Under the tiles was a layer of salt, to retain the heat from the fire. This layer would eventually loose its savor and no longer do its job. So the tiles would be taken up, and the old salt removed. Then a new layer would be poured in and the tiles replaced. The impurities looked white like sodium chloride, but did not have the qualities of salt.
Salt is soluble.
Insoluble salts, dirt, sand, especially dampness can leach away the salt leaving other minerals behind. When you lose Jesus you lose your saltiness. Be careful what you log on to on the internet. Don't allow something to sever you connection with Jesus. Keep up your wholehearted devotions to Jesus.