How bronze statues are made

Bronze swells slightly just before it sets, so it fills the finest details of the mold.

stations: 7
things upstream: 16
raw sources: 9
steps from the ground: 4

The short version

Most bronze statues are made by lost-wax casting. An artist makes a model, then a mold of it produces a wax twin. The wax is coated in ceramic shell and melted out, leaving a hollow. Bronze is poured in, the shell is chipped away, and the surface is welded, polished and patinated.

Sculpt the model

The artist builds the figure in clay or plaster on a supporting armature, working through smaller study models for large works.

Goes in
Clay
Make the wax copy

A mold is taken from the model and a wax copy is cast in it. For large sculptures the wax is hollow, with a core inside, and wax sprues are added to carry the metal in.

Goes in
Beeswax
Coat it in ceramic

Workers dip the wax tree repeatedly in a ceramic slurry and sand until a hard shell covers it.

Goes in
Silica sand
Burn out the wax

The shell is heated so the wax melts and drains away. The hollow left behind is an exact negative of the figure.
Pour the bronze

Molten bronze is poured through the sprues and fills the hollow from the bottom up so that it does not splash. Bronze alloys expand slightly just before setting and shrink after.

Goes in
Bronze
Chip, weld and chase

Once cool, the shell is chipped away and the sprues are cut off with a saw. Gas holes are welded, and tool marks are polished out.
Patinate the surface

Corrosive chemicals are applied, which give the artist control over the color and finish.

End of the lineBronze statue

Worth repeating

Investment casting has been used in various forms for the last 5,000 years.
During the Shang dynasty, ritual bronzes were buried in sets of up to 200 pieces in royal tombs.
From the ninth through the thirteenth century, the Chola dynasty in South India represented the pinnacle of bronze casting.