The hole in the middle

You pour a tray of tubes, they set, and each one has a crater in the middle of an otherwise perfect surface.

The dimple

A conical depression at the center of the top, sometimes with a deep channel running down inside the stick where you cannot see it.

It is a common cosmetic defect in hand-poured tubes and it is not a mistake so much as physics.

Why it forms

The balm shrinks as it solidifies. It sets from the outside in, so the last liquid is in the center, and when that finally contracts there is nothing left to fill the space it occupied.

Hotter pours shrink more, because there is more contraction between pour and set. A very cold setting surface can make it worse by freezing a crust early.

The second pour

The standard answer, and the reason it has its own page. Slightly overfill, let the tubes set until the dimple has formed, then top each one with a small amount of the same melt held at pour temperature.

Timing is the skill. Too early and the second pour merges and dimples again; too late and it sits as a visible disc that can lift off.

Keep the reserve melt covered and at temperature rather than reheating it, which costs you flavor.

When it is a tube problem

A tube with a wide internal shoulder or a poorly seated elevator disc gives the balm somewhere to sink into, and no amount of second pouring fixes a stick that is dropping down the barrel.

If the tops are sinking a day later rather than dimpling at set, look at the tube rather than the formula.

References