Red Brick Dust is a powder composed of pulverized red clay bricks which is used in the practice of hoodoo/rootwork/conjure. Some practitioners may add extra ingredients to the red brick dust to make it stronger, in a magical sense. Red brick dust is deployed by sprinkling a line of it in a doorway or entrance. This is believed to magically prevent evil, especially one's enemies, from being able to cross it and gain entry into one's home. Other means of deploying red brick dust are to add it to one's mop or scrub water or to add a pinch of it to a protection mojo to make it "brick-house strong".
Red brick dust became popularized due to the 2005 movie, The Skeleton Key, which featured some mightily-wicked hoodoo practitioners.