I smudge with white sage, sometimes its a mix of that and lavender or sweet grass. I smudge our home, at least once a year. Usually during spring cleaning. I also smudge to cleanse any new magical tools, stones or crystals.
I visualize the smoke binding any "negative energy" that has attached itself to whatever it is I am smudging, and then carrying it away in the wind/air.
This article from the GreenMedInfo suggests that the practice of smudging may actually purify the air of harmful bacteria. Read the whole story here: www.greenmedinfo.com/blog/killer-germs-obliterated-medicinal-smoke-smudging-study-reveals
It is alwas\ys interesting to find out that long forgotten practitioners of magic my have known more about "science" than we give them credit for.
