To measure how interesting a conversation is.
Slughorn's Hourglass was a clock made of two glass vessels connected with a narrow passage through which sand and a green fluid flowed, and green serpentine features adorning the outside. The serpent imagery in its design symbolized Slytherin house, of which Slughorn was the Head of. It was kept in Slughorn's Potions classroom on his desk, though it is presumed to also reside in his office.
The sand runs accordingly with the quality of the conversation: if stimulating, the sand runs slowly.
Horace Slughorn had this hourglass in his possession during the 1996-1997 school year, during which when he is explaining the properties of the hourglass to Harry Potter the sand runs quickly (as it isn't a very deep conversation) but later as they discuss Lord Voldemort and what he was like at school the sand moves so slowly that it seems to freeze.
Behind the scenes
- This hourglass is never mentioned in the books.
- Harry Potter and the Half-Blood Prince (film) (First appearance)