I went shooting in South Wales on Tuesday. I shared a peg with my father shooting driven pheasant, occasional woodcock and, on a couple of drives, mallard duck. We were sharing a shotgun that fires cartridges filled with shot. The diagram shows how they are constructed. When the primer is struck, it ignites the gunpowder which turns almost instantly from a solid to a gas. The hot expanding gas forces the wad forward, throwing the shot out of the barrel.
With skill, this expanding ball of fast-moving shot can be thrown into the path of the oncoming pheasant. More often than not I find that my shot arrives where the pheasant was fractions of a second earlier, and the bird flies on unmolested. Given accurate information about the pheasant’s trajectory and speed, my distance from the bird, the speed of the shot as it leaves the barrel and its deceleration, I could calculate the exact moment to squeeze the trigger. Sadly, this information is never available at the crucial moment so I have to rely on instinct.
Lead, the metal used to make the shot, is toxic. There have been concerns about the effect that increasing levels of lead in the environment is having on wildlife. There are even some concerns about traces of lead in game eaten by humans. Lead is known to inhibit mental development and it is thought to have a harmful effect even at below the recommended safety levels. In 1999 it became illegal in the UK to use lead shot in cartridges when shooting wildfowl – aquatic species which include woodcock and duck. The advantage of lead shot is that it is heavy (density 11.3g/cm3) so it carries more kinetic energy to its target. It is also cheap and plentiful. Lead is element number 82 on the periodic table and is the highest numbered stable atom (i.e. it is not radioactive).
Finding a suitable alternative to lead has not been easy. You can buy cartridges filled with steel which is also cheap and plentiful. Its lower density (7.9g/cm3) reduces its effective range and its extra hardness means that the gun’s barrels have to be specially tested as the shot can scratch and damage them when the cartridge is fired.
We were using cartridges filled with bismuth on Tuesday. Bismuth is also a very plentiful metal element sitting one place along from lead on the periodic table. Technically it is radioactive but with a half-life that is a billion times greater than the current estimated age of the Universe, it decays so rarely that it can be treated as a stable element. It is somewhat less dense than lead (9.8g/cm3) and rather brittle, but it is an effective replacement. It has a low melting point (271°C) and is non-toxic despite being surrounded by very toxic elements on the periodic table. Unfortunately, bismuth cartridges are very much more expensive. A box of 25 Ely Grand Prix bismuth cartridges costs £26.90 whereas the comparable lead filled cartridges are only £6.75 per box.
Most bismuth is produced as a by-product of lead smelting. Smelting is the method of extracting metals from their ores by heating the metal oxide in a furnace with coke (carbon) and limestone (calcium carbonate). Bismuth is occasionally found naturally in a pure form, it is more commonly found as bismuth oxide or bismuth sulphide. These compounds contaminate the ores of other metals which is why most bismuth production is done as part of extracting other metals from their ores. Demand for bismuth is very high at the moment, with its use as a lead replacement in more than just shotgun shells, and that is why its value has trebled in the last decade or so.
Bismuth is not a very metallic metal. It is a poor conductor of electricity, a poor conductor of heat and it is quite brittle. It is also the most diamagnetic naturally occurring substance on Earth. Diamagnetic means that, in the presence of a magnetic field, it creates an opposing field so it always repels a magnet. Many materials are diamagnetic but the forces they produce are so small that you don’t normally notice the effect. Man made materials called superconductors are the most diamagnetic materials of all. The two videos below show first, pieces of bismuth being used to make a small neodymium magnet levitate and second, a superconducting disc whizzing around a magnetic track.
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Questions…
- What word means changing from a solid to a gas (or from gas to solid) without passing through a liquid phase?
- How do you calculate time from speed and distance?
- Suggest three ways in which pheasant, woodcock and duck are typical examples of birds.
- What products form when calcium carbonate is heated?
- What word do we use for materials that do not conduct heat or electricity?
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