- Slides: 40
Analysis of Glass- Glass Evidence Chapter 4: Properties of Matter and the Analysis of Glass
Glass evidence can be found at many crime scenes. l l Automobile accident sites may be littered with broken headlight or windshield glass. The site of a store break-in may contain shards of window glass with fibers or blood on them. If shots are fired into a window, the sequence and direction of the bullets can often be determined by examining the glass. Minute particles of glass may be transferred to a suspect’s shoes or clothing and can provide a source of trace evidence linking a suspect to a crime.
How is glass formed? l l l Long before humans began making glass, glass formed naturally. When certain types of rock are exposed to extremely high temperatures, such as lightning strikes or erupting volcanoes, glass can form. Obsidian is a type of glass formed by volcanoes.
Timeline of Events l l l Prehistoric humans used obsidian as a cutting tool. The earliest man-made glass objects (glass beads) were found in Egypt dating back to 2500 BC. Glass blowing began sometime during the first century BC. By the 14 th century, knowledge of glass making spread throughout Europe. The Industrial Revolution brought the mass production of many kinds of glass.
How is Glass Formed? l l l Glass is a hard, brittle, amorphous material made by melting sand (aka silica, silicon dioxide, Si. O 2) lime (aka calcium oxide Ca. O) and soda, sodium carbonate (Na 2 CO 3) at very high temperatures. The lime (Ca. O) is added to prevent the glass from being soluble in water. The soda (Na 2 CO 3) is added to lower the melting point of silica (sand) and make it easier to work with.
Soda-lime Glass (amorphous solid) The atoms are arranged in a random fashion
Types of Glass: l Soda-lime glass: Mostly sand, sodium carbonate and calcium oxide: – Used for manufacturing most window and bottle glass
Float Glass l l Flat glass typically used for windows. Soda-lime glass that has been cooled on top of a bath of molten tin.
Borosilicates l l The common metal-oxides found in soda-lime glass are sodium, calcium, magnesium and aluminum. In addition, a wide variety of special glasses can be made by substituting in whole or in part other metal oxides for the silica, sodium and calcium oxides. Automobile headlights, heatresistant glass such as Pyrex are manufactured by adding Boron oxide to the oxide mix Lab glassware, thermometers, cookware.
Leaded Glass l l Fine glassware and decorative art glass, called crystal or leaded glass substitutes lead oxide for calcium oxide (lime). The addition of lead oxide makes the glass denser. As light passes through the more-dense glass, the light waves are bent, giving the glass a sparkling effect.
Tempered Glass l l l This glass is made stronger than ordinary window glass by introducing stress through rapid heating and cooling of the glass surfaces. When tempered glass breaks, it does not shatter but rather fragments or “dices” into small squares with litter splintering. Used for side and rear windows of automobiles sold in the United States.
Laminated Glass l l This glass derives its strength by sandwiching one layer of plastic between two pieces of ordinary window glass. The windshields of all cars manufactured in the United States are constructed from laminated glass.
Bulletproof Glass l l Bulletproof glass is a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic so it can flex instead of shatter. The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear view through the glass. Bulletproof glass varies in thickness from threequarter inch to three inches.
Properties of Glass and Comparing Glass Fragments l l l For the forensic scientist, comparing glass consists of finding and measuring the properties that will associate one glass fragment with another while minimizing or eliminating the possible existence of other sources. Considering the prevalence of glass in our society, it is easy to appreciate the magnitude of this analytical problem. Obviously, glass possesses its greatest evidential value when it can be individualized to one source.
Jigsaw Effect – Most Beneficial l When the suspect and crime-scene fragments are assembled and physically fitted together. Comparisons of this type require piecing together irregular edges of broken glass as well as matching all irregularities and striations on the broken surfaces. The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration. Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type
Density and Refractive Index l l l The physical properties of density and refractive index are used most successfully for characterizing glass particles. These properties are class characteristics which can not provide the sole criteria for individualizing glass to a common source. These properties do give the analyst sufficient data to evaluate the significance of a glass comparison, and the absence of comparable density and refractive index values will certainly exclude glass fragments that originate from different sources.
Measuring and Comparing Density l l Each type of glass has a density that is specific to that glass. One method of matching glass fragments is by density comparison. Density (D) is calculated by dividing the mass (m) of a substance by its volume (V). The formula for calculating density can be written as D = m V
Comparing Densities: Flotation l A solid particle will either float, sink, or remain suspended in a liquid, depending upon its density relative to the liquid medium. l Flotation = a standard / reference glass particle is immersed in a liquid; a mixture of bromoform or bromobenzene may be used. The composition of the liquid is carefully adjusted by adding small amounts of bromoform or bromobenzene until the glass chip remains suspended in the liquid medium. At this point, the standard / reference glass sample and the liquid each have the same density. Glass chips (same size and shape as reference sample) are added to the liquid for comparison. If both the unknown and standard / reference samples remain suspended, they have the same density.
Refractive Index l Refractive Index – When light travels from one medium to another its speed changes relative to the density of the medium. This can be observed as the light bends when traveling from one medium to another.
Index of Refraction (Refractive Index) l The speed of light in a vacuum is always the same, – l but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material. The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction (aka refractive index or n) for the substance.
Methods for Determining Refractive Index l The FBI has a database off over 2000 refractive indexes of different types of glass which shows that glass is very distinctive and helps assign an appropriate statistical probability that the two pieces of glass share a common source.
Snell’s Law l Snell’s Law describes the behavior of light as it travels from one medium into a different medium. Snell’s law can be written as – – n 1 (sine angle 1) = n 2 (sine angle 2) n 1 is the refractive index of medium 1 and n 2 is the refractive index of medium 2. Angle 1 is the angle of incidence and angle 2 is the angle of refraction. BOTH angles are measured relative to the normal or line drawn perpendicular to the surfaces where the two medium meet.
Determining and Comparing Refractive Index l l l Submersion method: Place glass fragment into different liquids of known refractive indexes. If a piece of glass and a liquid have the same refractive index, the glass fragment will seem to disappear when placed in the liquid. Submersion and Low Power of Microscope. Submerge fragment of glass in a liquid and then view it under low power using a compound microscope. If the refractive index (n) of the liquid medium is different from the refractive index of the piece of glass, a halo-like ring appears around the edge of the glass. This halo-like effect is called a Becke line. The Becke line appears because the refracted light becomes concentrated around the edges of the glass fragment A Becke line is visible under a microscope when the glass and liquid have different refractive indexes.
Becke line l l If the Becke line is located inside the perimeter of the glass fragment, than the refractive index of the glass is higher than the refractive index of the surrounding liquid. If the Becke line is located on the outside perimeter of the glass fragment, than the refractive index of the surrounding medium is higher than the refractive index of the glass.
Becke line – Indication of Refractive Index l l Notice the halo of light on the inside perimeter of the glass sample. When the Becke line is inside the perimeter of the glass fragment, the refractive index of the glass is higher than the refractive index of the surrounding medium.
Becke line Indication of Refractive Index l l Notice the halo of light (Becke line) is outside the perimeter of the glass fragment. When the Becke line is outside the perimeter of the glass sample, the refractive index of the medium is higher than the refractive index of the glass.
Annealing l l l When trying to make a distinction between tempered glass and nontempered glass particles a process known as annealing is used. Annealing- slowly heating and then cooling the glass. A heat treatment that alters the microstructure of a material causing changes in properties such as strength and hardness. . . The change in the refractive index value for tempered glass upon annealing is significantly greater when compared to nontempered glass and thus serves as a point of distinction.
Fracture Patterns in Broken Glass l l l Glass has some flexibility. When glass is hit, it can stretch slightly. When glass is forced to stretch too far, fracture lines appear and the glass may break. The fracture patterns on broken glass can provide clues about the direction and rate of impact.
Primary Radial Fractures l l When glass breaks, fracture patterns form on the surface. Breaks, called primary radial fractures, are produced. These fractures start at the point of impact and radiate (like spokes on a wheel) outward from there. Radial fractures form on the side opposite the point of impact.
Secondary Fractures l l Secondary fractures may also form. These fractures take the form of concentric circles around the point of impact. Concentric circles are circles that have the same center. Concentric circles form on the same side of the glass as the point of impact.
Analyzing Glass Fracture Patterns
3 R Rule- Determining Side of Impact l. Radial Cracks form a Right Angle on the Reverse side of the force.
3 R Rule l Radial cracks are at Right angles to the Rear (side opposite the impact) l Exceptions –tempered glass “dices” without forming ridges –very small windows held tightly in frame can’t bend or bulge appreciably – windows broken by heat or explosion no “point of impact”
Successive Penetrations of Glass l l When there have been successive penetrations of glass, it is frequently possible to determine the sequence of impact by observing the existing fracture lines and their points of termination. A fracture always terminates at an existing line of fracture.
Breakage of Glass from a Fire l l l During a fire, glass may break as a result of heat fracturing. Heat fracturing produces breakage patterns on glass that are different from breakage patterns caused by impact. Wavy fracture lines develop in glass that has been exposed to high heat. Glass will tend to break toward the region of higher temperature. If the glass was not broken before the fire, there will be no radial or concentric circle fracture patterns in glass that is broken by high heat.
Proper Collection of Glass Evidence l l l Standard reference glass should be taken from the crime scene (1 in 2) Package in solid containers to prevent breakage Preserve garment (shoe, pants, shirt) with glass on it All broken glass must be recovered and submitted for analysis when direction of impact is desired. Whenever possible, the exterior and interior surfaces of the glass must be indicated. The presence of dirt, paint, grease or putty may indicate the exterior surface of the glass.