Sinkholes

A candle that looked perfect can have a void inside it, and the first sign is the wick disappearing into a hole.

The cavity you cannot see

Wax shrinks as it solidifies. It sets from the outside in, and the last liquid is beside the wick, where it contracts into a cavity under an already-solid surface.

Sometimes it shows as a dimple. Often the top looks flawless and the void is underneath.

Why it forms

Pouring too hot, which means more contraction between pour and set.

Cooling too fast, which sets a crust before the middle has finished.

Cooling unevenly, which does the same on one side.

Some waxes shrink far more than others, and the supplier's pour temperature range exists largely because of this.

The second pour

The standard answer. Pour slightly under, let it set until the sink has formed, then top up with wax from the same batch at the same temperature.

Timing is the skill: too early and it merges and sinks again, too late and it shows as a visible ring where the two pours met.

Keep the reserve covered and at temperature rather than reheating it, because reheating changes the scent of the top layer relative to the body.

The dangerous version

A deep cavity beside the wick is not only cosmetic. The flame reaches it, drops, and burns inside a pocket where it cannot draw air properly.

In a container that means a concentration of heat in one spot on the glass, which is one of the ways a vessel cracks.

Which is why a sinkhole found after the fact is a candle to remelt rather than a candle to top up and sell.

References