When I boil potatoes, beans, or other vegetables, I often have issues with the water boiling over. Here’s a household hint in a hundred-year-old cookbook about how to prevent a pan from boiling over:
To Keep the Lid on a Boiling Pot – A teaspoonful of butter dropped into the water in which you are boiling dry beans, or other starchy vegetables will stop the annoyance of having the lid on the pot jump off, as it will otherwise do. The butter acts the same as oil on troubled waters and keeps it calm and manageable.
Cook Book (Published by Bethany Shrine Patrol No. 1, Rochester N.Y., 1923)