system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the purpose of the black dots on the edge of the windows of cars and trains?
gpt
So though all of these answers have some truth to them they are not completely correct. The black stuff is called frit, and it "fades" from heavy at the edge to light due to the tempering process. If it were a solid color it would heat the edge too fast and shatter in the oven.

The purpose is for the adhesive to have something to adhere to, and hide the glue edge for a more finished look.

I used to run the tempering oven that made windshields and we had to "print" the frit on in a very specific way to prevent breakage while tempering.