How big bells are made

The modern bellmaking process was developed centuries ago and has changed very little since the 1400s. Large bells are created using a "founding" process, in which hot liquid metal is poured into a mold. People who make things this way are called "founders," and they work in a foundry.

Here's how bellmakers create and tune a bell at the Verdin Company in Cincinnati, Ohio. You can see pictures of each step of the process at their web site: www.verdin.com/verdin. (Click on the "Casting Bells" option.)

First, a mold is made. The part of the mold that shapes the inside of the bell is created first. It's made out of brick and molding sand. Then a "false bell" is made of clay to fill the space that the real bell will occupy. The company name and any other inscriptions are added to the false bell.

Now the outer mold is built up, and an iron cage is added to the outside to support the molten metal. The outer mold is lifted up, and the false bell is broken away. The inner and outer molds are then cleaned and sealed together.

The bell is made of bronze: four parts copper and one part tin. (The smallest bells have a little more tin, to give a clear, sharp sound to the higher notes they produce.) The metals are heated to 2,000 degrees F. and poured into the mold. The metal-filled mold is buried in the ground so it will cool slowly and evenly for up to three weeks.

After the bell has cooled, it is removed from the mold and polished. Then it must be "tuned" to the exact pitch needed. Cast bells are made a little thicker than the final bell needs to be. This allows the tuner to shave metal off the inside of the bell until the sound is exactly right. He does this using a lathe or a boring mill.

Now the bell is ready to take its place in a carillon or church tower.

Carillons (KAIR-uh-lons), are sets of at least 23 tuned bells that can play melodies. Originally, small sets of bells were played with a hammer. By the 1200s, a mechanical linkage was used. The first keyboard-operated carillon appeared in Antwerp, Belgium, about 1480. The oldest existing carillon today, in Amsterdam, dates back to 1554.