Sandy,

Adding iodine to salt does nothing for the salt, but our bodies need trace amounts of iodine for the thyroid to function properly. Adding it to salt is an easy way for us to get our required amount and prevent thyroid disorders such as goiters (a swelling of the thyroid gland which is in the neck). I belive this began in the early 1900's.

I don't know why salt is associated with hospitality but I bet it has something to do with salt being used as currency by the Romans. Roman soldiers were paid with salt (hince salary). It was probably considered polite to bring a host a little salt when invited to a dinner party.

Good luck with your seach. Are you doing a paper for a class or is this a personal quest? I'd love to know what you find out. Trivia is my middle name.