If you were cut by a sword in 17th century New England, either a family member or a village healer would take care of the wound. They'd clean it with water, bandage it, and possibly smear it with a salve containing herbs and animal fat.
If you were lucky, a highly skilled doctor (like John Winthrop Jr.) would also apply a salve to the weapon that injured you. Putting the salve on the sword would magically heal the wound it made on you.
This seems a little implausible to a modern New Englander, but the so-called weapon salve was grounded in a widely held view of the universe. This worldview claimed that if everything in the universe is connected, then the sword that cut you had a particularly strong connection to you. The weapon salve takes advantage of this connection to heal you at a distance.
This blog has a recipe for a weapon salve from the 16th century text Archidoxis Magia. Ingredients include human fat, moss that has grown upon a human skull, rose oil, linseed oil, and human blood. The ointment should sit for a while after it has been mixed. When someone is wounded, the doctor should dip a stick in their blood and then insert the stick into the ointment. The ointment is now ready for use.
Doctors in Europe and New England debated the efficacy of the weapon salve. Some claimed it didn't work, but those who claimed it did had a variety of reasons as to why. Some claimed it worked through magnetism, while others claimed it was the harmony between the macrocosm and the microcosm. Some thought it was just the work of Satan.
I learned about the weapon salve a while ago and thought it was just another piece of folk magic that had long since disappeared. But I was wrong! The practice, or one very similar to it, survived until at least the 1980s in parts of Vermont.
Jane C. Beck, director of the Vermont Folklife Center, has interviewed many Vermonters about their traditional medicine practices. She collected the following piece of lore from a woman in Hyde Park, Vermont:
Similarly, it was believed that a nail that had been withdrawn from the foot, must be treated as well as the puncture wound itself. While salt pork was applied to the wound, the nail was carefully greased, wrapped up, and put into the warming closet where it would stay an even temperature. Today these supplementary measures are considered in holistic terms - treating the psychological mind as well as the body.
Although no skull-growing moss is involved, the theory is the same. Treating the item that injured you will cure your wound. I have no clue how the weapon salve evolved into this folk magic about the nail, but I think the continuity over 300 years of history is amazing.
I got the information about the weapon salve from Walter W. Woodward's Prospero's America: John Winthrop, Jr., Alchemy, and the Creation of New England Culture, 1606 - 1676 (pages 195 - 196). Jane Beck's article "Traditional Folk Medicine in Vermont" appears in Medicine and Healing. Volume 15 of the Proceedings of the Dublin Seminar for New England Folklife.