Rock Salt - the myths and the facts

What is road or rock salt and how does it work?

When cold weather arrives, stores may stock large bags of road or rock salt, which is often spread on pavements and roads to melt ice and snow. Road salt is halite, the natural mineral form of table salt or sodium chloride (NaCl). Unlike purified table salt, rock salt contains mineral impurities and typically appears brownish or gray. Machines mine the salt, which is then crushed and packaged for delivery. Additives like sodium hexacyanoferrate(II) and sugar are sometimes mixed with road salt to prevent caking and ease delivery using gritting machines.

How Road Salt Works

Road salt works by lowering the freezing point of water via freezing point depression. The salt breaks into its component ions in a small amount of liquid water, making it harder for the water to freeze into ice. Therefore, for road salt to be effective, there needs to be a bit of liquid water present. The salt will function down to -10°C if its concentration is high enough. The hard crust that sometimes forms is due to the salt absorbing water and creating a "saltwater liquor," which dries and reforms as a crust.

Myth: Salting a road prevents the formation of ice

Salt lowers the freezing temperature of water, preventing ice or frost from forming on the carriageway as temperatures fall. Generally, salt loses effectiveness below -10°C. Pre-salting the road forms a debonding layer so snow does not freeze onto the road surface and can be ploughed off or churned off by vehicle movements.

Myth: Spreading salt on ice or snow will melt and remove it quickly without any other actions

Salt comes in grain sizes of 6mm or 10mm and is spread at rates between 10 and 40 grams per square metre depending on the forecast road surface temperatures and whether snow is expected. Each grain melts surrounding ice, forming a pool of salty water that helps melt more ice. Without traffic to move the salt and salty water around, the melting process can take considerable time. Snow falling on salted surfaces begins to melt from beneath, and vehicular movement speeds up this process. However, initial vehicles compress the snow into ice, similar to creating a snowball.

Myth: It’s too cold for snow

While temperature affects the moisture air can hold, it must drop below -40°C before the air contains so little moisture that snow rarely occurs. In many regions, most rainfall begins as snow in the upper atmosphere and turns to rain as it falls through warmer lower layers. Winter brings colder lower atmospheric air, allowing snow to reach the ground. Slight temperature changes at ground level due to wind and altitude can change precipitation types over short distances.

Myth: All water freezes at zero degrees centigrade

Freezing rain occurs when precipitation starts as snow in the upper atmosphere, turns to rain in warm air layers, and cools below freezing point in a thin cold air layer near the surface. Supercooled water droplets freeze upon contact with surfaces, rendering grains of salt ineffective until temperatures rise. Freezing rain disrupts road travel and makes it unsafe for heavy gritting vehicles due to traction loss.

The salt bins are for public roads, pedestrian areas, and footways only, not private properties. Volunteers spread the salt, and the bins are checked and refilled at the start of winter. Usage is not monitored, so residents should inform the Clerk if bins are empty. Parish Council refills bins if possible, with each bin holding about 200kg. Excess salt supplied by Wiltshire Council is stored by Councillor Stephen Crossman and used to refill bins as needed.

Misuse of salt from bins can be reported to the Council confidentially. Theft of salt should be reported to the Police with details of any involved vehicles.