Physical Properties: Glass

1. Physical Properties: Glass Criminalistics Chapter 4

2. What is a property? • A property is an identifying characteristic of a substance. • There are two groups of properties that are important to forensic scientists when identifying substances: • Physical properties • Chemical properties

3. Physical Properties • A physical property describes a substance without reference to any other substance. • Physical properties can be measured without altering a substance’s chemical composition. • Physical properties are only associated with the physical existence of the substance • Examples of physical properties: • Weight, Volume, Color, Boiling Points, and Melting Points

4. Chemical Properties • A chemical property describes the behavior of a substance when it reacts or combines with another substance. • Examples of chemical properties: • Wood combining with oxygen in the air to burn or combust • Suspect specimen of heroin mixed with Marquis reagentColor change to purple indicates positive result for heroin

5. The Metric System • A system of measurement that uses a decimal relationship so that a unit of length, mass, or volume can be converted into a subunit by multiplying or dividing by multiples of ten. • Basic units of measurement in the metric system are as follows: • Length Unit: Meter • Mass Unit: Gram • Volume Unit: Liter

6. Metric System Conversion Values

7. Metric-to-English Conversion Values Example: Convert 12” into centimeters. 12 inches X 2.54 cm = 30.48 cm 1 inch

8. Physical Properties • Temperature: a measure of heat intensity, or the hotness or coldness of a substance • Two common temperature scales: • Fahrenheit scale: • Freezing point of water is 32º • Boiling point of water is 212º • Celsius scale: • Freezing point of water is 0º • Boiling point of water is 100º

9. Physical Properties • The Difference between Mass and Weight • Weight: The force in which gravity attracts a body to the earth • Mass: the amount of matter an object contains • Independent of its location on earth or any other place in the universe • Weight and Mass have a mathematical relationship W=mg We measure mass with a balance

10. Physical Properties • Density: mass per unit volume • Density is an intensive property of matter—this means that it is the same no matter the size of the sample • Mathematical relationship: D = m V The fact that objects will either sink, float, or remain suspended in solution depends on the density of the object and the density of the solution.

11. Physical Properties • Light can have the property of a wave. It travels in air at a constant velocity of nearly 300 million meters per second. It will continue to travel at this speed until it meets another object or medium. • Refraction: The bending of a light wave as it passes from one medium to another

12. Physical Properties • Refractive Index: The ratio of the velocity of light in a vacuum to the ratio of light in a medium • Refractive Index = Velocity of light in vacuum Velocity of light in medium Refractive index is an intensive property of matter and can characterize a substance.

13. Crystalline Solids • Crystalline solids have TWO refractive indexes • Crystalline solids have definite geometric forms because of orderly arrangement of the fundamental particle of the solid—the atom.

14. Amorphous Solids • Amorphous solids: will only have ONE refractive index • Amorphous solids: have atoms randomly arranged throughout the sample • Example: glass

15. Birefringence • Most crystals that have cubic configuration refract light at two angles and therefore have TWO refractive indexes • The difference between the two refractive indexes characterize the crystal based on the substance’s birefringence.

16. Light Dispersion • Dispersion: the separation of light into its component wavelengths • Light is often separated by a prism • The ability of a prism to disperse light into its different colors is due to dispersion • Each color of light, in passing through the glass prism, slows to a speed slightly different than the others and bends at different angles as it emerges from the prism. • The different wavelengths of light makes the colors

17. Glass Fragments • Glass: hard, brittle, amorphous substance that is composed of silicon oxide (sand) and other various metal oxides • By adding soda, Na2CO3, soda-lime glass is formed, which will not dissolve in water. • Most glass that is examined by forensic scientists is soda-lime glass, which makes up most automotive and bottle glass.

18. Typical Types of Glass • Pyrex: A borosilicate— boron oxide is added to the glass to increase resistance to heat • Tempered Glass: glass to which strength is added by introducing stress through rapid heating and cooling of the glass surfaces • When tempered glass breaks, it does not shatter • Usually found in car windows because of safety issues Laminated Glass: two sheets of ordinary glass bonded together with plastic film • Usually used in auto windshields in the United States

19. Glass Comparison • If glass fragments are too small to be pieced together, they are identified by their densities and refractive indexes. • Only identifies glass to a common source: Class Characteristic • Density: compared by a flotation method • Reference sample is suspended in solution • Glass sample is added • When the glass sample is at a equal point in solution as the reference sample, then the density of the glass equals the reference sample.

20. Glass Comparison • Refractive index is compared by an immersion method. • Glass particles are immersed in a liquid whose refractive index is varied until it is equal to that of the glass particles. • At this point, called the match point, the observer will note the disappearance of the Becke Line and the glass will seem to disappear. • Becke Line: a bright halo that is observed near the border of a particle immersed in a liquid of a different refractive index.

21. Glass Fractures • Glass bends in response to any force exerted on it. • When its’ elasticity is reached, the glass fractures. • Fractured glass reveals information that is useful for forensic scientists in reconstructing the force and direction of the impact.

22. Projectile Glass Fractures • Glass fractures based on a projectile, whether a stone or a bullet. • This type of fracture results in two basic patterns: • Concentric fractures: circular cracks in the glass • Radial fractures: straight cracks which radiate outward from the center of the fracture

23. Bullet Glass Fractures • Evidence of gunshot residue on the glass • Often leaves a round, crater-shaped hole that is surrounded by a nearly symmetrical pattern of radial and concentric cracks • The hole is wider on the exit side of the glass • When the velocity decreases, the fracture pattern increases and provides less information

24. How Does Glass Fracture? • Once glass reaches its’ elasticity point, it begins to crack. • Radial fractures develop first on the side opposite of the applied force • The continued motion of the force places tension on the front surface of the glass and results in concentric fractures • Examination of stress marks on the edges of the fractures reveals information related to the side on which the glass first cracked

25. Stress Markings On Fractures • Stress marks are shaped like arches that are perpendicular to one glass surface and curved nearly parallel to the opposite surface and are found on radial cracks. • The perpendicular end of the arch is always on the side opposite of the impact. • 3R Rule: Radial cracks form Right angles on the Reverse side of the force

26. Collection and Preservation of Glass • If glass evidence is to be pieced together, all glass must be collected. • If no fit can be completed, glass fragments must be submitted with reference samples found at scene. • When direction of impact need be determined, all broken glass must be recovered and submitted to the crime lab.