Here are some recommendations for maintaining your candle:
1) When you first light your candle you must let it burn for a minimum of one hour. This ensures a good first burn all the way across the candle. This will deter tunneling.
2) Before you light your candle (after the first burn) you need to trim the wick to 1/4 inch above the wax. This helps ensure a clean, lower flamed burn.
3) Never leave your candle burning for more than 4 hours.
If the wick is too long, or the candle is burned for too long, soot from the wick may accumulate on the jar. This blackens the jar. It is easy enough to wipe clean, but easier still to follow the above recommendations.