Everything you need to know about siccatives
A siccative is a dilution of metal compounds and is used to speed up the chemical drying of oil paint. Use very sparingly, keeping use to a minimum.
Preferably keep the use of siccatives to a minimum. Incorrect use of a siccative can cause problems. Apart from the drying process, adding siccatives also speeds up the painting's ageing process. To avoid problems, never add more than 2% Siccative Courtrai to the paint and never more than 5% Siccative Harlem.
The drying of oil paints can be roughly separated into surface drying and depth drying. Depending on which metal compounds a siccative is composed of, either deep-drying or surface-drying will be stimulated. The light siccative, Siccative Courtrai, primarily accelerates the depth drying process. The dark Siccative Harlem, on the other hand, speeds up surface drying and ensures depth drying. Link to text on siccatives.