Storage hardening is a phenomenon specific to natural rubber, which leads to an increase in bulk viscosity, usually characterized by the Mooney viscosity (MV) and Wallace plasticity (P0) (De Vries, 1927). The increase in the viscosity upon storage is not a desirable property of natural rubber as a raw material because this means a change in its processing behavior. However, the technological aspect of this has been overcome (Sekhar, 1964), and constant viscosity grade rubbers (CV rubber) are now available on the market. Although Sekhar (1961) reported that CV rubber has an average increase in Mooney viscosity of only 4-8 units after 4-5 years of storage at ambient temperature, but Yunyongwattanakorn and Sakdapipanich (2006) reported that STR CV60 showed increasing in Mooney viscosity, gel content, P0, and high PRI during long-term storage as real condition. The only difference from the other commercial rubber grades is the low Mooney viscosity at the initial stage (Figure 15).