Ironwoodwilson.files.wordpress.com



SHATTERED: Forensic Glass Analyses Apr 1, 2001 © Elizabeth Becka Lansky

Beat cops get used to seeing a lot of glass. The term “smash and grab” is still used, and quite literally—most burglars aren’t sophisticated enough to pick a deadbolt, when a nice handy brick through the window will do just as well. Then there’s the automobile glass, broken in accidents, shattered by gunshots, or just caved in with a baseball bat by someone who decided he doesn’t like you.

There are essentially two ways to analyze glass—by the pattern of breakage, or by the composition of the glass itself.

In the case of bullet holes or small punctures in glass, cracks will form around the hole. Radial cracks will radiate outward from the hole, like petals from a flower. Another set of cracks will develop as a series of concentric circles, like ripples. In either case, cracks will end at existing cracks. So if a radial crack from hole A ends in a radial crack from hole B, you know that hole B appeared first. This is absurdly easy to do and quite accurate.

If you take a piece of glass from along the line of a radial crack, the edge will show a series of wavy lines, extending straight from one side of the glass to swoosh along the inside of the other edge, forming a sort of loose L shape. These are called conchoidal fractures. The edge of the glass where the lines are straight (perpendicular) is not the side of the glass that the force came from. This is the forensic scientists’ little R rule—radial cracks make right angles to the rear. In a concentric crack, it’s the opposite—the chonchoidal fractures DO make a right angle to the side the force came from. This can be vitally important in determining if something was a shoot-out or a shoot-in.

Bullet holes through thick glass will also form a crater, with (as is usually the case in bodies) the larger hole on the side that the bullet exited, and the smaller hole on the side it entered. Higher-velocity bullets will leave a very neat hole. A gun held close to the pane of glass (again, like a human body) will completely shatter it, because the hot gases from the muzzle of the gun escape rapidly and with great pressure. A pane of glass broken with a large stone will look the same.

If you’re standing inside and you break a pane of glass, most of the glass will land outside—but some will fly backwards towards you, landing inside the house and depositing tiny shards on your sleeves and clothing.

To analyze the composition of the glass itself, characteristics such as density and color have traditionally been studied. Float glass is created by floating molten glass on molten tin; as a result, one side of the glass will fluoresce. Whether glass is tempered can be determined with a polarizing microscope. I laughed when I saw people on the TV show “CSI” compare the densities of two pieces of glass by using two test tubes filled with a combination of chemicals to create a density gradient, because this is an extremely old-fashioned technique. It’s still quite valid, but very rarely used for the simple reason that glass, these days, is all pretty much the same.

Up one notch from using a density gradient is determining the refractive index of glass by using a microscope and a hot stage. An attachment warms the slide with the piece of glass and a liquid; when the liquid reaches a certain temperature, it and the glass will have the identical refractive index and the glass will seem to disappear. This is a technique guaranteed to ruin your eyesight. After staring at a faint image under a colored filter for so long that you no longer trust your own eyes, well…I could never do it. Most modern labs will have a wonderful machine called a GRIM II which will do this automatically. However—the same qualifications apply.

Glass is usually too similar, and while this technique can tell you with certainty if two pieces of glass are different, it cannot be so sure that they are the same. That takes you to the next stage, the current state of the art in glass analysis—ICP. This stands for Inductively Coupled Plasma, and it is a large, deceptively simple looking, and very expensive piece of machinery. Very few labs will have equipment like this unless some administrator got liberal with the funding, but your detective could always send the sample out to a larger, more well-equipped lab, which would most likely welcome the submission to justify their expenditure. The ICP can reportedly detail the composition of glass, down to trace elements.

Forensic Science Name:______________________________

Shattered: Forensic Glass Analysis use w/book Pd:____ Date:________________________

1. What are “essentially the two ways to analyze glass”?

1.

2.

2. Label the radial cracks and the concentric cracks in the image (

3. I f a radial crack from hole B ends in a radial crack from hole A which hole (A or B) appeared first? (yes, read carefully I AM trying to trick you here – because as the author of the article says it is “absurdly easy to do.” Which is it? And can you explain with the DRAWING above (label hole A and B)

4. What are conchoidal fractures?

5. What do they look like? – draw or describe in the space (

6. The edge of the glass from a radial crack where the lines are straight (perpendicular) is _________________ glass that the force came from.

7. What is the Forensic Scientists little “R” rule?

8. However, the chonchoidal fractures _______________________________to the side the force came from.

9. In a bullet hole through thick glass the larger side of the hole will be on the side that the _______________

10. A gun fired in close proximity to glass will ___________________ it.

11. What will a pane of glass look like if shattered by a large rock?

12. The author says if you’re standing inside and you break a pane of glass, most of the glass will land outside—but some will fly backwards towards you, landing inside the house and depositing tiny shards on your sleeves and clothing. Do you know what this is called? If I didn’t mention it – make me (

13. The author said she laughed when she saw the people on “CSI” compare the densities of two pieces of glass with two test tubes filled with a combination of chemicals to create a density gradient. By looking at the lab activities in our book (blue shaded areas) can you guess what the (two) chemicals may have been and what this “old fashioned” method is called? (

Two chemicals: ____________________________ and ___________________________

Method: _________________________________________

14. What is the GRIM II – and what could it have done for you on our latest blundering foray ( into the world of Forensics in glass analysis?

15. What is the ICP?

16. What does it do?

17. In conclusion: What do you think? Glass is most often class or individual evidence?

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download