How bronze bells are made

A bell's sound comes from its shape, so it is cast mouth-down in a pit and then shaved to tune.

stations: 6
things upstream: 13
raw sources: 8
steps from the ground: 3

The short version

A bell is cast from bronze with a high tin content, often about 80 percent copper and 20 percent tin. The founder builds a clay core and an outer mold with a gap between them, lowers the case over the core, buries it in a pit and pours metal into the gap. The cooled bell is tuned.

Build the core

A core is built up from brick or coke on a base plate, then covered with loam, a clay mix with straw. It is shaped to the inside profile of the finished bell and dried with gentle heat.

Goes in
Clay
Make the outer mold

Loam, a mix of clay, sand and straw, is packed inside a perforated iron case and shaped to the outside profile, then dried and smoothed. Graphite and whiting give the surface a final smooth coat.

Goes in
Clay Silica sand
Close and bury the mold

The case is inverted and lowered over the core, leaving a hollow in the shape of the bell, and clamped down. It is often buried in a pit to hold the weight of the metal and let it cool evenly.
Melt the bell metal

Copper and tin are melted together in a furnace to make bell metal, a hard bronze that rings clearly.

Goes in
Copper Tin
Pour it mouth-down

The molten bronze is poured into the gap between core and case. After it cools slowly, the mold is dug up and broken away.
Tune on the lathe

Tiny amounts of metal are shaved from the inside of the bell until its harmonics ring in tune with each other.

End of the lineBronze bell

Worth repeating

The traditional metal for bells is a bronze of about 23% tin, known as bell metal.
The St. Petersglocke at Cologne Cathedral weighs 24 tons and was cast in 1922.
Bellfounding in Europe dates to the 4th or 5th century.