Tunnelling — a narrow shaft burning down the middle with a wall of unused wax around it — is the most common candle complaint there is. It has two causes.

Cause one: the wick is too small for the vessel

The flame has to generate enough heat to melt the wax all the way out to the glass. If it cannot, it never will, no matter how long it burns.

This is decided by the wick calculation before pouring, and nothing the customer does afterwards can rescue it.

Cause two: the first burn was too short

Wax has a memory. The first burn sets the diameter of the melt pool, and every subsequent burn tends to follow it.

Light a new candle for twenty minutes and you have taught it to tunnel permanently. The first burn should run until the melt pool reaches the edge, usually two to three hours.

How to tell which one you have

If the candle tunnelled from the very first long burn, it is the wick. If it burned cleanly to the edge once and only started tunnelling after some short burns, it is the memory.

The second can sometimes be partly recovered with a long burn; the first cannot be recovered at all.

What a good supplier does about it

Sizes the wick per vessel and per fragrance load, test burns every new combination, and prints the first-burn instruction on the label rather than hoping the customer knows.