In Yasaka Fusion three different synthetic rubbers are mixed with natural rubber for the outside rubber. Each three kinds have individual characteristics for : Speed, Spin and Durability That is the basic technology for Yasaka's new concept "Energy Fusion Rubber". The sponge fusion also contains a certain level of synthetic rubber, which support a good speed glue effect. When using Fusion with speed glue you will feel a very elastic and easy playing rubber with a strong sound effect! Sponge hardness: 38 degrees