What was the process of ladling salt water alluded to in the noh play Matsukaze?

I am reading the script of Matzukaze (松風, 14th century), where the protagonists are the ghosts of two ladle-women that pull a ladle cart behind them. Out of curiosity I looked for a description of that process. I have read that getting salt out of sea water back then was actually very labor-intensive, but I cannot find a description of the process itself. Would anybody here happen to know?