How old is the belief that spilling salt is unlucky and should be followed by throwing a pinch of salt over the shoulder?
Brewer's Dictionary of Phrase and Fable claims the superstition dates to the Romans, when salt was used in sacrifices and spilling it from the head of the victim would be considered a bad omen:
Spilling salt was held to be an unlucky omen by the Romans, and the superstition remains to this day, though, with us, the evil may be averted if he who spills the salt throw a pinch of it over the left shoulder with the right hand. In Leonardo da Vinci's famous picture of the Lord's Supper, Judas Iscariot is known by the salt-cellar knocked over accidentally by his arm. Salt was used in sacrifice by the Jews, as well as by the Greeks and Romans; and it is still used in baptism by the Roman Catholics. It was an emblem of purity and the sanctifying influence of a holy life on others... Spilling the salt after it was placed on the head of a victim was a bad omen, hence the superstition.
There are many who believe that the spilling of salt is a sign of bad luck and has its' origin in Leonardo da Vinci’s painting of the Last Supper!
The Last Supper has given us two common superstitions: the first is that you should never seat 13 at dinner, and the second is that spilling salt brings bad luck.
If you look closely at Leonardo da Vinci’s painting of the Last Supper, you can see that Judas has knocked the salt cellar over with his elbow. Thanks to Judas Iscariot, spilled salt is associated with treachery and lies. If you do spill salt, a pinch thrown over your left shoulder is supposed to blind the devil waiting there. - Daily Mail
In this image of the left side of Leonardo da Vinci's painting of the Last Supper, one is able to observe Judas spilling the salt on the table!