1. Glass Analysis Part 2
Mrs. Ashley

2. Review of Glass Analysis Part I
Glass is made up primarily of three substances
Sand
Soda
Lime
Most glass is Bottle Glass or also called Soda- Lime glass (90% of all glass)
Windows are made from Float Glass which is soda lime glass that is allowed to cool on top of a liquid

3. Fractures
Concentric
Radial
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4. Direction of Penetration
A projectile hole is inevitably wider at the exit side
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5. 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

6. Successive Penetrations
A fracture always terminates at an existing line of fracture
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7. 3R Rule
Radial cracks are at Right angles to the Rear (side opposite the impact)
Exceptions
–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”

9. Glass Analysis Part 2
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.
Most often it is the composition of the glass that is examined and the method of how glass was broken.
The general composition of most glass is the same.
Density
Refractive Index

10. Measuring and Comparing Density
Each type of glass has a density that is specific to that glass. One method of matching glass fragments is by density comparison.
Density (D) is calculated by dividing the mass (m) of a substance by its volume (V). The formula for calculating density can be written as D = m V

11. Measuring Density
Measure the density using a scale (be sure to zero scale before using and put to correct units – grams)
Measure the volume by measuring the amount of water displaced using a graduated cylinder in milliliters.
Use your calculator on phone to find M/V to find the density

12. Flotation Method for Denisty

13. Refractive Index Refractive Index
When light travels from one medium to another its speed changes relative to the density of the medium. This can be observed as the light bends when traveling from one medium to another.

14. Index of Refraction (Refractive Index)
The speed of light in a vacuum is always the same,
but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material.
The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction (aka refractive index or n) for the substance. This measurement does not have units

15. Methods for Determining Refractive Index
The FBI has a database off over 2000 refractive indexes of different types of glass which shows that glass is very distinctive and helps assign an appropriate statistical probability that the two pieces of glass share a common source.

16. Determining Refractive Index
Submersion method: Place glass fragment into different liquids of known refractive indexes. If a piece of glass and a liquid have the same refractive index, the glass fragment will seem to disappear when placed in the liquid.
Submersion and Low Power of Microscope. Submerge fragment of glass in a liquid and then view it under low power using a compound microscope. If the refractive index (n) of the liquid medium is different from the refractive index of the piece of glass, a halo-like ring appears around the edge of the glass. This halo-like effect is called a Becke line

17. Becke line – Indication of Refractive Index
Notice the halo of light on the inside perimeter of the glass sample.
When the Becke line is inside the perimeter of the glass fragment, the refractive index of the glass is higher than the refractive index of the surrounding medium.

18. Becke line Indication of Refractive Index
Notice the halo of light (Becke line) is outside the perimeter of the glass fragment.
When the Becke line is outside the perimeter of the glass sample, the refractive index of the medium is higher than the refractive index of the glass.

19. Determining Refractive Index
You will use the submersion method by placing the piece of glass in substances with a known refractive indices.
When the glass and the substance have the same refractive index, the glass will appear to disappear.
In reality, if a piece of glass is immersed in a liquid with the same refractive index, the glass usually does not completely disappear. The reason is that there are other effects caused by light passing through glass. These are more pronounced when the glass is thick. The result is that we can still see faint borders of the glass even if they have the same refractive index.

20. Lab Procedures
Remember to wear goggles and gloves until your lab is complete and area is clean
Use forceps to pick up glass particles
Once lab is complete, clean up your area and return all materials to the proper location.
Complete the Study Guide on Glass Analysis, due Monday, April 28.
Test on Glass Analysis Monday April 28, after we go over the study guide at beginning of class.