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In a shot tower, lead is heated until molten, then dropped through a copper sieve high in the tower. The liquid lead forms tiny spherical balls by surface tension, then solidifies as it falls. 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 remelted. 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 1782. The same year, Watts extended his house in Redcliffe, Bristol to build the first shot tower. 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.
The "wind tower" method, patented in 1848 by the T.O LeRoy Company of New York City, which used a blast of cold air to dramatically shorten the drop necessary meant that tall shot towers became unnecessary, but many were still constructed into the late 1880s, and two surviving examples date from 1916 and 1969. Since the 1960s the Bliemeister method is used to make smaller shot sizes, and larger sizes are made by the cold swaging process of feeding calibrated lengths of wire into hemispherical dies and stamping them into spheres.
The Jackson Ferry Shot Tower, located in Wythe County, Virginia began construction around the end of the Revolutionary War in this rural area. It was built of stone with walls almost a meter (3.28 feet) thick, as it was not practical to use brick in that region for such a large structure. The 23 meter (75 foot) tower was built at the edge of a cliff and utilized a subterranean shaft of the same length to double the overall height the lead would drop.
In 1855–56, the architect James Bogardus built two shot towers in Lower Manhattan, the 175-foot McCullough Tower on Centre Street and the 217-foot Tapham Tower on Beekman Street. Both structures were demolished in the first decade of the 20th century.
The Shot Tower on the South Bank of the River Thames in central London, England was constructed in 1826, and was in use until 1949, but was demolished to make way for the Queen Elizabeth Hall (opened in 1967), after being retained as a feature in the 1951 Festival of Britain. Another square shot tower was also located not far away downstream along the Thames.
Another shot tower is the Taroona Shot Tower situated 11 km out of Hobart, Tasmania. This shot tower is the tallest stone shot tower in the southern hemisphere. Other surviving examples of shot towers include:
- Cheese Lane Shot Tower, Bristol, UK, 1969 reinforced concrete tower built to replace the very first shot tower built in Redcliffe by William Watts – and now part of an office development called 'Vertigo'
- Chester Shot Tower, Boughton, UK – circular brick tower, built in 1799; the oldest surviving shot tower in the UK
- Clifton Hill Shot Tower, Melbourne, Australia
- Coop's Shot Tower, Melbourne, Australia
- Daugavpils Shot Factory Tower, Daugavpils, Latvia, still in operation
- Drochtersen Shot Tower, Drochtersen, Germany
- Helena Shot Tower, Spring Green, Wisconsin, US – built in 1831, now part of a state park.
- Dubuque Shot Tower, Dubuque, Iowa, USA
- New Vienna Shot Tower, New Vienna, Iowa USA
- Jackson Ferry Shot Tower in Wythe County, Virginia, USA – built in the 1790s, now part of a state park and open to the public during the tourist season.
- Peters Shot Tower, Kings Mills, Ohio, USA
- Pispala Shot Tower, Pispala, Tampere, Finland
- Rossie Shot Tower, Rossie, New York, USA
- Phoenix Shot Tower at Baltimore, Maryland, USA
- Remington Shot Tower, Bridgeport, Connecticut, USA
- Shot Ball Tower, Berlin-Lichtenberg, Germany – built in 1908
- Sparks Shot Tower, Philadelphia, Pennsylvania, USA – built in 1808
- Stelco Shot Tower, Montreal, Quebec, Canada
- Taroona Shot Tower, Hobart, Australia
- Winchester Shot Tower, New Haven, Connecticut, USA
- Colonial Ammunition Company Shot Tower, Mt Eden, Auckland, New Zealand - 20th century, steel framed
- Brussels Shot Tower, Brussels, Belgium
- Drop tube, a similar concept, but used for scientific experiments
- Prill, a small granule of material formed by a similar process to shot-making. Often used in the chemical industry for solid chemicals.
- "Up a shot tower". The Strand Magazine. 1891. p. 205.
|Wikimedia Commons has media related to Shot towers.|
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- Tower Hill State Park Shot Tower, WI, US.
- Shot Tower Historical State Park, VA, US
- "Colonial Ammunition Company Shot Tower". Register of Historic Places. Heritage New Zealand. Retrieved 2010-03-02.
- "Gravity Molds Shot In A Modern Tower", October 1944, Popular Science detailed and large article on subject of Shot Towers