Grumbacher Cobalt Drier is the only siccative that has been scientifically proven to be the least harmful to use for fine art painting. Cobalt Drier speeds drying of oil colors and oil painting mediums. Because it is extremely strong, Cobalt Drier should be used sparingly by applying it in drops. It should not be mixed directly into oil paints, but first mixed into an oil painting medium, which can then be added to the paint. Cobalt Drier should always be tested before using it in a painting.