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What could be the cause of this glass crack in a window?

2 months ago

I live in a rented apartment and have noticed a glass crack in a window, so I'm trying to figure out whether I damaged the glass or it somehow got damaged on its own (for example, by thermal stress), in order to decide how to talk to the landlord about it.

The crack is in the inner pane of a double-paned window and looks pretty unusual, unlike a "normal" glass crack. Here's a photo (in which I put a piece of paper on the glass to make the camera focus on the glass): quZmL.jpg (905×2185) (

The photo was taken when the camera was oriented parallel to the window – that is, the line of sight was perpendicular to the window.

As you can see in this photo, the crack is an ideally straight line that goes exactly vertically, getting thinner towards the ends, and is about 15 cm long and 1 mm wide. The crack looks as if it were a long cavity filled with air or water, no matter the angle you look at it, so I'm unsure whether it's really a crack or something else. And when I touch it with my finger, I don't feel any surface irregularity whatsoever, so the damage seems to be entirely inside the glass. The crack is near the center of the window, so both ends of the crack are far from the frame.

I wouldn't suspect myself of damaging the window if I hadn't recently killed an insect on it with my finger. The impact of my finger was so mild that no one would ever expect any glass damage, and I didn't see any crack appear when I killed the insect. Neither did I hear any crack sound. But the crack is pretty much where I killed the insect, and I noticed the crack about an hour after I killed the insect, so I'm at a loss as to whether the crack is due to that impact. I've never really paid attention to that window, so I can't exclude that the crack was already there when I moved in.

My question: Based on what the crack looks like, can I exclude the possibility of it having been caused by that physical impact of my finger? Could a glass crack due to an impact look like this? What could have caused such a crack?

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