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Most of us are familiar with the use of rock salt to de-ice an area and provide some traction to make it less slippery but have you ever wondered where the rock salt comes from, how it works and how to use it effectively? Here is some background information about rock salt and how best to store and spread it so you can stay safe during the winter months. Rock salt used for de-icing is mainly sourced in the UK from salt mines in North West England. The scientific term for rock salt is halite and this particular salt differs from other forms of salt because it is classed as a rock rather than a mineral. The salt is formed when large bodies of water that contain salt evaporate leaving the rock salt behind. The salt then becomes buried over time, which is why it must be mined in order to retrieve it. In fact the salt that is mined in Britain dates back some 250 million years ago when the waters were surrounded by desert! Fortunately there is a plentiful supply of rock salt and in fact the same mines have been used since the 1800s, providing over 100 million tonnes of salt. The method by which it is extracted has progressed from picks and explosives to far safer processes that use boring pneumatic drills. Salt melts ice by lowering its freezing point. When you add salt particles to water the freezing point will continue to lower until the salt can no longer dissolve. In a lab this occurs at around –21 degrees Celsius but when used out on the streets to melt ice this freezing point is much higher at around –9 degrees Celsius. If temperatures fall below this then the salt itself will begin to freeze becoming ineffective. Once the rock salt has been spread on the ice and melted it the freezing point of the water will remain much lower so provided temperatures do not fall too low it will also prevent the ice from re-forming. When storing salt it is important to remember that the salt will freeze solid at temperatures below –7 degrees Celsius so it may be best to keep it indoors or in a garage that will not be too cold. A grit bin is generally the safest way to store salt because it is specially manufactured from non-corrosive polyethylene. A large handful of rock salt per square metre should provide some traction although for areas of a high footfall you may need a little more. Try to ensure that the salt is spread evenly using a shovel or spreader and make sure the snow and ice has been cleared as much as possible prior to spreading. Be prepared for the unpredictable UK weather and make sure you have a plentiful supply of rock salt along with a grit bin to store it safely.
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