Tunneling

Tunneling happens when a candle burns down through the middle only, leaving a ring of solid, unmelted wax along the inside of the container. It is one of the most common complaints from candle buyers and one of the most common troubleshooting topics for makers.

The usual causes are an undersized wick for the container, a first burn that was stopped before the melt pool reached the edges, or drafts that pull the flame to one side. Once tunneling starts, it tends to get worse with each burn, because the walls of hardened wax insulate the remaining wax from the flame's heat. Left unaddressed, tunneling wastes a large share of the wax in a container and can leave the wick sitting in a deep, narrow well later in the candle's life, which changes flame height and burn behavior.

Preventing tunneling starts with correct wick sizing and a first burn that runs long enough for a full melt pool. If a candle has already started tunneling, gently warming the surface, trimming the wick, and shielding the candle from drafts can help later burns recover some of the lost width, though a badly undersized wick will keep tunneling until it is replaced in a future batch.