These vessels can be likened to kettles where the wort is boiled along with the hops and any sugar included in the recipe. The fact that these vessels were originally heated by coal and without tops must have made for some fairly unpleasant working conditions.

 The wort is boiled for three reasons:

1. To extract bitterness from the hops.
2. To kill any bacteria present.
3. To remove protein from the wort which may otherwise cause haze in the beer at a later stage.

These days the coppers are heated by steam.