Our sages recommend the best dessert: After every time you eat - you should eat salt; after every time you drink - you should drink water (B. Brachot 40a). The salt and water dessert is not a culinary recommendation, it is health advice: Salt or water effectively nullifies any possible harmful effects of the food just ingested.
The passage continues with a similar source that expresses the idea in more threatening terms: If you ate any food but did not eat salt, or if you drank any drink but did not drink water - during the daytime you should be concerned about the possibility of bad breath and during the nighttime you should be concerned about the possibility of askera. Askera is a horrible death, commonly identified as diphtheria. It involves the inflammation of the digestive tract up to the throat, resulting in strangulation (Rashi, 11th century, France).
The talmudic post-meal salt and water requirements are codified in the Shulhan Aruch (OH 179:6) where we are further instructed to dip our middle finger or third finger in the salt; dipping the thumb - we are warned - may result in burying children; dipping the pinky may bring about poverty and dipping the forefinger may bring about severe boils (Shibbolei Haleket, 13th century, Italy).
Apparently connected to this salt-eating custom, elsewhere in our tractate we are instructed to wash our hands at the end of meal before reciting Grace After Meals (B. Brachot 53b). This ritual is known as mayim aharonim (last water). Elsewhere in the Talmud, mayim aharonim is described as an obligation (B. Hullin 105a) and the mayim aharonim requirement is codified as law (Shulhan Aruch 181:1).
In a third talmudic passage relating to mayim aharonim the Talmud asks why washing the hands at the end of the meal is obligatory, and here the connection between the salty ending of the meal and the compulsory hand wash is stated clearly (B. Eruvin 17b; B. Hullin 105b): "Why did the sages declare that mayim aharonim is obligatory?" The Talmud explains: "Because of Sodomite salt that blinds the eyes." Sodomite salt is so potent that one sage explained that it is found in the concentration of one grain per kor (430 liters) of ordinary salt, and it is still harmful.
The danger of Sodomite salt led the sages to rule that a soldier at the front was released from the obligation to wash his hands before eating bread, but must still wash his hands at the conclusion of his meal; evidently Sodomite salt was a clear and present danger on the battlefield.
Other contact with Sodomite salt also requires washing of the hands. Thus if Sodomite salt was measured to give to an animal, for instance, the one who served it must also wash his hands even though he didn't ingest any of it.
Sodomite salt is presumably salt from the area of Sodom, that is, the Dead Sea plain. Nowadays, salt from this area has high concentrations of minerals that can be harmful to the eyes. Salt from the Dead Sea area is also extremely fine and a person may not realize when there is salty residue on his hands that may eventually reach the eyes. Nevertheless, the potency of Sodomite salt to blind upon contact with the eyes is not something we need to be concerned about today.
Many do not wash mayim aharonim at the end of a meal, and even fewer people are careful to have a lick of salt after eating. Codifiers explain that the salt elixir is effective even if salt is eaten as part of the meal with the food. The food we eat generally has some measure of salt and this satisfies the talmudic requirement of salt at the end of a meal (Rema, 16th century, Krakow). Other authorities explain that human nature has changed and a dip of pure salt at the end of the meal is no longer sound health advice (Rabbi Avraham Abele Gombiner, 17th century, Poland).
As to mayim aharonim - talmudists of the Middle Ages in Western Europe explained that the salt that was prevalent on the Continent was from underground mines and was not the dangerous Sodomite salt. Hence the need to wash hands at the end of the meal before reciting Grace was no longer necessary (Tosafot, 12th-14th centuries, France-Germany).
Despite these attempts to explain why mayim aharonim is no longer necessary, some codifiers - particularly those who were conscious of the mystical tradition - stated that the obligation remained. This position saw the washing as having mystical value as well as an hygienic purpose.
Though this stance was favored by mystics, its roots can be identified in the Talmud: Our sages distinguished between the requirements of mayim rishonim, washing the hands before eating bread, and those of mayim aharonim, washing hands at the end of the meal. Mayim rishonim may be poured into a vessel or onto the ground; mayim aharonim may only be poured into a vessel and then should be removed from the table. These last waters are not only repulsive since they have been used to clean dirty hands, they also contain an evil spirit (B. Hullin 105a, b). If someone was to walk over used mayim aharonim it could be harmful; it should therefore be disposed of in a location where people are unlikely to tread (Rabbi Avraham Abele Gombiner).
Thus in general terms we can identify two different traditions that have developed: Those who follow the Ashkenazi rite do not have the custom of washing mayim aharonim before Grace. Those who follow the Sephardi rite or Ashkenazi Jews who are sensitive to the requirements of the mystical tradition are careful to wash mayim aharonim at the conclusion of the meal.
Regardless of what tradition your family follows, it is hardly appropriate to thank the Almighty for the food we have eaten with soiled hands. Thus, regardless of whether your desert is sweet or salty, everyone agrees that if your hands are dirty, they should be washed before addressing God in the recitation of Grace After Meals.
The writer is on the faculty of Pardes Institute of Jewish Studies and is a rabbi in Tzur Hadassah.