Spilling Salt Was a Bad Omen
In the Middle Ages, salt was a precious resource. It was very expensive and it was believed to have medicinal properties. If salt was ever spilled, it was no longer able to be used for medicine and therefore it was gathered up and thrown over the left shoulder in order to blind the evil spirits that were said to constantly follow people around.
There is even older reasoning behind the spilled salt superstition. In the da Vinci painting of the Last Supper, Judas Iscariot is portrayed as having knocked over the salt. This led many to see the spilling of salt as a bad omen and something that was likely to cause bad luck. Salt was known to make soil barren for a long length of time, and this is the basis for the belief that spilling salt is akin to cursing the land.
The Romans had their own beliefs about salt. They believed that it was a symbol of friendship because of its lasting quality. However, it was very expensive and useful for preserving food, so if someone spilled salt on the table it was considered to be very ominous. In contrast, it was considered propitious to spill wine on the table.
Over the centuries there have been numerous accounts and writings from historians that relate the bad omens associated with spilling salt. In addition, since the Roman Catholic Church used salt to make Holy Water, it also had a religious significance which made spilling it a bad omen.