Proposed Canon
Cleansing is a natural form of DirtamancyWB2014 Digdoug - Episode 5 that happens for every unit at dawn, regardless of turn order. It cleans a unit of any dirt or grime on their body and in their raiment.TBFGK 37
As a natural form of Dirtamancy, Dirtamancers can use juice to cleanse themselves or others.
Cleansing happens at dawn, along with healing any lost hits suffered in combat, and the poping of rations. It's possible all three events happen courtesy of a Ruler (or Barbarian) paying a unit's upkeep.