shattered glass remaining at the crime scene. Glass is a hard, brittle, amorphous substance composed of sand (silicon oxides) mixed with various metal oxides. -When sand is mixed with other metal oxides, melted at high temperatures, and then cooled to a rigid condition without crystallization, the product is glass. -This manufacturing process produces flat glass typically used for windows. 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 oxide for the silica, sodium, and calcium oxides. These glasses are therefor known as Borosilicates Tempered glass is glass that is strengthened by introducing stress throughrapid heating and cooling of the glass surfaces -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 little splintering Laminated glass is two sheets of ordinary glass bonded together with a layer of plastic -This glass derives its strength by sandwiching one layer of plastic between two pieces of ordinary window glass Comparing glass consist of finding and measuring the properties that will associate one glass fragment with another while minimizing or eliminatingthe possible existence of other sources.
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-Glass processes its greatest evidential value when it can be individualized to one source -Comparisons of this type requires piecing together irregular edges of broken glass as well as matching all irregularities and striations on the broken surfaces. Most glass evidence is either too fragmentary or too minute to permit a comparison of this type. -Example: the general chemical composition of various window glasses within the capability of current analytical methods has so far been found relatively uniform among various manufacturers and thus offers no basis for individualization. -Trace elements present in glass have been shown to be useful for narrowing the origin of glass specimen The physical properties of density and refractive index are most widely used for characterizing glass particles. (Class characteristics) In a method known as flotation, a standard/reference glass particle is immersed in a liquid; a mixture of bromoform and bromobenzene may be used -The composition of the liquid is carefully adjusted by the addition of small amounts of bromoform or bromobenzene until the glass chip remains suspended in the liquid medium -At this point, the standard/reference glass and liquid each have the same density -Glass chips of approximately the same size and shape as the standard/reference are now added to the liquid for comparison If both the unknown and the standard/reference particles remain suspended in the liquid, their densities are equal to each other and to thatof the liquid.
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