About a week ago, the rear window of my 91 Caprice exploded as I was driving downtown in an industrial area. Glass particles showered the back seat and the only remaining part of the window was a 3- or 4-inch fringe around the edges. I drove farther down the street then pulled over and dialed 911, thinking the window may have been shot out. The police came and looked over the car but couldn’t say what the cause may have been.

Is there a way to know how the glass was broken? Could it have been something hitting the glass or a short in the rear window defogger, which was on at the time? The window has been replaced but my curiosity is still strong.

– Matt B., San Jose, Calif.

Only with “CSI”-like attention to detail could we be sure of the cause of your broken window. Since it already has been replaced, we’re unfortunately left to guess as to the cause of this frightening situation.

The rear and side windows of most cars are constructed of tempered glass, and the windshield is laminated safety glass. Tempered glass is heated, then cooled in a special way to make the glass very strong. Also, should it break, it crumbles into supposedly harmless small pebbles. Tempered glass (labeled AS-2) has been in use for many decades and is favored by automakers for cost and weight reasons.

Laminated safety glass is used for windshields, the side and rear windows of very old cars, some 1980s-era vans and recent luxury vehicles. Laminated glass (labeled AS-1) actually is two sheets of glass fused together with a vinyl layer between. When broken, laminated glass holds together like a springy spider web, reducing the chance of occupant ejection or an outside object entering the vehicle. It also is quieter than tempered glass and offers greater protection against break-in.

Many believe laminated glass should be used in all vehicle windows, as the shatter-and-crumble nature of tempered glass increases the chance of injury and ejection during rollover accidents and often the flying pebbles cause a surprising number of lacerations.

I ran your situation past my glass guru, John Morello of Campbell Glass, and learned a lot. Morello says the vast majority of rear window replacements he performs are impact-originated, but he sees perhaps a dozen a year that are believed to result from a rear window defogger malfunction.

Thermal shock, caused by an overheated electrical connection at the glass/wire connection, can shatter the window. Typically the glass remains in place, due to the arch effect of the curved surface, but the crumbling radiates throughout the entire window. The defroster wire connection (there’s one on each side of the window) is often jostled during window cleaning, which can result in the connection coming loose, or the conductor tearing loose from the window. Discoloration of the electrical terminal provides the clue. It’s too bad in your case that the glass is no longer available for inspection.

The odds lean toward a defogger malfunction, since it was in use at the time of the incident. Perhaps an inspection of the defogger wire terminals may put this question to rest.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.