Thats called a tempetture stress crack, if it broke all on its own, such as had no water in it was sitting all buy its self undisturbed, then that is a flaw of the glass maker or blower. They did not properly annel it. that is the process where you have to raise the tempurature of the entire peice and keep it their for a certain amount of time, then steadely and slowly bring the temp bakc down to room temp. this is what strengthens the peice. I have seen strees cracks happen when people put hot water in it then use ice or snow or take it out side in the frezzing cold. the extream temperatur shocks the glass and stress crack happen. I would take it back and tell em what happend if they dont replace it, then you could ask the blower to fix it. it takes about 10 minutes to fix it like brand new, however it will take about 12 hours to annel it. Depending on how skilled the blower is he might not even have to stick the peice in a flame. If I were to fix something like this I would just stick it in a oven or kiln and leave it and let the oven fix it. it would take a bit long er but that would free up my time to do real work.