A compound obtained from several metals including lead, iron, mangonese and cobalt it accelerates the drying process when added to oil paint. However, pigments affect the drying process so some colors with driers added respond more quickly than others. Cobalt drier is regarded as the most effective. Siccative is another word for Drier, and Retardent is the opposite of Drier. Source: Ralph Mayer, "A Dictionary of Art Terms and Techniques"