Presentation on theme: "Glass Analysis Mrs. Ashley. Glass Analysis How broken? Link a suspect to a crime scene Fingerprints Blood Match pieces by density, refractive index, thickness,"— Presentation transcript:
Glass Composition There are thousands of different uses for glass Formers-major portion of what makes up the glass Fluxes- change the temperature at which the formers melt Stabilizers- strengthen the glass and make it resistant to water
Glass Ingredients – SAND = silicon dioxide (Major Component) – SODA = Sodium carbonate to lower the melting point – LIME = Calcium oxide. This is a METAL oxide and is added to prevent the “SODA-LIME GLASS” from dissolving in water.
6 Major Types of Commercial Glass Soda-lime glass 90% of all glass, container glass and flat glass, resistance to high temperatures and sudden changes in temperature not good Lead glass will not withstand high temperatures, art glass, electrical insulating properties Borosilicate glass – high resistance to temperature change; Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware
Types of Commercial Glass Aluminosilicate glass used as resistors for electronic circuitry Ninety six percent silica glass- resistant to heat shock up to 900 degrees Fused silica glass very difficult to make and can withstand temperatures up to 1200 degrees for short periods
Glass Uses Windows are made from FLOAT glass. Float glass gets its name because the SODA-LIME Glass is cooled on top of a bath of molten tin Tempered glass is used in the side and rear windows of automobiles. Laminated glass is two sheets of ordinary glass bonded together with a plastic film. Windshields of automobiles are made with laminated glass.
Properties of Glass Mechanically Strong – Glass has great inherent strength and is weakened only by surface imperfections, which give everyday glass its fragile reputation. Special tempering can minimize surface flaws. Hard surface – Glass resists scratches and abrasions. (Because the composition can vary, so can the hardness. On average the hardness of glass is about 5.5 on the Moh’s scale) Elastic – Glass “gives” under stress – up to a breaking point – but rebounds exactly to its original shape Chemical corrosion-resistant – Glass is affected by few chemicals. It resists most industrial and food acids.
Properties of Glass Thermal shock-resistant – Glass withstands intense heat or cold as well as sudden temperature changes. Heat-absorbent – Glass retains heat, rather than conducts it. Glass absorbs heat better than metal. Optical Properties – Glass reflects, bends, transmits and absorbs light with great accuracy. Electrical Insulating – Glass strongly resists electric current. It stores electricity very efficiently.
Additives and Color Oxide Iron Manganese Cobalt Gold Antimony Copper Lead/antimony Color green, brown Amethyst deep blue Red White light blue yellow
Direction of Penetration A projectile hole is inevitably wider at the exit side bsapp.com
Direction of Penetration Concentric Radial bsapp.com
Comparison of Radial and Concentric Fractures Radial FractureConcentric Fracture When Formed?First (primary)Second (secondary) On which side of the glass Opposite the side of force or impact Same side as force or impact DescriptionLines originate from point of impact and extend to edge of glass Series of circles one inside the other sharing the same center
Crime Scene Backscatter- when window breaks, most of the fragments will be carried forward, but some will be projected backwards, called backscatter Heat fracture-In a fire, glass may break due to heat fracture that looks like wavy fracture lines towards the heat source
Stress Marks The perpendicular edge of stress marks always face the surface on which the crack originated bsapp.com
Successive Penetrations A fracture always terminates at an existing line of fracture bsapp.com
Forensic Examination of Glass Review definitions and map out a plan of analysis. The FBI definition of glass is "...an inorganic product of fusion that has cooled to a rigid condition without crystallizing.” For the analytical methods the FBI recommends, see below. Collect, handle, and identify the evidence. Make your initial examination. If possible, you'll want to determine the color, fluorescence, surface features, curvature, and thickness of the glass. Examine fractures. Are they radial or concentric? Crater or hackle? A fracture match is an absolute means of identification
Forensic Examination of Glass Now measure the density of the sample. Density tells you something about the composition and thermal history of the glass. Measure the refractive index of the evidence. Refractive index is the most commonly measured property in forensic glass analysis. It can give you the same information as density, but you only need a very small piece of glass. Testing both density and refractive index tells you more than just checking one or the other. Determine major, minor, and trace elements in the glass. These methods are destructive methods so the FBI recommends measuring density and refractive index first (both methods are destructive). – You can use scanning electron microscopy, – X-ray fluorescence spectrometry, inductively coupled plasma-optimal emission spectrophotometry, – inductively coupled plasma-mass spectrometry, – laser ablation-inductively coupled plasma-mass spectrometry, – atomic absorption spectrophotometry.
What is the only way to individualize glass fragments found at a crime scene to a single source? The ONLY way to individualize glass fragments found at a crime scene to a single source is to be able to put the pieces of crime scene glass fragments together like a jig saw puzzle with suspect glass fragments.
What physical properties are used most often to characterize glass particles? What is the main drawback of using these properties to characterize glass? Do the glass particles from suspect and crime scene fit together? – Most of the time the glass evidence is too fragmentary or too minute to permit a comparison of this type. - Composition of the glass The general composition of most glass is the same. Density Refractive Index
Refractive index is represented by the letter n. The refractive index is a known quantity for different types of glass. The FBI has a data base of over 200 refractive indexes for different glass types and compositions. The higher the refractive index, the denser the glass. The higher the refractive index, the more the light will bend toward the normal (produce a smaller angle)
Immersion Method of Refractive Index The immersion method submerges the glass fragments in a liquid with a known refractive index. The temperature of the liquid is then increased or decreased until the glass fragments disappear. Increasing or decreasing the temperature of the liquid produces predictable changes in the refractive index. When the glass fragments disappear- THE INDEX of REFRACTION of the glass is the SAME as the index of refraction of the liquid.
Becke Line A Becke line is a bright halo observed near the border of a particle immersed in a liquid of a different refractive index. When the Becke line is located INSIDE the glass fragment, the fragment has a higher index of refraction than the immersion liquid. When the Becke line is located OUTSIDE the glass fragment, the fragment has a lower index of refraction than the immersion liquid.
Your consent to our cookies if you continue to use this website.