Quote Originally Posted by lunarose
O,

They are made with heat tempered glass in canning you typically use a process called pressure canning where you use a pressure cooker which heats the sealed filled jars to 240 degrees F under 10lbs of pressure. The only real danger of the glass breaking is if when you remove the jar straight from the oven and place it on a cold wet surface. I have done it this way a half dozen times now.
Sharon
Sharon (lunarose) is right. My mom uses them for canning and they are put into a pressure cooker full of boiling water as part of the canning process. Rarely does one ever break, and when it does it is user error not a canning jar issue...