What Is Rock Salt?
Rock salt is the common name for halite. It is a rock, rather than a mineral, and this is what makes it different to the salt you may find on your dinner table, although they do share many characteristics.
Where Does Rock Salt Come From?
Rock salt can be found all over the world. There are deposits ringing dry lake beds, inland marginal seas, and enclosed bays and estuaries in arid regions of the world. At various times in the geologic past, very large bodies of water (such as the Mediterranean Sea and a huge body of water that sat where the Atlantic Ocean sits now) also evaporated and made enormous deposits of rock salt. These deposits were later buried by marine sediments, but since halite is less dense than the materials that make up the overlying sediments, the salt beds often “punched up” through the sediments to create dome-like structures. These are now mostly buried by additional sediments.
Rock salt in Britain was laid down around 200 million years ago, when Britain was a shallow inland sea surrounded by desert, near the Equator.
History of the Rock Salt We Supply
Salt is a vital part of human life. Although over-consumption has led to well-advertised dangers, we need salt – or sodium – to live. Salt is one of the oldest food flavourings and has been mined for thousands of years; the oldest known saltworks is in Xiechi Lake in China and dates back to 6000BC. Until relatively recent innovations in methods such as canning and freezing, salt was also one of the major food preservatives. Salt was originally panned from sea water or lakes, but rock salt is traditionally mined.
Rock salt was first discovered in Winsford in Cheshire in 1844, which is the mine used by Online Rock Salt. Local prospectors were originally searching for coal – which, ironically, would be used to heat the brine-filled pans that made salt. The value of their find did not go unnoticed, however, and during the nineteenth century mining methods led to one million tonnes of rock salt being mined between 1844 and 1892.
Mining methods were still relatively basic in this era, however. Shovels, hand picks and black powder explosives were used to excavate the rock, which was then transported to the surface in wooden barrels. The work was back-breaking and often dark and uncomfortable. Up until the introduction of electricity to the mine in the 1930s tallow candles were stuck to the rock and used to light the working face. Bundles of unused candles can still be found in the old cavities of the mine to this day.
The Winsford mine was closed in 1892 to make way for the local Northwich mines. The salt market in the UK at the time was overcrowded and there was no room for two such close competitors. However, the Northwich mines were flooded in 1928 which led to the re-opening of the Winsford mine – and it remains in production today, making it the oldest working salt mine still in production today.
How Is the Rock Salt Mined?
These mines vary in depth from 100 metres to a mile or more. Within the mines there are networks of pathways formed by the areas from which salt has already been extracted, and they are used as roads for mining vehicles to move from one part of the mine to another, and also used by the personnel to move from the shafts to the working face.
A machine similar to the pneumatic drill for digging up roads is used on the working face. It has a rotating head carrying tungsten-carbide tips, which bores into the salt. The lumps are then taken to a crushing and screening plant, without the need to be crushed by a feeder-breaker first. Care must be taken to ensure that the mine is stable by leaving substantial pillars of salt to support the mine roof. This technique has been used for hundreds of years, and at Winsford you can see the pillars left by the original miners of the 1840s.
Before storage, the salt is treated with an anti-caking agent which will stop the pieces coagulating. This ensures that it can be held in local storage depots, ready for use as soon as the bad weather sets in!
What About White Salt?
Unlike traditional rock salt, white marine salt is not mined from underground. Instead it is harvested by evaporating sea water, which leaves behind a cleaner product.
White salt works just as well as brown rock salt but does not leave behind the same residue that is often traipsed into carpets and so is a better option for those who have areas with a high footfall such as schools and shopping centres.
The evaporation process requires a warm climate - much warmer than we can achieve in the UK which is why white salt is imported from places such as Spain, Egypt & Sardinia.
Some More Salt Facts
- Many of the towns in Cheshire such as Northwich, Middlewich and Nantwich take their names from the production of salt. The endings ‘wych’ or ‘wich’ mean ‘brine town’.
- 87% of the nations salt was produced in Cheshire by the close of the 19th century.
- Salt is essential to life, the average human adult has 250g of salt in their body.
- Salt is used in the production of over 14,000 products ranging from food to chemicals.
- Salt has been important for the ritual of society for hundreds of years, it has been used to preserve mummies in Egypt and even as part of religious ceremonies to symbolise purity.
- The world’s oceans contain a huge amount of Salt; if all of this salt was on land it would be enough to cover the uk with a layer of salt 50 miles deep!
- The UK’s salt mines have around 140 miles of tunnels – that’s the same distance as from London to Brussels!