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Chapter 4 Glass. Properties of Matter Review. Forensic scientists determine the properties that impart distinguishing characteristics: Physical Properties – weight, volume, color, boiling/melting point Chemical Properties – how a substance reacts/combines with other substances.
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Properties of Matter Review • Forensic scientists determine the properties that impart distinguishing characteristics: • Physical Properties – weight, volume, color, boiling/melting point • Chemical Properties – how a substance reacts/combines with other substances
The Nature of Matter Elements & Compounds States of Matter Theory of Light Temperature Weight & Mass Density Refractive Index
Glass Fragments • Glass that is broken and shattered into fragments and minute particles during a crime can be used to place a suspect at the scene • For example, • Chips of broken glass from a window may be lodged in a suspect’s shoes or garments • Particles of headlight glass found at the scene of a hit-and-run accident may offer clues that can confirm the identity of a suspect vehicle
Collection & Preservation of Glass Fragments • Forensic scientists need to find and measure those properties that will associate one glass fragment with another while minimizing or eliminating the possible existence of other sources • Glass will possess the greatest evidential value when it can be individualized to one source. Such a determination can only be made when the suspect and crime scene fragments are assembled and physically fitted together. Comparisons of this type will require piecing together irregular edges of broken glass as well as matching all irregularities and striations of the broken surfaces • The majority of glass evidence presented to the forensic scientist is either too fragmentary or too minute to permit a comparison of this type
Glass Composition • Glass is a hard, brittle, amorphous substance that is composed of silicon oxides (sand) mixed with various metal oxides • Amorphous – a solid in which the molecules are arranged in random or disordered position; there is no order • When sand is mixed with metal oxides, melted at high temperatures, and then cooled to a rigid condition without crystallization, the product is glass • By adding soda (Na2CO3) to the sand, its melting point and viscosity are both lowered, making it much easier to work with • Lime (CaO) is added to the sand and soda mixture so that the “soda-lime” glass will not dissolve in water • The soda-lime glass is used for manufacturing most window and bottle glass • The common metal oxides found in soda-lime glass are sodium, calcium, magnesium, and aluminum
Glass Composition • Glass is considered trace evidence • Because breaking of glass can occur at a variety of crimes, forensic scientists should be able to distinguish between various types of glass such as automobile, window, and household • When collecting glass samples from a scene, it is important to take samples from the intact source for comparison • Investigators label the outside surface and inside • When looking at glass fragments, investigators will try to determine the orientation of the glass before it was broken; dirt, paint, or putty can indicate orientation
Glass Properties • Tempered glass – • Glass that 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 little splintering. Used in side and rear windows of automobiles • Laminated glass – • Used in windshields of all cars in the US. • Derives its strength by sandwiching one later of plastic between two pieces of ordinary window glass.
Glass Properties • Automobile headlights /Heat-resistant glass /Pyrex • Types of specialized glass made by substituting in whole or in part other metal oxides for the silica, sodium, and calcium oxides. • These glass types are manufactured by adding boron oxides to the oxide mix. • Therefore they are called borosilicates.
Glass Properties Tempered glass
Glass Properties Laminated Glass
Comparing Glass Fragments • Physical properties of density and refractive index and used to characterize glass particles • Refractive index – the ratio of the speed of light in a vacuum to its speed in a given substance • These are class characteristics and cannot be used to individualize glass to a common source
Comparing Glass Fragments • Once you know the refractive index of liquids, you can immerse glass fragments in the liquids • If you can distinctly see the glass in the liquid, then it has a different refractive index and bends the light making it visible • If the glass disappears in the liquid, then it has a refractive index close to that of the liquid and it not reflected by the glass • “Becke” lines are the visible outlines of the glass sample in the liquid due to difference in refractive values
Becke Lines http://www.youtube.com/watch?v=O-KALJ3p26Q
Density A solid particle with either float, sink or remain suspended in a liquid, depending on its density relative to the liquid medium. This allows the criminalist a rather precise and rapid method for comparing densities of glass!
Density Flotation This method uses a standard/reference glass particle in an immersed liquid (often a mixture of bromoform and bromobenzene). The composition of the liquid is adjusted by adding some amount of the liquid until the glass chip remains suspended in the liquid.
Density Flotation When the chip is suspended then standard reference glass and the liquid have the same density. Glass chips of the same size and shape as the standard or reference are then added to the liquid for additional comparison.
