Under what conditions can glass crack spontaneously?

[davenn]

Some of these lines may be crinkles in the label, but others look suspiciously line small cracks in the windshield.

Follow the paralleled white line that I drew on the image just below the crack ... that is the ONLY crack in the glass ... a single clean curved crack
That label has been on since the car was manufactured 5-5.5 yrs ago the cracks in it is because of the hot Aussie summers often exceeding 40C

To the left of this nice smooth curved section is the more meandering crack that expands in little bursts every few days or so
that is shown in the first image in my previous postDave

 

[collinsmark]

As @CWatters mentioned, the front windshield is usually made from a different process than other car windows. The front windshield is usually not tempered, and instead made from laminated glass. Laminated glass is made by sandwiching plastic in between two sheets of non-tempered glass.

There's good reason for that. If the front windshield was tempered glass and did shatter, it would heavily obstruct the view in front of the driver. Therefore, laminated glass makes the better choice for this window (or windshield). When it does crack, the cracks are typically isolated, clean cracks rather than the spiderweb pattern of tempered glass. 'Makes it much easier to see through when cracks happen. Additionally, any debris that hits the windshield might only break the outer layer (of the laminated glass) and not puncture through the window and hit the driver who is in control of the vehicle.

 

[davenn]

The front windshield is usually not tempered, and instead made from laminated glass. Laminated glass is made by sandwiching plastic in between two sheets of non-tempered glass.

yup, that appears to be the way these days ... but it wasn't always like that ..

There's good reason for that. If the front windshield was tempered glass and did shatter, it would heavily obstruct the view in front of the driver.

and I have been in that situation a few times in years gone by ... down the road at 100km/h and boom, zero visibility ... not fun!