The Artist's Handbook of Materials and Techniques: Fifth Edition, Revised and Updated (Reference) Usually, if I'm trying to alter the drying time of oils, it's to speed it up to meet a deadline or to make changes over dry paint. But once in a while, like in portraiture or figure painting, it's nice to slow down the drying time to achieve subtle changes over a longer period of time. Oil of Cloves is one way of slowing the drying time. It's a preservative that's used to cover up the odor of decomposition. It has a pretty strong odor but it's tolerable. A few drops in a paint mixture retards the drying time. Ralph Mayers' book The Artist's Handbook of Materials and Techniques: Fifth Edition on materials talks about other ways of retarding color as well as answering every question you ever had about artists materials. It's an exhaustive manual on techniques old and new as well as supplies and materials.