What is Glass? An amorphous fusion of mineral compounds that produces a transparent solid when cooled. A 3D network of atoms which lacks the repeated, orderly arrangement typical of crystalline materials. Physical properties: hard, elastic, brittle, insulative (thermal and electrical) Chemical: resistant to all but fluorine and very strong bases
Types of Glass by Composition Soda-Lime Glass Most common and least expensive Windowpanes and containers 60-75% quartz sand (SiO 2 ) 12-18% sodium oxide (Na 2 O) reduces melting point 12-15% quicklime (CaO) increases durability Colors are due to impurities
Types of Glass by Composition Lead glass ( crystal ) Decorative glassware, radiation protection Lime replaced with >20% lead oxide (PbO) increases refractive index reduces viscosity and melting point
Types of Glass by Composition Borosilicate Glass (Pyrex ) Cookware, laboratory glassware, light bulbs Lime replaced with boron oxide (B 2 O 3 ) increases resistance to thermal shock
Types of Glass by Manufacturing Laminated glass Two sheets of glass with plastic between them. Used in windshield and skylights
Types of Glass by Manufacturing Tempered glass Treated by heating and quickly cooling float glass OR chemically treating the surface, replacing the Na+ with K+ Designed to be much tougher and shatter into tiny pieces rather than fracture Used in auto side windows, high stress areas
Kendall/Hunt 8 Publishing Company 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
Why Measure Density? Can be used as a screening technique with large numbers of fragments. Useful in identifying multiple sources present in the known and/or questioned samples. It is nondestructive and an intensive property (not dependant on sample mass). Need to measure very precisely in parts per hundred or thousand or better.
Glass Density Density can be measured by: directly determining mass and volume (usually by displacement) comparison by flotation comparison using a density gradient column Density gradient column method: Fragments of different densities settle at different levels in the column of liquid of varying density. Technique is not accurate for fragments that are cracked or contain an inclusion.
Density by the Flotation Method A glass particle is immersed in a liquid. The density of the liquid is adjusted by the addition of small amounts of another liquid until the glass chip remains suspended. At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.
Kendall/Hunt 12 Publishing Company Density of Glass Type of Glass Density window 2.46-2.49 headlight 2.47-2.63 pyrex 2.23-2.36 lead glass 2.9-5.9
How Do Glass Windows Break? Each force causes a deformation that may leave a visible mark or fracture the glass. This can be used to determine the direction and amount of force. Glass acts initially as an elastic surface and bends away when a force is applied. When the force increases beyond its tensile strength, it cracks.
How Does Glass Break? Radial cracks form first and are propagated in short segments on the side opposite the force. Concentric cracks come later from continued pressure on the same side as the force applied.
How Does Glass Break? Edges of broken pieces of glass will show stress marks Wallner lines. In a radial crack, the stress marks are perpendicular to unloaded side and parallel to loaded side. The arrow shows the side that received the impact. 3R rule: Radial cracks form at Right angles on the Reverse side of the force
Stress on Flat Glass https://www.sqaacademy.org.uk/pluginfile.php/34022/mod_resource/co ntent/2/glass/images/breaking%20glass.jpg
More Glass https://deathbetweenthecovers.files.wordpress.com/2013/04/glass1.jpg
Exceptions to the Three R Rule Tempered glass dices without forming ridges Very small windows held tightly in frame can t bend or bulge appreciably Windows broken by heat or explosion no point of impact curved, smooth edges at break points
Types of Fractures by Projectiles Bullets are a projectile force (load) that can pass through glass. Load side is the entrance side; unloaded side is the exit side. Low-speed projectiles: rib marks may indicate where breaking force was applied As the bullet s velocity increases, the central hole becomes smaller, cracking patterns become simpler, and the exit hole becomes wider than the entrance hole.
Fractures by Projectiles Which side was the bullet fired from? Exit (unloaded) side is wider than entry (load) side. Stress lines on the glass edge of radial cracks form a right angle on the reverse side from the force. Stress lines on the glass edge of concentric cracks form a right angle on the same side as the force.
Which Bullet Hole Was First? The sequence of impacts can be determined since crack propagation is stopped by earlier cracks. In the figure above, which impact occurred first?
Putting it Back Together Again? Examiners can fit together two or more pieces of glass that were broken from the same object. Because glass is amorphous, no two glass objects will break the same way.
Learning Check In the figure below & left, which impact occurred first? In the figure above & right, from which side did the impact occur?