I was wondering about that. I have tested the Cauldron in one of my cities. I used two ingredients to obtain a certain effect. The preparation was successful for only 2 out of 5 goblets, and only one of the two goblets contained the potion I had aimed for. The second potion was good too, but not exactly what I wanted.
What I had not understood and wanted to test was whether the preparation was a separate process from the actual drinking of the potion. It seems that they are one and the same process. Once you prepare and obtain a certain potion, it is considered that you have already ingurgitated it and the effect starts to apply. Now that I know this, I will use the Cauldron appropriately in my other two cities.