CINNABAR is composed of Brimstone and Mercury. When it is reduced to a subtil Powder in a Marble Mortar, it acquires so lively and so high a Colour, that it is called Vermilion. Some Ladies mix it with Paint wherewith they rub their Cheeks, which is very dangerous, for by using it frequently they may lose their Teeth, acquire a stinking Breath, and excite a copious Salivation." Did people "back then" know that certain materials were harmful? The answer is more complicated than "yes" or "no." In some cases, as above, explicit warnings were written about substances like mercury and lead. However, many people still used things that they knew to be harmful. A few modern correlations immediately spring to mind!
Brimstone, if you were wondering, is sulphur. Rather than being harmful, it is the active ingredient in a number of topical medications. (Only use it under the direction of a doctor, of course.)
You can read more about mercury poisoning here: http://www.medicinenet.com/mercury_poisoning/article.htm
Source: Abdeker: or, the art of preserving beauty. By Antoine Le Camus, 1761