This medium grinding stone is molded with a mixture of abrasive(alumina) and bonding(phenol resin)agents, then fired at moderate temperature(approximately 120-200℃).Just add water and the grinding stone can be used right away. The composite is a natural grinding stone. Since it is hardened at moderate temperature, the abrasive and bonding agents are still effective. As a result, the stone could be damaged if left for long periods(several years or up to a decade), or if it is stored while wet. Also, a half-sharpened knife left to sit could start to rust, so after sharpening, be sure to rinse and wipe the knife. Since this is rougher than most medium grinding stones, it can work as a substitute for rough grinding stones used with less worn knives. It is optimal for times when you want to quickly sharpen a knife.