Strangely enough, in my 10 years of candlemaking, I've never had a jar crack. Until now! I never ever re-use the jars, as glass tends to weaken. Had a regular customer give a friend a candle for Xmas. Customer tells me her friends jar cracked. The candle was about gone, did not do anything different from my standpoint, did not re-use the jar, etc. No freakin clue what happened! She was not upset that it cracked... it didn't hurt anything.....she caught it right away......she was only mad because there was "still some candle left" ......her only complaint.