The 14 December 1886 issue of the Arizona Gazette reports:
 
"The tartar which so generally accumulates at the back of the teeth, forming oft
en a thick crust, may speedily and effectually be removed by brushing it with a
soft brush dipped in fresh flour of brimstone. After a few applications the tart
ar will crumble away."
 
Yikes, sounds rather unpleasant. Any guesses what "flour of brimstone" would be?
 (Baking soda???)