Glossary
Melt pool
The area of liquid wax that forms around a lit wick, used to judge whether a candle is burning evenly.
Melt pool is the pool of liquid wax that forms around a burning wick as the candle heats up. It grows outward from the wick with each hour of burning and is one of the clearest visual signs of whether a wick is sized correctly for its container.
On a well matched candle, the melt pool should reach the edges of the container within the first few hours of the first burn, then stay roughly full width on later burns. A melt pool that stalls partway to the edge points to an undersized wick, and that pattern is one of the leading causes of tunnelling, where the candle burns a narrow well down the center and leaves solid wax untouched along the sides.
Watching the melt pool during a candle's first burn is one of the simplest checks a maker or customer can do, since it needs no special tools beyond patience and a safe place to let the candle burn. A common mistake is extinguishing a candle as soon as it looks lit rather than letting the first burn run long enough for the melt pool to fully establish.