A-level Applied Science/Colour Chemistry/Paint/Catalysts
A catalyst is a chemical which makes a chemical reaction go faster, but is not itself used up in the reaction.
Siccatives (Paint and Ink Driers) are metal carboxylates ('soaps') used to accelerate the drying process in, for example, linseed oil. The metal can be a transition or a group II metal and the carboxylate can be naphtenate, 2-ethyl hexanoate (widely known as 'octoate') or C7-C11 salts.
Cobalt (II) octoate is the most reactive and most commonly used siccative. It catalyses the oxygen uptake and accelerates peroxide formation. Cobalt octoate causes a surface film to form rapidly. Other 'auxiliary' driers are added which either reduce the activity of cobalt (e.g. zinc octoate) or enhance the drying of the interior of the paint (e.g. lead octoate). 
Retardants may be added to artists' acrylic paint to slow the drying process. Retardants are essentially 'negative catalysts' - slowing down a reaction rather than speeding it up.