making the impression
Effects of weather:
 Temperature
 Rain/snow
 Wind
Gypsum manufacturing processes
110-130 Deg C
Calcium Sulfate Dihydrate
Open Oven
130-200 Deg C
Calcium Sulfate
200-1000 Deg C
Calcium Sulfate
Plaster of Paris
Dental Stone
Requires more water
Irregular Crystals & Porous
Consistency > 50
Requires less water
Uniform Crystals
Dense Crystals
Consistency < 50
Choosing Dental Stone
Several kinds of dental stone … always check the
compression strength measured in psi
(pounds per square inch).
Regular plaster of Paris is 5,000 psi
compression strength and will chip fairly
Hydrocal is around 8,000 psi and is more
Merlin's Magic is around 14,000 psi and is
even more durable yet.
Excalibur and Die-Keen are 18,000 psi
and dry so hard it's almost like a ceramic
Casting Footwear Impressions
Gypsum Consistency vs Compressive Strength (psi)
Plaster per pounds of
Dry compressive
Water-to-Powder (W:P)
Lbs per 100lb plaster
W:P –
The quantity of water (by wt.) per quantity of powder (by wt.)
A “30 Consistency” means – 30 parts of water/100 parts powder
Higher the consistency – longer setting times – lower strength - softer
Mixing Merlin's Magic
 Merlin's Magic: Special type of casting material.
 Made to pour into molds easily with very few air bubbles.
 Mix differently than for regular plaster. Below are instructions to mix up enough
plaster to fill one regular size mold.
First you need to make a measuring cup that can be reused.
Procedure for Merlin’s Magic
 Need two disposable plastic cups - nested.
 Pour 2 ounces (60 ml) of water into the top cup and place a black mark
on the outside of the bottom cup where the water line is.
 Place an additional 2.5 ounces of water into the cup (for a total of 4.5
ounces or 135 ml).
 Place another black mark on the outer cup at the water line.
 Remove the inner cup and you have a reusable measuring cup.
Here is how you mix the plaster
 Insert a new cup into your measuring cup.
 Pour in water until it reaches the first line.
 Carefully shake in the powder until the mixture reaches the second line.
The powder must be absorbed into the water before you can determine if the
second line is reached.
 Remove the inner cup, mix up the plaster and pour it into your mold.
Mixing by weight,
Ounces (or
of water
Amount of
(by weight)
2 ounces
186 grams
4 ounces
372 grams
6 ounces
558 grams
8 ounces
744 grams
 Use table @ right. Need scale to Number of
molds to fill
measure the weight of the powder.
 "ounces" shown here are a
liquid measurement (not weight).
 Mixing instructions on the
package of Merlin's Magic will be
 Their instructions are used
for dental castings, which use
a vibrator to shake thicker
plaster into their dental molds,
which can give mix that is too
thick to pour into a mold.
Casting Using Dental Stone
Photographing the Impression
Proper positioning
of scales
of Retainer
Placing the casting frame in place
Mixing water (3/4 cup/lb stone)
and dental stone
Pouring mixture onto impression
Curing the cast
Allow to sit 30-40 minutes before lifting
Do not remove adhering soil
Final cast
 Allow to cure up 24-48 hours
 Carefully remove soil
 Save Soil for comparisons
Casting Underwater Impressions
Drain or remove excess water – Good – BUT
- Not Necessary
 Pipette or syringe
 Carefully Absorb with paper towel
 Pour casting material as usual
If water can’t be removed?
 Frame the impression
 Sift dry casting powder gently into water above
 Allow to fall to bottom
 Sift until 1” of powder covers the area
 Powder builds up & saturated with powder
 Use additional dental stone slurry to fill the framed area
 Will settle into the water later & into impression – cover entire
Casting in Snow
Casting Procedure
 Spray Impression with Snow
Print Wax or Dust with Snow
Print Powder
 2-3 layers
 Allow to dry for 2-5 minutes
between applications
 Slowly Add slightly cooled
dental stone
 Cooled dental stone
minimizes melting the snow
 Non-Casting Procedure
 Auto primer spray paint
before Snow Print
 Better contrast for
 Photos critical
 Can’t directly cast with
primer paint
 Many snow prints not cast
Snow Casting Procedure
 Spray the impression lightly with the Snow Print Wax at an angle in order to
highlight the raised areas of the impression.
 Do not cover the entire area of the impression. This is the best time to
photograph the sprayed impression.
 Do not hold the spray too close to the impression because the blast from the
aerosol can damage the impression’s detail.
 Spray the impression with 2 or 3 more applications, ensuring that the entire
impression is covered in wax. A properly sprayed impression will not reveal much
detail when viewed from above.
 The red (or other colored sprays) attracts heat:
 Shield the impression from the sun.
 Allow the wax mold to dry for approximately 5-10 minutes.
 Prepare a dental stone slurry and allow it to sit longer than usual so that when the
slurry begins to harden, the heat generated will not melt the impression.
 When the cast is hardening, scratch initials and date into the cast.
 Allow the impression to sit for approximately 60 minutes before lifting.
 After lifting, immediately photograph the cast.
 Snow print casts are fragile, and the wax molding is easily destroyed. Keep the cast
away from sunlight. If melting occurs, cast detail may
3D Scanning
Snow Print Wax
Casting with plaster
3D surface scanning
GOM ATOS II system (Gesellschaft fu¨r Optische
Messtechnik mbH, Braunschweig, Germany).
3D model of the sole
Packaging Casts
Allow cast to dry for 24 hours or longer
 Do not attempt to remove soil or clean until cured
 Will destroy impression
 Soil used for comparison purposes
Loosely & individually wrapped in paper or paper
 Do not wrap in tight plastic
Place wrapped cast in cardboard box & tape
Place in shock absorbent or porous packaging
Illinois State Police
Packaging Procedure for Cast Impressions
 Identification:
Before cast hardens, place CSI or investigator’s
initials, date and case & number on back side of cast.
Amount Desired: Standard -Evidence - Up to 2 ft.
 Preservation:
Use mesh reinforcing and let dry 24 hours before
putting in package.
 Wrapping & Packing:
Surround with packing material in box too prevent
shifting or
breakage. Avoid sealing in plastic bags.
 Miscellaneous:
Dental stone is the preferred casting material. Take
photos of impressions
before casting. Mark package FRAGILE. Do not clean.
Do not use twigs for reinforcement.
Impression Evidence
On-Scene Overview
And Surface
Wet soil or Mud
(fine, even
1. Dust with
black/fluorescent or
bicolor magnetic
Cast using dental
1. Quickly dust with
black/fluorescent or
bicolor magnetic
1. Re-photograph
2. Cast using
dental stone
In dry soil
(Hard, packed or
loose. Varying in
consistency from
coarse to powderfine)
1. Stabilize with hair
spray or lacquer
2. Spray with
automotive primer
paint of appropriate
contrasting color
1. Re-photograph
2. Cast using
dental stone
(variable texture –
retains small
amount of
impression detail)
1. Stabilize with spray
paint (black) or “snow
print wax” or “snow
print powder”
2. Spray with
automotive primer
1. Re-photograph
2. Cast using
dental stone
Adapted from Hilderbrand, Dwane S. Footwear, The Missed Evidence, Skaggs Publishing 1999, pages 61-62.
And Surface
Category of
(Temperature affects
textures, cohesiveness and
impression detail)
1. Stabilize with automotive
primer paint (gray) and/or
“snow print wax or powder”
2. Spray with automotive
primer paint.
1. Re-photograph
2. Cast using dental
stone or Jade Stone
Dry Residue - Dust on
Hard Surfaces
(Produce highly detailed
impressions on hard
1. Lift using electrostatic or
gelatin lifter)
2. Chemical enhancement.
Wet Residue – Dust or
Mud on Hard Surfaces
(Highly detailed
1. Cannot be lifted using
electrostatic lifter. Can be
lifted using gelatin lifter
2. Dust with magnetic powder
(dual color or black)
3. Treat for the presence of
iron or other metals
4. Cyanoacrylate fume
followed by powder dusting or
fluorescent staining (surface
1. Re-photograph
2. Lift with gel lifter
Dust – Fabrics
(Carpet, upholstery &
bedding can produce
detailed impressions within
the fabric nap or on the
1. If dust – lift as above.
1. Re-photograph
2. Cast using Jade Stone
… or
3. Lift with gel lifter
(Can produce high-detail
1. Light surfaces – Stain using
Acid Violet 17.
2. Dark surfaces – stains using
Basic Yellow 7 or DFO.
1. Re-photograph
2. Lift using gelatin
Exemplar Impressions
Exemplar Footwear Impressions
Many people leave impressions @ scene
 Categorize designs of shoes
 Victims
 Witnesses
 Medical personnel
 Other innocent people
 Officers
 Suspects
Known impressions made
 Specialty impression materials available
 Biofoam
Inkless Lifter
Forensic Footwear Databases
 The number of shoe prints at a crime scene can be so large that the process of impression recovery becomes
very time-consuming.
 Commercial product using four parameters—pattern, size, damage, and wear—to identify individual outsole
 Compared with shoe print data from two sources: suspects in custody and crime scenes.
 A match could yield the name, date of birth, criminal record number, places of interest, and similar offenses for
possible suspects.
How does TreadMark™ work?
 Impressions from crime scene obtained using photograph, gel lift, dust lift, and adhesive lift.
 Input directly into the analytical system by high-resolution digital imaging. Same procedure used with
impression of a suspect’s shoe print:
 Operator measures, analyzes, and compares crime-scene and suspect images.
 Both image sources can be searched within themselves and against each other, allowing such images to be
transmitted to other users. Exit Notice.
 Commercial database contains information—manufacturer, date of market
release, an image or offset print of the sole, and pictorial images of the uppers—for
more than 12,000 sports, work, and casual shoes.
 Sold on DVD, updated and distributed to subscribers every 3 months.
 Limitation is that different manufacturers often use the same sole unit. Therefore,
it may be difficult to determine the exact make and model of a shoe. The software
links such records, however, so that all footwear that might match a crime-scene
print can be considered.
How does SoleMate work?
 The pattern of an unidentified shoe print is assigned a set of codes to isolate
basic features, such as circles, diamonds, zigzags, curves, and blocks. Options, with
variations, are presented pictorially, allows investigator to code features that best
match the shoe print.
 These codes form the database search, with results presented in descending
order of pattern correlation.
Foster & Freeman USA Inc., at 888-445-5048.
 Maintained by the same United Kingdom company that markets SoleMate, this
database contains information:
5,000 vehicle tires and tire tread patterns,
Date of market release,
Pictorial image,
Pattern features.
 Because manufacturers sometimes use the same tread, it may be difficult to
find the exact make and model match of a tire. In these cases, records are
linked so that all tires that might match a crime-scene tire mark may be
How does TreadMate work?
 The pattern of an unidentified tire mark is assigned a set of codes for pattern
features, such as waves, lines, diamonds, zigzags, curves, and blocks, which then
form the basis of the database search. Results are presented in descending order
of correlation.
Foster & Freeman USA Inc., at 888-445-5048 or [email protected]
Tire Print Evidence
Evidence Often Overlooked
Vehicle-Involved Scenes
 Vehicle-involved scenes run the gamut of scene types:
homicides, sexual assaults, burglaries, drive-by shootings,
terrorist events, etc.
 Identifying vehicle should be a critical aspect of any onscene investigation.
 In typical homicide investigation, investigators must consider
the possibility that specific categories of physical evidence
related to the crime are present.
 Hit and run crimes, whether vehicle-vehicle, vehicleperson, vehicle-other object, involve vehicles that leave the
 In these crimes, damage creates physical evidence
 Evidence could prove the culprit vehicle was at the
o This evidence should be collectable.
The following article from the New York Daily News is such an example[1]. In this
case, the suspect and his vehicle were found. The NYPD crime scene unit had the
responsibility of working the car to prove it was the vehicle that struck the victim.
Case Example
 Hypothetical - Kidnapping
 Involves a vehicle for transport
 Critical to find physical evidence that the child had been inside the car,
 Reality: Such evidence may not be present or had been removed.
The abducted child case.
 No evidence found inside the suspect vehicle proving the child there …
 Must consider other, indirect, avenues to move investigation forward.
 The abductor vehicle had been at the scene.
 Success includes thorough investigation of the outside where the vehicle
might have been parked
 Physical evidence collected and scene archived.
 One example of physical evidence vehicles leave behind is tire track
Tire Track Impression Evidence
Evidence Often Overlooked
 Footwear Impression:
 Dealt with locating, enhancing, photographing, etc, footwear impression
 Much of that discussion is applicable to tire track evidence as well.
Similarities with Footwear Impressions
 Tire track evidence:
 Classified as two-dimension (2D) or three-dimension (3D).
 Commonly in dust or are otherwise contaminated 2D impressions or
impressions in a soft surface.
 Considered Class or individualizing:
 Physical characteristics needed for meaningful criminalistic-quality
 Former provides information about the tread design
 Latter provides information imbedded into the tread of the tire from daily
Footwear & Tire Track Differences
Their Intrinsic Forensic Values
 Provides evidence of the individual (shoe
impressions) or the vehicle (tire impressions) being at
the scene,
 For tire tracks, suggests the individual vehicle was
used in the crime.
 Tire track impressions geared to identify vehicle …
not the person,
 Although the person might have been driving the
How Important is Scene Evidence?
 Identifying specific vehicle requires recovering it and
making direct comparisons with physical evidence from
 Scene data allows investigators and laboratory
analysts to narrow the search among universe of
 Until suspect vehicle is located and impounded,
all scene data must be archived, collected and
 Determining which tire tracks to photograph,
enhance and or cast is critical
Critical Vehicle Information
Crime Scene Procedures
How vehicle was
Track measurements
Tread wear
Wear bars
No. vehicles & no.
Were objects loaded
or unloaded?
 Direction of travel
 Relationship of impressions
@ scene to arrangement of
tires on suspect vehicle
 Position of front of vehicle
 Which impressions made
by front & rear tires
 Which impressions to
 Locations where vehicle
track measurements will
be recorded
 Other relevant evidence
 Footwear impressions
 Fluid spills
On-Scene Considerations
Success At the Scene
One Shot at It
 Unknown:
 Whether something seemingly unimportant and ignored will
be important as evidence AND suddenly plays a prominent
role in the investigation.
 Consider everything at vehicle-involved scenes as potentially
 Misperception that tire track impressions have little forensic
or investigative value.
 General scene investigative principles apply equally to vehicleinvolved scenes;
 Management, archiving, searching, etc, are an integral
aspect of the investigation.
Crime Scene Procedures
 Secure the area
Discrete Security Areas at the Scene
 Tire prints protected
 Obtain information
 Case information
 Vehicle information
 Scene information
 Establish safe path to view
 Archive
 Collect/package/preserve
Command Center
Secure Travel Routes for
Emergency Equipment / Personnel
 Should employ guidelines
previously discussed
 While these principles are
 Unique characteristics for
vehicle-involved scenes.
 Specifically,
identifying, archiving,
characterizing and
preserving the physical
evidence associated with
identifying a suspect
 Investigative Questions
 Archiving
 Sketching and critical on-scene
 Vehicle information
 Suspect information
 Reconstruction
 Photographing, sketching, video, 3D-Imaging tiretrack and other vehicle-involved scene evidence
 Much like for footwear impression evidence.
 For tire track impressions – Document at least 24” of
 For comparison purposes
Photography of Tire Track Impressions.
Photos must Include markers that
Identify the tire position on vehicle
– Establishing or Overview Photographs
• Tell the story and relationship of how the tire tracks relate to the overall
scene. Photos with and without evidence markers in place.
– Midrange
• Tell the story of how the tire track evidence relate to each other and to
nearby objects. Photos with evidence markers are important to establish
evidence-to-evidence relationships.
– Close-ups
• These include at least 24” of all 4 impressions, appropriately identify
which tire, e.g., right front, etc, being photographed. Photos with and
without scales must be included.
Considering Lighting and Glare
3-D Impression
Blocking Sunlight Using Ambient Light
Photographing 3-D Impressions
 Photograph before casting
 Reproduces class characteristics
Accidental characteristics are often lost
Spray paint may enhance sufficiently
 Outdoor lighting may make it necessary to
block direct sunlight
 Sometimes sunlight may be superior to oblique
 Use polarizing filters to eliminate glare
 Look for sidewall information in impression
Photography – Effect of Lighting
•Existing light blocked out and oblique light provided with off camera Flash
Archiving 3D Impressions
Archiving 3D Impressions
Tire Impressions in the Snow
Reverse Sidewall Impression
Marking Tire Impression Evidence
Vehicles Still at the Scene
 Photographer’s name, Date, Time
 Use scales in plane of impression
 Mark tire position on vehicle & on scene
 Use spray fluorescent paint for all wheels
 Rt front, etc
 Arrow pointing to front of vehicle
 Establishes inside & outside edge of impression
 Impression number
 1st or 6th impression recorded @ the scene
 N/S directional
Tire Track Impressions
 Plan sketches: Overview of impressions
 In-Depth detail not important
 Measurements
 Photography
Archiving – Sketches
Include Vehicle-Critical Measurements
 Sufficiently detailed to permit a determination of
specific vehicle characteristics
 Dependent on the amount and detail of the
tire track impressions present.
 Appropriate measurements must be made.
 The measurements are the critical data
needed to compare the on-scene tracks with a
suspect vehicle.
Anatomy of the Tire
Tread Design
Eliminating Road Noise
Vehicle Information
How Was the
Vehicle Maneuvered?
Direction of Travel
 Closely examine tire tracks
 Using known factors
 Common sense
Direction of Travel
 Spinning tires
 Striations by sidewall in furrow
 Shows which way tire rolling
 Location where vehicle stopped,
backed up to change direction
 Overlapping front & rear tire
 Look to see if coming or going
from scene
 Grass or small plants
 Direction they were flattened
 Directional tire tread patterns
 Damp soil or snow
 Tires lifted soil or snow slightly in
direction of travel
 Deposition of transferred material
in direction of travel
 Mud, dirty water or fluids
splashed or thrown in direction of
Make the Appropriate
Tire Track Measurements
 Wheelbase
 Front track width
 Rear track width
 Turning diameter
 Tread design width (arc width)
 Tread depth (skid depth)
 Tire circumference
Defined as the distance between the leading edge of
the front and rear tires.
Front Tire
Track Width
Front Tires Turned
Front Wheels Turned
Inside leading edge of
track will be rounded
 Make rt. Angles to
center line of impression
Will run laterally across
the width of impression
Project line along inside
edge – parallel to center
o Intersection is fixed point
on inside leading edge
o Repeat of all 4 tires
Wheelbase in Mud & Snow
Tilt” of the tires as they
rest on the ground.
 Mechanical issues with vehicle affect how
impressions appear at scene
 Differences between normal and abnormal
camber (positive or negative) signifies
 improper alignment or worn front-end
parts of the vehicle.
 Positive camber: tires tilted further apart,
“out,” at the top.
 Negative camber tires tilted closer, “in,” at the
Normal Camber
Positive Camber
Tire tilted “out” @ top
Negative Camber
Tire tilted “in” @ top
Track Width or Stance
Front and Rear
Track Widths
 Defined as distance between
middle of leading edge of the front
and back tires.
Front and Rear Track Widths
• Track widths
– Midpoint of leading edge of
 Mechanical issues can affect front and rear tire track
how these impressions appear at impressions
the scene.
 Among others, one is
camber, which is the “tilt” of the
tires as they rest on the
Front Tire
Track Width
Track Width
Toe In or Out
 Difference between
front & rear of front
Normally set “in” only
a few millimeters
o Compensate for
normal front end
tendency to toe “out”
at highway speeds.
Improper alignment
Worn front end
Vehicle Information
Turning Diameter
Turning diameter
 Diameter is defined as the diameter of the
circle made when the vehicle is driven in a
 Determined from measurements taken @
 At the scene
 Don’t know if the turn is full-lock
 Curb-to-curb for the outer tires
 Not wall-to-wall for outside
edge of vehicle
Turning Diameter
 Procedure
 Select segment reflecting
sharpest portion of turn
 Measure imaginary line
between 2 points of arc
 Measure outer margin to outer
 A-AA
Calculating Turning Diameter
 Bisect line & draw line to outer
margin of track arc –
 (C-B)
 Calculate diameter
Rear Tire
 (A-C)
 Draw another line between the
bisect point and the inside of
the track arc –
Front Tire
Tire Impression
BD Bisects A – AA @ C: Line E= Distance B to C
Distance C to AA = Line F (= distance A to C)
Turning Diameter = (F / E) + E (11)
Alternatively: Turning Diameter = (F 2/8E + E/2) (12)
Tire circumference
 Approximate the tire circumference by finding an accidental, repeated characteristic,
such as a gouge or cut, along the imprint.
 The distance between these repeated marks is the rolling tire circumference.
 Investigators must understand that measured value – accidental characteristic-toaccidental characteristic – different from value obtained by wrapping the measuring tape
around the tire in its center line,
 Larger
 Measure distance between repeated accidental characteristics on impression
 Considerations
 Measured value is less than when the tape is wrapped around the tire
o Tires have a curved arc width, impressions do not.
 Measured circumference is larger in the center line of the tire than at the outer
Vehicle Information
Tread Design
Tread design width (arc width)
 Tread design width:
 Measurement from one edge of
the design to the other.
 These measurements must be
measured at the scene from the
 Information important so
manufacturers can help
investigators identify an unknown
Non-dirt Impressions
 2D impressions visible because of contaminants adhering to the tread; dirt
and dust the most common.
 Other contaminants also create impressions
 water, grease (oil) or blood.
 Each leaves visible impression
 Each can be enhanced, depending on the contaminant.
 Consideration of Enhancement Choices
 Variety of choices and investigators must make the proper decision.
 Archiving the impression photographically is step-one
 Enhancement choices and the variables offered by the scene:
o Surface material,
o Chemistry of the impression material (dirt, oil, blood, etc), and
o Chemistry of the enhancement method.
Tread Wear
Tread Wear Indicators
 Tread wear indicators (wear
 Located in grooves of tread design
 Run laterally across tread
o 1/16” above base of the groove
 Useful for comparison purposes
o Divide tire into useful segments
 Cars & light trucks
 6 evenly spaced
o Rim diameter of 305mm/12
inches must have @ least 6
o Less than 305mm must have @
least 3/12 inches
Wear Bar Indicator
Tread Wear
 First thousand miles
produces the fastest tread
 As elements become
 Flex less & squirming is reduced
 Slows considerably
 Accidental characteristics last
longer on well-worn tire than on a
new one
Tire wear indicator
Factors Affecting Tread Wear
Excessive under-inflation
Excessive over-inflation
Improper toe-in-out
Improper camber
Faulty breaks
Combined suspension
steering issues
Bad repairs
Measuring Tread Depth
Non-Skid Depth
 Equipment:
 Tread depth gauge (Measures in 1/32nds)
or 15mm ruler
 Newer tires: 11/32 to 16/32”
Maximum depths suggest newer tires
Wear bars are set @ 2/32nds,
Wear patterns appear as solid bar in
the tire impression
 Procedure
 Estimate height of groove in scene
Set depth gauge on impression groove
Carefully lower gauge into impression &
read depth from the scale
 Alternatively, measure depth from cast in
dental stone
Tread Depth - Some Statistics
 When tires near end of life,
 U.S. Lincoln penny can be used to confirm the tire's tread depth.
 If Lincoln's entire head is visible, the tire is worn to approximately 2/32" and is
considered legally worn out in most States.
 Average new tires used on cars typically start with 10/32" to 11/32" of original
tread depth.
 Dedicated winter / snow tires and light truck tires typically are deeper - how much
deeper depends on tire's tread type ... Highway Rib, Highway All Season, Off Road
All Terrain or Off Road Maximum Traction).
 Means - original tread depth has only 8/32" of useable tread depth.
 Useable tread depth calculated by subtracting a worn out tire's 2/32" from the
new tire's original depth of 10/32".
 The final 2/32" of a tire's tread depth isn't part of the equation when it
comes to calculating tread depth percentages because the tire is already
legally worn out.
 Remaining tread depth used to calculate tread wear percentages. …
oTire that started with 10/32" of original tread depth and has worn off
4/32" (down to 6/32" of remaining tread depth) is 50% worn.
o 2/32" legally worn out tread depth
 A tire that starts with 10/32" of original tread depth has 12.5% wear for every 1/32"
that is worn away, and a tire that starts with 12/32" " of original tread depth, has 10%
wear for every 1/32" that is worn away,
Identifying Tread Wear
Exposed tire bars
2/32 Tread
Legally Worn
 Tire bars recessed in
sipes & grooves
become exposed
4/32 Tread
• Circular wear
 Wear around
circumference w/sipes &
grooves indicates wear
6/32 Tread
Enhancing Tire Tracks
Enhancing Tire Track Impressions
 Involves many of the same principles as for footwear
 Photography :
 A method for archiving AND enhancing.
 Chemical enhancements.
 Chemical treatments take advantage of the inherent
chemical signature of the impression
o Minerals, such as iron and aluminum, etc,
organics, or bio-materials – to more clearly
visualize the impression.
 The chemical enhancements are essentially the same
those used to enhance footwear evidence.
Chemical Enhancement
Metals in Soil
Impressions with Metals
 Ammonium or
Potassium Thiocyanate
 Tests for Iron
 Reddish brown
 8-Hydroxyquinoline
 Iron, magnesium + other
 Fluorescent under UV
Impression w/oil or fats
 Iodine fuming or Iodine
 Oils & Other organic
 Iodine absorbed
 Enhancement
 Benzophenone
o Forms stable blue
 Thin layer of starch
o Subsequent steam
treatment turns
starch blue
Chemical Enhancement
Wet Tires with Salt Deposits - Winter
• Silver Nitrate
– Converts NaCl (salt) to AgCl (Silver
• NaCl + AgNO3
Road Salt 2% Silver Nitrate
In Methanol
AgCl + AgNO3
Ag Metal
– UV or sunlight darkens impression
Tire Tracks in Blood
Chemical Enhancement
 Blood reactive chemicals
Leucocrystal Violet
Acid Violet 17
Acid Yellow 7 – dark surfaces – yellow fluorescence
Amido Black
Crowle’s Stain
Coomassie blue
Nile Red
Hungarian Red
Ashley’s reagent
Investigative Evidence
Identifying Unknown Vehicle
Tire Impressions
as Investigational Aids
 Identifying manufacturer & brand
 Done after processing the scene
 Tread design
 Who makes it & where
 Tire guides – Boca Raton, FL
Shows tread patterns for all types of tires sold in the U.S.
Comprehensive Visual Identification of all types of Tire Tread Pattern Designs.
Covers Passenger, Light Truck and Medium Truck vehicles.
Includes Off-Road and Agricultural Tread Patterns.
Also covers Motorcycle and retread tire tread designs.
Provides information on how to read a tire sidewall and on the different Tire
Sizing Systems.
Gives detailed information on Speed-Rating and Load Indexes as well as lug nut
torque and tightening sequences.
On-Scene Information
Limited Slip Differential (LSD)
 Posi-traction
 One wheel was spinning
o Movie: My Cousin Vinnie
o Can narrow type of vehicle
Odd mixture of worn tires
 Suggests an older vehicle
 Possible alignment problems
Stolen items - gasoline
 Location of gas tank
o Check footprints @ side of car where gas tank located
 Estimate from amount taken whether a truck
Mechanical Problems
Mechanical Problems & Vehicle
Design Features
Fluid leaks @ scene
 Oil, antifreeze, brake fluid, trans fluid
 Collect for chemical analysis @ lab
Vehicle Location History
Foreign Material on Suspect Vehicle
Notice unusual material in tread
 Collect & take to lab
Identifying The
Correct Impression
Age of Tire Impression
How Long @ Scene
• How long an object (driven over) has been lying
on the ground
– Complainant may have this information
• Transferred substance
– Recently spilled Fluid
• Knocked over objects
• Weather conditions prior to offense
– Help date time of deposit of scene impression
Known Tire Impressions
Known Tire Impressions
 Need one full circumference of the tire
Complex pitch in modern radials
Might miss important
accidental characteristics
Taking Tire Impressions
Methods & Materials needed
 Large area to make
 Broom
 Vehicle lift
 Tire marking crayons
 Rag to clean dirt
from tires
 Roll of paper towel
 Large jar of
Petroleum Jelly
 Gloves
 Art board 15” x 40” (3
 Lg & sm felt marking
 Magnetic jet black print
 3” wide magna brush
 Strips of polyester
plastic 12” x 40” to
protect impression
 Scotch tape to attach
plastic to art board
Information From Each Tire
• ID each tire later
• Photograph Tire
Exterior sidewall
Tread design
Serial number
Design, mold &
drawing numbers
• Show up after
rubbing w/crayon
Tire brand name
Tire size
Type of tire
• Summer, etc
– Construction
• Radial, bias ply, etc
– Other numbers
• Mold, design, etc
– Wear
– Number of ribs in design
Recording Known Tire Impressions
Recording known tire impression
Art board is superior to
 Small debris on surface
can’t come through
 Reduces/eliminates
wrinkles/tearing common
with paper
 Use gloves to keep board
 3 pieces of 40” Board
sufficient to record 1 full
circumference of tire
Tread Wear Indicators
Recording Known Tire Impressions
Petroleum Jelly/Magna Brush Method – 2D
 Place 2 pieces of art
board in path of vehicle
 Tape on reverse side to
hold pieces together
 Mark adjoining pieces
 Push car over recording
 Vaseline leaves light
brown impression
 Mark on art board as
each tread wear indicator
(marked on sidewall)
rolls by
 Mark spaces on art
board to correspond to
numbers on known tire
Tire Wear Indicator Marks
Place locations on sidewall
Recording Known Tire Impressions
Petroleum Jelly/Magna Brush Method – 2D
 Cover art board in front & behind
impression w/paper towel to protect it
 Clean area of tire that was on the floor
when Petroleum Jelly first applied
 Apply Petroleum Jelly to this area and push
car again
Recording Known Tire Impressions
Vaseline/Magna Brush Method – 2D
 Near end of second art board
Stop vehicle
Remove 1st piece of art board
Add thin coat of Vaseline to tire
Place 3rd piece of art board in path of vehicle
Continue pushing until known impression is
transferred to this 3rd section
Recording Known Tire Impressions
Petroleum/Magna Brush Method – 2D
• Dust impression as soon as possible
– Waiting too long causes defined areas of
tread to diffuse into art board surface
• Brush with magnetic brush @ rt. Angles
w/swirling action
– Avoid dusting clear areas of board
– Black magnetic Dusting Powder
Recording Known Tire Impressions
Vaseline/Magna Brush Method – 2D
 Protecting the impression
 Cover with clear acetate to prevent smearing or
damage during transit & comparison process
 Allows examiner to draw lines during comparison
Recoding Known Tire Impressions
Inking Method I
 Apply ink to art board for 1 full
Fingerprint Ink works
 Push car (tire) over inked art board
This is the inking process
 Push car over clean strip of white art
board to record the impression
Recording Known Tire Impressions
Inking Method II: Transparent Method
 Ink the tire as in inking method
 Drive tire over transparent acetate or
other transparent media (Mylar)
Tape to art board - suitable solid backing
 Can be rolled up for storing
 Unrolled @ later time for comparison
 Can be compared directly to impression when
searching for
Comparing Scene Cast
with Acetate Exemplary Overlay
Comparing Scene Cast
Actual Exemplar Tire
The area defined by blue tape is
the segment of the right rear
tire which corresponds with the
crime scene cast.

similar documents