Uneven temp. Distribution on old glass. Temp. Of Glass resting on roof varied enough as opposed to glass not touching anything. Or Smurfs.
X2 what he said.
if there was (and usually is) a quenched stress on the glass,
imagine a balloon that has the outside layer of water frozen and putting pressure on the liquid center section, yup glass is considered to be "not quite solid" so outside is smaller than the inside. poking it anywhere in the surface tension and it breaks. according to an old timer in the glass pant 45 years ago, breaking me in said "the inside is slightly bigger than the outside and trying to get out, you make one spot weak and it's coming out"
Placing it on a large heat sink (roof) did the damage. it "cold shrank" that part of the glass and stressed it beyond it's limits. when you get the replacement one put it on a wooden or HDPE cutting board where it would not be subject to the temp changes . rear windows (called backlights in the glass plants) ALWAYS had to be resting on rubber or wood after they went through the quenching process. Even brand spanking new ones would blow up from edge temp differentials even less (20 degrees would do it sometimes)than metal roof of car and ambient air. For tempered "quenched" class it creates a stressed structure. A sharp pressure sudden almost Anywhere (edges are better for center punch or pressure breaking) will break the glass. it takes more on the center section on the glass to make it pop but it will given time. (heat vs cold differential, not using a center punch to pop it.)
all my windows sit on (2) 2"x 4" with holes drilled in them for 2'+ long wooden dowels to separate the glass pieces. if yours had broken but stayed together , like in the rear opening then you could have looked for the "butterfly wings" pieces that would tell you exactly where the stress got too bad and let it break.
on a side note when I worked in the glass plant we used to run flat pieces through the furnace for testing and one of the lab tech would ....ummm run a few extra each time and take them home. he would then get 2 more pieces cut the same way (not tempered though) and sandwich the tempered piece of glass in between. he then taped it solid around the edges to hold the glass together and center punched the middle tempered piece on the taped edge. it shattered but stayed in the square shape. looked sort of cool. he then made a rim and legs for it and sold them as coffee tables or end tables sometimes pouring coloring fluid (auto candy tint and clear lacquer) into the middle section giving them a water look,cracked ice,lava or any color you wanted. they sold very well even into the late 90's at a couple hundred each.
sort of like this (they still sell well today)
View attachment cracked glass.jpg
and from what I can see they are MORE than a couple hundred each now!
darn I do tend to run on don't I? oops
ops: sorry bout that have a great holiday.
