This is an excerpt from Das Kuchbuch der Sabina Welserin
(Germany, 16th century - V. Armstrong, trans.)
The original source can be found at
David Friedman's website
33 To prepare dried cod, from the gracious Lord of Lindau, who was Bishop in Constance. First take river water and ashes and add caustic lime, which should be rather strong, and soak the dried cod therein. Allow it to soak for a day and a night, afterwards drain it off and pour on it again the previously described caustic lime solution. Let it soak again for a day and a night, put it afterwards in a pot and wash it off two or three times in water, so that the fish no longer tastes like lye. Put it then in a pot and put water therein and let it slowly simmer so that it does not boil over. Allow it to only simmer slowly, otherwise it becomes hard. Let it cook approximately one hour, after which, dress and salt it and pour salted butter over it and serve it. Also put good mustard on the outside in about three places. One must also beat dried cod well before it is soaked.