Japan Explained- FAQs and SAQs Part 16
Why did Japanese women blacken their teeth and why did they stop doing it?
Showing your white teeth was thought to be like showing the white of your bones. The blackening liquid was also thought to preserve the teeth. The practice was outlawed in the Meiji era in order to not offend foreign visitors.
If you want anything else Nipponese explained, it might already be on the Japan Explained blog.
