History is littered with innovative ideas and processes which we often take for granted today but were groundbreaking in their day. Something as simple as producing lead shot, in an effective and efficient manner, goes back to 1782 and the creation of “Shot Towers”. Have you ever sat back and thought how the perfectly spherical lead shot we see today was produced 200 years ago? How did they create lead shot in the volumes required to load the guns to fight wars?
History of the Shot Tower
The original Shot Tower was created by William Watts of Bristol even though there are others around the world who unfairly claimed his idea. Legend has it that William Watts stumbled across the process when lead falling from a burning church roof fell into a puddle of water. Whatever the truth, his unique process for creating shot was patented back in 1782. This was the year that he extended his family home in Redcliffe, Bristol to accommodate the first Shot Tower. Over the years variations on the original design have emerged such as the “wind tower” process, which simply blasted c
old air into the tower to reduce the necessary drop, and towers which were partially underground – making use of old mineshafts.
There are still original Shot Towers located around the world which have become symbols of the industrial revolution and groundbreaking technology of the time. They seem basic and simple to us today perhaps because they have been around for so long that we take them for granted?
How does a Shot Tower work?
There is obviously a lot more behind the creation of lead shot but in essence a Shot Tower is a method of cooling molten lead in a fashion which creates perfectly spherical lead shot in the most efficient and effective manner. Previous to the creation of Shot Towers moulds were used to produce lead shot but this was extremely expensive, time consuming, highly inefficient and very dangerous. The Shot Tower process revolutionised the creation of lead shot at a time when it was used as bullets in historic wars amongst other things.
The process is fairly simple in that a furnace at the top of the tower is used to heat lead until it is molten. This molten
material is then fed through a copper sieve high up in the tower. Using surface tension, and gravity, the liquid lead forms spherical balls as it falls through the sieve and the material solidifies as it drops down through the Shot Tower. At the bottom of the tower there is an area filled with cold water into which the lead shot falls bringing to an end the cooling process.
The physics behind manufacturing shot
At first glance the process of creating lead shot seems extremely straightforward but the detail behind the production of different sized lead shot is fascinating. While the variation in actual shot size is created using different sized holes in the copper sieve, the cool down process is a little more complicated.
The height of the Shot Tower is perfectly calibrated to the size of lead shot required. To give you an example, a Shot Tower which has a 40 m drop is able to produce up to #6 shot which has a diameter of 2.4 mm. While you might expect an 80 m drop Shot Tower to be able to produce shot with double the diameter, it is not that straightforward. A Shot Tower with an 80 m drop can produce up to #2 shot with a nominal diameter of 3.8 mm.
There is a fairly complicated formula used to calculate the required drop of the molten lead spherical balls to perfect the process. It is worth noting that while the molten material is constantly cooling as it falls down the tower there is a moment, when the molten material turns to a solid, that the rate of fall in temperature slows as more energy is required to solidify the lead. The height of the Shot Tower also takes into account the fact that when hitting the cold water at the bottom the lead must have a temperature less than the boiling point of water. This ensures no steam and therefore no loss of water into the atmosphere.
Collecting the lead shot
There is a collection system at the bottom of the Shot Tower which separates the perfectly spherical balls from those with any deformations. Those which are not perfectly spherical are collected, re-melted and go through the process again. The idea of using cold air, as opposed to earlier processes, to cool the lead shot as it falls through the Shot Tower is simple but highly effective. It reduces not only the time required to create the lead shot but also any additional energy requirements to cool the material.
Since the 1960s the Bliemeister method has been used for small shot sizes with larger diameter shot manufactured using hemispherical dies to stamp the molten material into perfect spheres.