You need to have the right information and tools if you are going to break a glass window or door to make an exigent entry into a structure or to rescue a trapped motorist. There are three types of glass you will encounter: tempered glass, non-tempered glass, and laminated glass.
Tempered glass is special heat-treated form of glass. Tempered glass is used where there is a likelihood of human contact and as such is made to shatter into small ¼” pieces when broken. The small pieces are not very sharp and are relatively safe to handle.
Non-tempered glass is a different animal. When broken, non-tempered glass makes large, irregular, jagged pieces that are razor sharp. Non-tempered glass is much more dangerous than tempered glass and should be handled with caution, regardless of its thickness.
Laminated glass has plastic layers sandwiched between glass layers. Because of the plastic sheeting, laminated glass tends to puncture under impact (see video where Mr. Summers tries to break the double laminate glass). Although the glass breaks, the plastic will hold the laminate together. High impact and/or cutting forces are needed to adequately defeat laminated glass. Fragmentation and spalling will also send glass flakes and powdered glass inward, which can be a danger to those on the opposite side.
Here are some rough generalizations about glass. Car side and rear windows, newer storefront windows, and modern sliding glass doors are made of tempered glass. Many building codes require tempered glass be installed in residential and commercial windows that are within 48” of doors. Tempered glass normally has manufacturers’ watermarks at all four corners, however it can be special ordered without the watermarks.
Non-tempered glass is most often encountered in ordinary residential windows, whether jalousie, casement, Miami-style, single or double hung. Older sliding glass doors and older storefront windows were made from non-tempered glass and can kill you or cause severe lacerations when shattered. These break into very large, very heavy, and very sharp shards. I once worked for a window company and all of us dreaded replacing large non-tempered glass. It is extremely dangerous to work with.
Car windshields, skylights, and some commercial storefront doors and windows are constructed with laminated glass, which is the safest but toughest of the three glasses mentioned here. The problem is that you have to punch out the entire plastic sheet to clear a door or window and it can take time, tools, and technique.
Another type of laminated glass is wire glass. This is made for security purposes and is very hard to defeat, as the wire mesh is anchored to the window or door’s frame. This glass has the visible crosshatch of the wire when you look through it. Bring a siege engine, because wire glass will laugh at your hammer.
If you have the time, you should always wear gloves, eyewear, and protective gear when breaching glass. If you have to break out a large window in an older structure, you should not break it at the bottom. If it is non-tempered glass, it will rain down heavy fragments that will injure you badly.
The safest place to strike large glass is near an upper corner, while you are standing as far to the side of the window as you can. Once the glass is broken, use a longer item, like your baton or a long handled tool, to clear the rest of the glass from the opening. Pay particular attention to clearing out the glass above you.
Window punches used for car side glass are used effectively at a lower corner of the window. There is less danger from the safety glass when it comes down. We were issued some spring-loaded plastic window punches for TAC. When tried on a crashed car one night, they did not work reliably. If you have a punch such as this, you need to test it on real windows.
Many things can be used to break glass. There are better tools than your standard issue baton. I have a Bust a Cap window-breaking device mounted on the end cap of my Streamlight Stinger. I have tested it and it works just as well as in Mr. Summers’ video.
Glass can be a very dangerous substance, and is one we are sometimes forced to work with on the street. Do your best to respect the glass and use as much safety gear as you have available.
Finally, while not a glass, polycarbonate sheeting is used in place of glass, but if you are attempting to physically bash out the bullet-resistant windows of a bank or on your light armored vehicle, you need serious help.