For some reason my jars keep cracking. I cannot figure out why. I am not screwing a lid on them, and it always seems to be the ready to use jar. I do set a lid on the top, but it's not even a lid that really fits the jar, so it won't screw on anyway. I have gone through at least 7 jars now, all cracked in a U shape at the bottom. These are in a controlled temperature room. I am confused . . . Has anyone else had this issue? Perhaps I need to change to plastic because I will soon be out of glass.