- "The Hardening Charm will turn an object into solid stone. This can be surprisingly handy in a tight spot. Of course, most students only seem to use this spell to sabotage their fellow students' schoolbags or to turn a pumpkin pasty to stone just before someone bites into it. It is unwise to try this unworthy trick on any of your teachers."
- —Miranda Goshawk, Book of Spells[src]
During the 1986–1987 school year, the Hardening Charm was taught to third years in Transfiguration class at Hogwarts School of Witchcraft and Wizardry. Jacob's sibling briefly used this spell on a Jumbo Dungbomb, as a part of Tulip Karasu's plan to prank on Merula Snyde, Barnaby Lee, and Ismelda Murk.
During the Battle of Hogwarts on 2 May, 1998 Hermione Granger cast this spell at a tapestry, turning it into stone and wounding the two Death Eaters who were pursuing her, Harry Potter, and Ron Weasley.
Duro is Latin for "I harden" or "I stiffen"; the same term is Spanish and Portuguese for "hard" or "rough"; it makes sense given J. K. Rowling's relation to the language.
- Harry Potter and the Deathly Hallows (First appearance)
- Wonderbook: Book of Spells (First identified as Hardening Charm)
- Harry Potter: Hogwarts Mystery
- Harry Potter: Magic Awakened (Mentioned only)
Notes and references