The stiffening and embrittlement of oil paints over time has been a real concern for those responsible for the long-term care and preservation of paintings. This paper examines the effects of time, pigments, relative humidity (RH), temperature and solvents on the mechanical properties of traditional oil paints. In this way it is possible to determine the role of each factor in causing the paints to become brittle. Even after fourteen years, the oil paints show little evidence that the long term maturing processes have begun to slow down. It is shown that there seems to be little correlation between the time paint requires to dry-to-the-touch and the longer-term mechanical properties. Both low and high temperature levels can increase the stiffness of the paints though the mechanisms are quite different. Considerable hydrolysis of the paints occurs early in their history and the ones that hydrolyze most quickly are the ones that remain the most flexible. -- ICCROM