A pin thrust into "Witch's Butter" would cause the witch to undo her work.
"Welsh Folk-Lore a Collection of the Folk-Tales and Legends of North Wales" by Elias Owen
If the butter did not form from the milk, some witch was in the churn.
"The Witch of Salem" by John R. Musick
Witches' influence on the butter and milk of cows.
"Traditions, Superstitions and Folk-lore" by Charles Hardwick
It is of a yellow colour like gold, and is called butter of witches.
"Demonology and Devil-lore" by Moncure Daniel Conway
A hot heater, put into the churn, kept witches and evil beings from spoiling the cream or retarding the butter.
"Lancashire Folk-lore" by John Harland