SHATTERED: Forensic Glass Analyses

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.

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