Which Way Does Glass Fall? (Hint: It’s a Trick Question)

This is a great question that comes up from authors now and again. Usually it refers to broken glass at what we call the “point of entry”. This is the location where the crook breaks into the residence, business, or vehicle.  There are a lot of ways to break glass. You can use a tool like a glass cutter, hammer, or simply throw a rock through it.  So when a CSI responds to a crime scene and finds a broken window one of the questions that we must try to answer is “was the glass broken from the inside out or the other way around?” Now if you see a huge rock sitting in the living room surrounded by broken glass it’s a pretty easy call. But what if the tool isn’t present? One thing to consider is where the glass fell. There is a more definitive examination we can do but I’ll cover it in a future post. For now let’s just consider the issues of glass being inside or outside.

A lot of authors (and police) believe that if a majority of the glass is inside the point of entry then force came from the outside. If most of the glass is outside then the force came from the inside and might suggest staging. One of the important aspects to consider is the type of glass. Generally speaking the glass will be either “plate/sheet” (also called pane or flat glass) or “safety”. Plate glass is not typically found in new US homes as most local codes call for safety glass. Safety glass is also required in all new vehicles. Plate glass can be commonly found in older windows (1980′s and earlier). There are a number of manufacturing processes but CSIs are more concerned with how it breaks. You’ve probably all seen this but when plate glass breaks it tends to form large (sharp) irregular pieces.  Safety glass on the other hand is designed to shatter into small thumbnail sized fragments. You may have seen these at the scene of an auto accident or vehicle break in. Safety glass generally produces more glass fragments (that more easily fall away from the window plane) that may be found at the point of entry.

Notice the broken safety glass both inside and outside the point of entry

The other major consideration is how entry was made. Did the suspect break a window and crawl in? Did the suspect break door glass and reach in to unlock a door? If so, which way does the door open? Consider a theft from a vehicle. Your bad guy breaks a small portion of the window out and reaches in to unlock the door. Some of the glass will undoubtedly fall inside on the seat and floor. But what happens next? He opens the door to the outside. Now any glass that falls will land on the concrete “outside” the point of entry. Finding a majority of the broken glass outside the vehicle may falsely suggest to some that the force came from the inside.  So while common sense suggests that more glass will be found on the side opposite of force that may not be the case because the event of falling glass is not strictly limited to that moment in time when the window is broken. Other activities like opening the door or even brushing past it may cause additional (even a majority) of the glass to dislodge and fall.  Remember that suspects may have to make several trips through the entry way to move stolen merchandise or in their general search of the premises.

Good investigators know this but a rookie or private investigator may not. As authors you may choose to use the “dominant” glass position to pit different characters and theories against each other. One camp can argue staging while the other thinks it’s legitimate. Just don’t make the mistake of concluding direction of force from the position of glass fragments alone. I’ll post an article soon on the physical examination of glass fragments in the lab (and field) to verify the true direction of force.