Glass is a vital component of any car. It’s how we’re able to glance up at the road from our phones without our faces getting blasted with wind. If someone drives enough, they’ll become very aware of every line, scratch and smudge on their windshield. They might even notice a strange pattern of light spots on their rear window under certain circumstances.
Those spots, officially referred to as quench marks, are ultimately the result of how the light reflects through the rear window. Most vehicles use tempered glass for the rear window. During the manufacturing process, the panes are heated to over 1,000 degrees and then quenched to strengthen the glass. However, the quenching jets can’t uniformly cool the panes, according to glassmaker Pilkington North America. The uneven cooling causes uneven shrinkage and density across the surface, leading to uneven light reflection.
Quench marks are completely normal, but car owners are understandably concerned when they notice them. Automakers occasionally have to send out service bulletins because of this. In 2021, General Motors sent out a bulletin concerning every model it has ever built simply reading:
Some customers may comment on spots showing on the rear and side windows when the vehicle is in the sunlight. When wearing polarized glasses/sunglasses, these spots may be more magnified.
The spots are due to the manufacturing process and are “quench marks” in the glass. Quench marks are an unintentional side effect of the tempering process.
Technicians should explain to the customer that this is a normal condition and part of the manufacturing process for tempered glass. Do not replace the window(s) for this condition.
Typically, windshields don’t show this same phenomenon because of differing manufacturing processes. Laminated glass is used for windshields, where a layer of polyvinyl is sandwiched between two layers of glass. It’s designed to be difficult to punch through cleanly, but can still crack. One Tesla Cybertruck owner found that out the hard way after walking over their windshield. Please don’t walk over any glass surface on your car!