Impressions Evidence
Impression Evidence
An impression is made when one object makes physical contact with another, leaving some of its physical characteristics on the recipient.
Soft or pliable materials leave three-dimensional impressions. Examples include putty, mud, concrete, and soft dirt.
Hard surfaces have two-dimensional impressions. Examples include dirt, dust, blood, and ink.
All impression evidence has a donor and a recipient.
The donor contains some three-dimensional markings.
Common donors include shoe soles and heels, tire treads, fingerprints, footprints, lip prints, tools, metal dies, and ribbing and texture in fabrics.
The recipient is made of material that can form and hold a negative image of the donor markings.
Common recipients include soft plastics, soil, putty, paint, dust, metals, plastics, and wood.
Significance of Impression Evidence
The factor that determines if one object can be associated with another is the presence of unique characteristics in the donor.
A three-dimensional pattern or marking must contain some characteristics that are unique to that object.
The characteristics could have come about as an object is used or could be random.
This type of evidence is valuable in associating a particular shoe or tire, for example, to a crime scene.
Serial numbers can also be used, which are unique to an object.
Not all impression evidence is unique.
For example, a fabric pattern from denim jeans impressed in the paint of a car could have come from numerous pairs of jeans.
Footwear Impressions
Footwear impressions may be left at crime scenes and in entry and exit locations of scenes.
Footwear evidence may be overlooked because:
it may be nearly invisible
it may be on uneven ground
it may be trampled by other footwear
Footwear evidence is misunderstood or undervalued by police, detectives, judges, and juries.
Footwear evidence can be fragile—photography and casts should be made of the impressions.
When footwear touches the ground:
A static electricity charge can be applied to the impression if the shoe is clean and dry.
This helps in the transfer of trace residues and dust to the impression.
The pressure exerted by the foot will cause the surface to deform and take on the contours of the surface of the footwear.
This may be permanent or temporary.
A mixture of static electricity and pressure help footwear to be impressed into a surface.
Types of Footwear Impressions
There are positive and negative impressions.
A positive impression is made because of the residue on the surface of the footwear that touches the surface. [4]
This is the most common type of footwear impression.
A negative impression forms if the shoe is clean, and the recipient surface contains a lot of dust or residue.
Negative impressions result when someone tracks through blood, wet paint, or grease with clean shoes.
A positive impression may be deposited after a negative impression if sufficient material is present.
For example, if a person tracks through a liquid and leaves a negative impression (a shoe mark in the liquid), a positive impression made with the liquid of the shoe mark can also be left on a clean surface.
An imprint is made when there is enough residue on the footwear to leave an impression on the recipient surface.
Information From Footwear Impressions
If enough unique characteristics are present, the impression can be matched to the actual footwear that made the impression.
Footwear impressions can provide information about:
the type, manufacturer, model, and exact size of the footwear
the route taken into and away from the crime scene
activities that took place during the crime
the number of people and suspects at the scene
characteristics of the gait of the wearer
Finding Footwear Impressions at the Crime Scene
The first problem with footwear is finding it.
Usually, entries and exits are examined.
Finding footwear evidence may involve:
a systematic search that includes the route of entry and exit and the crime scene
utilization of strategies for detecting latent or invisible prints
oblique lighting and physical methods, similar to those used to detect fingerprint residues
a search that encompasses two- and three-dimensional impressions
Preserving Footwear Impressions
Photograph footprints.
Photograph the footprint with a ruler or measuring tool for court testimony.
In two-dimensional footprint cases, a photo will be used for comparison.
Three-dimensional footprints are casted using material that fills the impression and then hardens to capture the impression image.
General Treatment of Footwear Impressions at the Scene
First, complete a visual record.
Use digital still and video photography, classical film, and tape methods.
This provides a record of the position of all footwear impressions and their general condition.
Complete notes and sketches that document exact locations and circumstances.
Take photographs that can be used for examining the individualizing characteristics.
Take close-up photos with sufficient resolution and lighting.
Second, determine how best to preserve and/or enhance the impression.
Avoid altering an impression until examination-quality photographs have been taken.
The impression and substrate should be removed and transported to the laboratory if possible.
If removal is not practical:
a cast should be made if the impression is three-dimensional
it should be lifted if it is two-dimensional
Casting Footwear Impressions
The primary material of choice to cast footprints is dental stone due to its hardness and ability to capture detail. [9]
It captures detail of the footprint to a much greater extent than other materials, such as plaster of Paris. [9]
Improved casting materials are drying faster and showing a great deal of detail. [9, 10]
There are also several advantages of casting over photography: [9, 10]
Photography requires a level, two-dimensional subject to be effective.
This may be difficult to gain a proper perspective.
Photography also makes it difficult to locate a measuring device and get a proper measurement of the size of the shoe that made the impression.
Oblique lighting may obscure details in photographs.
Casting was heavily used when photographic techniques were lacking, but then higher-resolution films decreased the use of casting of impressions. [9, 10]
Presently, casting is making a bit of a comeback. [10]
Plaster of Paris: [9]
soft, even after drying
made up of large particles that may cause the loss of crucial detail
requires the use of forms and reinforcing materials
Dental plasters and stones: [9]
most commonly used
more dense, uniform, smaller particle size
quick drying and show more detail
Lifting Footwear Imprints
Imprints in materials like dust or powder can be lifted using tape, as is done with fingerprints. [11]
Gelatin materials also can be used for lifting such imprints. [11]
The most popular technique involves the use of an electrostatic lifting device. [11]
A large, static electricity charge will strongly attract dust and other fine powders. [11]
The low-current, high-voltage charge is put across a film that attracts the particles from the impression, which is then a transferred image that can be photographed. [11]
Impressions in grease, oil, blood, cushions, or carpeting materials may be impossible to lift. [12]
Impressions made in materials that deform when impressed but then bounce back and lose the impression, like cushions and carpeting, can be lost when the substrate regains its shape, but residues embedded in the impression that forms an imprint may be lifted. [12]
Comparison of Footwear Impressions
Footwear impressions can be individual evidence if there are sufficient, unique characteristics present. [12]
There is no set number of unique characteristics necessary for a positive association to be made between an impression and a particular shoe. [12]
Tire Tread Impressions
Tire tread is the part of the tire in contact with the road. [13]
It serves to increase friction at the point of contact and minimize slippage. [13]
Tires are mass-produced; there are few unique or individual characteristics. [13]
With time and wear, tires pick up details that set them apart from other tires. [13]
Tire Impressions as Evidence
More than two-thirds of major crimes in the U.S. involve an automobile. [14]
Tire impressions are not recorded as often as they should be. [14]
Crime investigators overlook this evidence. [14]
Three major methods for recording tire impressions at a crime scene: [14]
Photographs and drawings are the best methods of faithfully recording the overall scene.
Tire impressions must be documented with photographs or casting quickly before weather factors destroy them.
Evidentiary Value of Tire Impressions
Tire impression casts made with dental stone are much bulkier and harder to handle due to their size. [15]
Three-dimensional casts make negative (opposite) impressions. [15]
Document with photographs. [15]
Use casting wax on tire impressions in snow. [15]
Casts of tire treads can be made, examined, and compared.
There are difficulties with tire tread impressions that are not seen with footwear, with size being the main issue.
Casts should be taken of three-dimensional impressions.
Tire impressions often cannot be taken up and moved to the laboratory.
Several disadvantages of casting tire impressions that are not issues with footwear impressions:
difficulty casting impressions on steep inclines
tire impressions may be many feet long
Negative impressions should never be compared to positive impressions.
Casting materials are the same as those used with footwear impressions.
Tire impressions can also assist in determining other information about the vehicle:
Wheelbase may be determined (distance from the center of the front wheel hub to the center of the rear wheel hub).
Stance may be determined (distance from the centerline of the right tire to the centerline of the left tire).
Impressions are compared to inked treads of known vehicles for identification.
Collecting Tire Impressions
Lifts can also be made of tire treads.
Lifts of two-dimensional tire impressions are made in the same way as with footwear impressions.
Several lifts of the same impression may be made in order to get the whole impression.
Other data, including wheelbase and tire tread stance measurements, can be collected.
This can be used to determine the make and model of a car or truck.
Serial Number Restoration
Serial numbers are stamped into an object that is usually metal.
When a serial number is stamped into metal, the crystal structure is affected.
Metal is compressed, making it more dense than the surrounding metal, and the metal-to-metal bonds are disrupted, and the structure becomes weakened when a serial number is stamped into the surface.
Filing or grinding may only eliminate enough metal to remove stamped numbers, but the altered metal underneath remains and can be used for restoration.
Restoration involves polishing the surface with a fine abrasive and then slowly treating it with a corrosive acid.
During the restoration process, the stamped metal will dissolve more quickly than the adjacent metal, and the serial number will be pressed into the metal once again.
Different types of metals require different acids and conditions for restoration.
It is extremely difficult to restore serial numbers in plastics.
Plastics are treated differently than metals.
Hydrochloric acid, water, and cupric chloride solution is a common means to recover serial numbers.
Once the serial number is visible, it must be photographed, as it will fade from view with time.
A camera should be used to take pictures of each number as it is restored.