Shot towers were used in the 18th and 19th centuries in the manufacture of lead shot for weapons. In a shot tower, lead is heated until molten, then dropped through a copper sieve high up in the tower. The liquid lead solidifies as it falls and by surface tension forms tiny spherical balls. The partially cooled balls are caught at the floor of the tower in a water-filled basin. The now fully cooled balls are checked for roundness and sorted by size; those that are "out of round" are re-melted. A slightly inclined table is used for checking roundness. To make larger shot sizes, a copper sieve with larger holes is used. However, the maximum size is limited by the height of the tower, because larger shot sizes must fall farther to cool. A polishing with a slight amount of graphite is necessary for lubrication and to prevent oxidation.
The process was invented by William Watts of Bristol, UK, and patented in the late 18th century. Watts extended his house in Redcliffe, Bristol to build the first shot tower in 1782. Shot towers replaced the earlier techniques of casting shot in moulds, which was expensive, or of dripping molten lead into water barrels, which produced insufficiently spherical balls. Large shot which could not be made by the shot tower were made by tumbling pieces of cut lead sheet in a barrel until round. Shot towers were replaced by the "wind tower" method by the end of the 19th century, which used a blast of cold air to dramatically shorten the drop necessary. Today the Bliemeister method is used to make smaller shot sizes, and larger sizes are made by the process of feeding calibrated lengths of wire into hemispherical dies and stamping them into spheres.