Jesus said, “Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men" (Matthew 5:13)
Table salt is the simple chemical called sodium chloride. It is responsible for the salty taste of, for example, the sea. No other chemical tastes quite like salt, so it is chemical illiteracy to say that salt can lose its saltiness. Other (similar) compounds formed from sodium chloride can never taste as salt does.
Salt was a valuable commodity in Palestine during Roman times, so we know a lot about its production: salt came from two sources, either from salt mines or by evaporating salt-containing water. Rock salt is quite pure and in Roman times was more precious than salt from the evaporators. It was ground into small lumps ca. 1-3 mm across. Most of the salt used, however, was the cheaper evaporated material. Production of evaporated salt was as follows: the water at the edge of a lake was sectioned off, leaving a wide but shallow pool. The water would soon evaporate in the warmth of the sun to leave an off-white powdery solid. This ‘salt’ was very impure and contained many other substances such as chalk and sand. Water from the dead sea also contains gypsum. The crude material could be used as produced, or could be purified by dissolving in a small quantity of pure water. Impurities being less soluble would remain solid while the salt dissolved. The second stage of purification was thus a simple process of filtration followed by a second evaporation. This purified salt was as almost as pure as rock salt.
Most of the poor (and therefore most of Jesus’ audience) would have used the cheaper, impure salt. If this salt was stored in a damp place then the soluble (real) salt would leach from the powder leaving behind the less-soluble impurities. Although looking like the real thing, this salt-free ‘salt’ would thus have lost its saltiness. This white powdery impurity was not very soluble, had no taste and only a slight preservative ability, and would have to be thrown away.