Chapter 22 Impression Evidence PDF

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CHAPTER 22 – IMPRESSION EVIDENCE  Many objects have a texture or pattern on their outside surface. These include fingerprints, many shoe soles, motor vehicle tire treads, & even markings imparted by tools onto objects. When one of these objects comes into contact with a recipient object or material & force is applied, an impression may be left on the surface.  Impressions can be 2-D or 3-D. most common impression evidence types are footwear, tire treads, & serial numbers.  Impression evidence can be individualized to one particular object if there are sufficient unique characteristics present. TYPES OF IMPRESSION:  Impression evidence involves a donor & a recipient. The donor contains some 3-D markings & the recipient is made of a material that can form & hold a negative image of the donor markings.  Donors include shoe soles & heels, tire treads, fingerprints & other friction ridges such as footprints & lip prints, tools that leave markings on the objects, ribbing & texture in fabrics.  Recipients include soft plastics, soil, putty, paint, dust, metals, & some soft plastics. FOOTWEAR IMPRESSIONS:  Left by soles & heels. Footwear & Criminal Activity:  Most people have on footwear when they commit a crime. The potential is great that footwear impressions will be left at the scene. Difficult to locate. Suspect's footprints may be mixed with those of other people. Information That Can Be Derived From Footwear Impressions:  Can indicate type, manufacturer, model, & often the exact size. Can indicate the route(s) taken into & away from the crime scene, can also indicate some of the activities that took place, number of people at the scene. When Footwear Touches the Ground…  A static electric charge can be applied to the impression if the shoe is clean & dry.  If the surface is soft or pliable, pressure exerted by the foot will cause the surface to deform & take on the contours of the surface of the footwear. If the surface is dirty or snow, the impression may be long lasting. If its grass or carpet, impression is quite transitory.  An imprint is where there is enough residue on the footwear to leave an impression on the recipient surface. This would be a positive impression, which are the most common type of footwear imprints.  If the shoe sole is clean & the recipient surface contains a lot of dust or residue, then a negative impression can form. Footwear Impressions at the Crime Scene:  Detection: o Problem with footwear evidence is finding it, is a systematic search. Impressions may be latent or invisible.  General Treatment of Footwear Impressions at the Scene: o Avoid altering an impression until photographs are taken. Digital still & video photography will provide a permanent record of the position of all footwear impressions & their general conditions. o Complete notes & sketches. o Photographs need sufficient resolution & lighting. o Make a decision about how to best preserve and/or enhance the impression, this depends on where the impression is, how easy it is to remove the recipient object & the impression from the scene, & whether the impression is 2 or 3-D. o If possible, impression should be physically removed & transported to the lab. If removal is not practical then a cast should be made.  Casting 3-D Footwear Impressions: o Casting is depended on the quality of photograph. Casting was used more when photographs weren’t good (old days). At that time, the major method for casting was to use plaster of Paris. o Improved casting materials such as dental stone. Advantages of modern casts over photography – if an impression is imbedded deeply into a substrate it will be difficult to gain a proper perspective photographically. o Casting Materials: - Plaster of Paris was soft even after drying, made up of large particles. Requires cast to be 2 inches thick. - Today, dental plasters & stones are used more often, they are denser, have more uniform, smaller particles than plaster of Paris, they dry quickly & show more detail.

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Footwear Impressions in Snow: - The most popular & successful method for making casts of snow print impressions is by snow print wax. Comes in a spray can & is sold in bright red & brown. It is sprayed onto snow print & dries in minutes. Shows excellent detail but is fragile. Keep direct sunlight away as it might melt. Lifting Imprints: - An imprint in a material like dust or one that has been visualized using a powder technique analogous to fingerprints can be lifted from a surface. - Most popular method used today involves one of a number of electrostatic lifting devices. A large, static electricity charge will strongly attract dust & other fine powders. A low current, high voltage charge is put across a film that attracts the particles from the impression, thus affecting a transfer. - Some imprints are difficult or impossible to lift, this includes those in grease/oil or blood. There are also impressions made in materials that deform when impressed eg cushions or carpeting materials. Comparison of Footwear Impressions: - Impression must be good enough quality & the lifting or casting technique must be done properly in order to record the impression for comparison with the actual footwear. - There is no set number of unique characteristics that must be present on an impression, each case must be considered on its own merit & will have its own facts & circumstances. Once a shoe has been worn, it starts gaining unique characteristics.

TIRE IMPRESSION EVIDENCE:  The impressions are similar to footwear impressions.  The pattern of the modern tire that is in contact with the road is called the tread – have many intricate designs that serve several functions. Treads wear with time & pick up unique characteristics. Tire Treads:  Firestone (1907) developed a traction design in the tread for the 1 st time. Carved "firestone" & "non-skid" in the tire. They were raised & lowered into the tread.  Treads are designed not only for traction, but also for channeling away water to prevent hydroplaning, for noise reduction, & for comfortable driving.  P235/75 R 15  P = tire is built for passenger car. If it was a pick up truck it would have "LT." "235" is the cross-section width or diameter of the tire in mm. "75" is called the aspect ratio, which is the height of the tire, this is actually a percentage (75%). "R" designates how the tire is manufactured, R is for Radial. "15" is the diameter in inches of the rim that the tire was designed for. Tire Impressions as Evidence:  2/3 of major crimes in US involve an automobile, if only as a getaway car.  A tire impression is the most effective way of positively linking a motor vehicle that has been at a crime scene.  3 methods of recording tire impressions  photography, drawings, & 3-D casting.  With tire impressions there is a need to make 3-D casts at the scene because unlike footwear impressions, tire impressions cannot really be taken & moved to the lab.  Disadvantages to casting tire impressions relative to photography: o It can be difficult to make a cast on a steep incline because the casting material may tend to flow downhill & part of the cast made at the top of the hill may be too thin & fall apart. o Unlike footwear impressions that are usually about a foot long, tire impressions may be many feet long & require very large casts that can be bulky, much easier to take a series of photos of a long impression. o There is a problem with 3-D impressions being negative; the raised areas of the treat become depressions in the cast.  Dental stone is the preferred medium for casts in soil, & snow casting wax is best for snow.  A measuring instrument should be placed in all photographs. Ordinary objects such as coins that provide perspective, not measurement, should not be used.  Lifts of 2-D tire impressions are made in the same way as footwear impressions. Because of the great length of many tire impressions, several lifts of the same impression may have to be made in order to get the whole impression.  Other data can be derived from tire impressions  Wheel base (the distance from the center of the front hub to the center of the rear hub) & tire tread stance measurements (distance from the centerline of the right tire to the centerline of the left tire).

BITE MARK EVIDENCE:  Bite marks can be left in virtually can pliable material. Human skin can be a good medium although the healing process & elasticity of human tissue means the bite mark may only be visible for a certain time.  Bite mark evidence in skin are analysed in 2-D fashion.  Known bite marks are obtained by use of an impression of the teeth in dental stone. Material for impression of bite marks must be fine-grained so that small details in the surface of the teeth will show up.  In cases where there is sufficient depth to the bite mark, the comparison with dental impressions can include the 3-D structure of the teeth in addition to the biting surface. SERIAL NUMBERS RESTORATION:  Many consumers products & commercial items are identified by a unique serial number. In some cases, this is required by law – firearms & certain auto parts are examples. If the object is metal, the serial number is stamped into the surface of the metal.  Most people, when faced with the task of wiping out a stamped serial number on metal will use a file or grinder.  It is possible & quite common for forensic chemist to restore an obliterated serial number impression.  When a serial number is stamped into a metal, 2 things happen to the metal under the number: 1. It is compressed, making it denser than the surrounding metal. 2. Metal-metal bonds in the stamped area are disrupted & the metal structure becomes weakened.  Once the metal surface surrounding the numbers becomes level with the stamped numbers, then they cannot be seen anymore. However, the compressed, deformed metal under the numbers is still there unless the perpetrator continues the grinding process beyond where the numbers disappear. To restore the serial number, the metal surface that has been abraded is polished with a fine abrasive & then slowly treated with a corrosive acid.  2 possible ways that the stamped metal can behave differently: 1. It has become sender when compressed. Metal would be expected to dissolve more slowly than the less dense metal surrounding it. 2. Metal-metal bonds of the stamped metal have been disrupted by the stamping process, & thus weakened.  The best way is to observe the restoration process using a low-power stereo-microscope. This means that the weakened bond theory must be most responsible for the dissolving process of the acid.  Once a serial number is restored, it will eventually disappear & then will be gone forever.  Acid is generally applied with a cotton swab. When a number appears, the acid is washed off quickly to minimize further dissolving while the operator photographs it.  Each type of metal requires different acids & conditions. Acidified, aqueous copper chloride solutions are used to restore serial numbers in iron & steel. If the metal surface is stainless steel, a more powerful acidic solution is needed. Acids are too strong for aluminum surfaces. In such cases, mild alkaline solutions are used.  Serial numbers are extremely difficult to restore in most plastics because they are insoluble in most solvents. SIGNIFICANCE OF IMPRESSION EVIDENCE:  3-D pattern or markings must contain some characteristics that are unique to that object.  No 2 tires or shoes will wear the same way, so each will change with time & become unique.  There is no agreement on what constitutes sufficient resolution or the number of features that must be present in order for a conclusion of individuality to be supposed.  There is the issue of the concept of individualization itself. Conclusions that individuality require more validation research.  When a car strikes a pedestrian, an imprint of the fabric pattern in the victim's clothes is impressed in the car's paint, do not lend themselves to individualization. LECTURE:  Forensic science = application of science to criminal/civil cases in order to provide information suitable to courts of justice. Any science can become a forensic science, eg: geology  forensic geology  Primary crime scene VS secondary crime scene  remember that where the body is found is not always the crime scene to investigate  Locard's Exchange Principle = every contact leaves a trace – whenever someone enters or exits an environment, something physical is added to & removed from the scene.  What/How did it Happen? Accidental death, suicide, homicide? Who is responsible for it? – autopsy, crime scene reconstruction.

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Analysis of physical evidence: o Identification: - Use of standardized tests o Comparison: - Determine common origin - Match characteristics of evidential sample with a reference - Determining the probability that the evidence has come from a particular crime scene/source - Individual characteristics – physical characteristics are unique - Class characteristics – can only be associated with general type (eg: type of can used to make a homemade explosive object) Types of prints: o Fingerprints o Foot, ear, hand o Eye/iris o Tyre o Shoe o Bitemarks o Toolmarks...