lead, vinegar, and horse manure
As far back as Roman times, white has been considered one of the most dangerous paints, due to its high lead content. The formula for creating white paint involved placing thin shavings of lead over bowls of vinegar, then collecting the white deposits of lead carbonate. Which was nothing compared to the Dutch technique for preparing white paint around Rembrandt’s time, described in Victoria Finlay’s Color:
The Dutch or “stack” process involved using clay pots divided into two sections— one for the lead and the other for the vinegar. The apprentices would line up several dozen of these, and then they would add the secret ingredient— great bucketsful of manure straight from the farm, which would be heaped all around the pots to produce not only the heat to evaporate the acid but also the carbon dioxide to transform the substance from lead acetate to basic lead carbonate. The room would be sealed, and was left closed for ninety days, after which the apprentices would no doubt draw straws to see who got the unpopular job of going in to get it…. in those months, the stagnant heat, gurgling excrement, sour wine and poisonous metal would have worked their alchemical magic, the dirt and the smells metamorphosing into the purest and cleanest white, which formed in flakes or scales on the gray metal. It was one of the many small miracles of the paintbox, the transformation of shit into sugar.
White lead was replaced by the much safer titanium dioxide in the 1920s, but many artists still insist that nothing can compare with the opaque quality of lead white. More on the origin & history (including Vermeer’s usage) of lead white can be found here.