Scholars point out that in the pre-restoration version of the Last Supper, the renegade apostle can be seen spilling the table salt to indicate that a great evil was about to be perpetrated against Jesus. Salt used to serve as the Roman soldiers’ salary (accounting for the phrase “salt money”), and spilling salt means simply wasting money and bringing bad luck on the person doing so.
Salt is also a purifier, a preservative, and it symbolizes the good and lasting qualities of life. It was mixed into the foods used in the religious ceremonies of both the Greeks and Romans. One source of this superstition is Leonardo da Vinci's painting of the Last Supper--the betrayer Judas has accidentally spilled salt onto the table.
Before Leonardo’s time, the practice was to depict Judas as physically isolated by placing Christ and the 11 faithful disciples on one side of the table and Judas by himself on the other. Leonardo decided to seat Judas on the same side of the table as the others but cut him off psychologically with the subtle use of shadows, facial expressions, gestures and symbols, achieving a stronger effect of isolation by portraying the traitor as spiritually alone—a dark, staring figure who, while nervously clutching a moneybag, leans away from Christ and upsets the salt while doing so.