Warm Up Objective: Scientists will describe glass evidence by taking notes and analyzing the activity. What is the topic? What will you be doing? Why is this important? How will you know if you have done well?

Warm Up Objective: Scientists will describe glass evidence by taking notes and analyzing the activity. What is the topic? Glass evidence What will you be doing? Taking notes; analyzing activity Why is this important? Evidence used for convictions; future career choice How will you know if you have done well? Able to describe glass evidence; took notes and analyzed the activity

Glass Evidence

CHARACTERISTICS OF GLASS Hard, amorphous solid Usually transparent Primarily composed of silica, with various amounts of elemental oxides Brittle Exhibits conchoidal fracture Curved breaks that look like concentric ripples

COMMON TYPES OF GLASS Soda-lime—used in plate and window glass, glass containers, and electric lightbulbs Soda-lead—fine tableware and art objects Borosilicate —heat-resistant, like Pyrex Silica —used in chemical ware Tempered —used in side windows of cars Laminated —used in the windshield of most cars

PHYSICAL CHARACTERISTICS Density—mass divided by volume Refractive index (RI)—the measure of light bending due to a change in velocity when traveling from one medium to another Fractures Color Thickness Fluorescence Markings—striations, dimples, etc.

FORENSIC USE OF REFRACTIVE INDEX The submersion method is used to determine the refractive index of small glass fragments. The FBI has a database with over 2,000 types of glass to assist in matching glass found at a crime scene with known sources

GLASS AS EVIDENCE Class characteristics: chemical and physical properties such as refractive index, density, chemical composition, color Individual characteristics: the source can be considered unique if the piece can fit together to complete the puzzle.

GLASS AND CRIME SCENES For glass comparison at a crime scene: Find and measure properties Associate one glass fragment with another Minimize or eliminate other sources Collection Collect all glass found at the scene for piecing Submit all glass evidence found in the possession of the suspect Packaged in solid containers to avoid further breakage Suspect’s shoes and/or clothing are to be examined Individually wrapped in paper Transmitted to laboratory

ANALYZING CRACKS CRACKS Radial fractures: radiate outward Concentric fractures: encircle hole

ANALYZING CRACKS Sequence of impacts: Successive penetrations of glass Fracture always terminates at existing line of fracture Figure 4–20  Two bullet holes in a piece of glass. Which bullet hole came first? The left hole preceded the right hole.

EVIDENCE FROM BULLET FRACTURES The angle at which a bullet enters window glass can help determine the position of the shooter.

Glass Activity Work in groups of 2-3. Get two microscope slides for your group. Tape both sides of the microscope slide as demonstrated. Put on your goggles. Using the hammer, hit the glass slide on the left side and then the right side. DO NOT BREAK THROUGH! BE CAREFUL! Using the screwdriver, hit the glass slide on the left side and then the right side. DO NOT BREAK THROUGH! BE CAREFUL! Record your observations as indicated and answer the questions.

Glass Activity Observations and Analysis For both the hammer slide and screwdriver slide: Draw the fracture patterns that you see in the space provided. Label the radial and concentric fractures on your drawing. Label the order in which the impacts to the glass occurred. Explain your reasoning for labeling them this way. Answer the analysis questions: How could investigators use glass as evidence? Look at sample A. What tool created these markings? How do you know? Look at sample B. What tool created these markings? How do you know? How might the fracture pattern be different if a different tool or a human hand was used? Explain.