Why wax shrinks as it cools
Explains how wax contracts as it sets, why the top of a moulded candle sinks, and how pour and room conditions change the dip.
Wax takes up less space as it cools and sets. This shrinkage causes the sink, the shallow dip that forms in the top of most moulded candles. This page explains why the dip forms, what changes its depth, and how to reduce it and fill what remains.
How the wax sets
A candle sets from the outside inward. The wax touching the mould cools first and forms a skin. The centre stays liquid for longer. When the core finally solidifies, it pulls inward, and the surface above it drops into a hollow.
The top has no mould wall to hold it level, so the drop shows there first.
What changes the depth of the sink
- Pour temperature: a hotter pour holds more heat, so the core sets later and the sink tends to be deeper.
- Room temperature: a cold room chills the outer wax quickly, so the skin hardens before the core has finished shrinking.
- Draughts: moving air cools the top unevenly and can crack the surface.
- Candle size: a thick candle holds more liquid in its core, so it sinks more than a thin one.
Steps to reduce the sink and fill the hollow
- Pour at the lower end of the temperature range given for your wax, as described in Pouring a moulded candle.
- Keep the room at a steady temperature and out of draughts until the candle has fully set.
- Wait until the candle is cold and firm all through. Do not top up while the wax is still soft.
- Melt a small amount of wax in a water bath, following the temperature limits in Melting wax in a water bath. Never heat wax over a direct flame.
- Pour the top-up slowly and let it set without draughts, so it bonds to the surface without a visible line.
What a normal sink looks like
A sink that is small, smooth and evenly placed is normal. A deep hollow, a crack in the top, or a ring of pale wax at the edge means the cooling was too fast or too uneven. Check the pour and the room before making the next candle.
Shrinkage cannot be avoided. Every wax shrinks, and every moulded candle sinks to some degree. The aim is a smooth top with a shallow dip, not a perfectly flat one. Terms such as skin and core are defined in the glossary of candle terms.