Candle glass may crack due to an array of reasons. The most common cause of glass cracking is if there wasn't enough wax left to absorb the heat of the flame, making the glass very hot, which can cause the glass to crack after an extended period.
To prevent this we recommend you stop burning when there is 1cm of unmelted wax remaining in the candle. In other cases, cracking may happen when the wicks travel and come in contact with the glass. This can happen if the candle has been burning longer than the recommended burning time, as the wax pool increases and the wicks become less stable and the flame moves towards the glass. If your glass has cracked, and you don’t believe the reason to be stated above, please take images of the candle and email them to firstname.lastname@example.org and we will investigate further.