Answer

We recommend that all Go players should wash their hands before playing the game. And after the game, if you clean white stones with dry cotton cloth one by one, you can keep as pure white as the new stones for eternity.
However, white stones may change its color to yellowish or brownish if you leave them for a long time without using it. This may happen when the fats and dirt of player's hands have attached to white stones.
In this case, please clean these stones as follows.
1. Put several drops of mild detergent into water and clean the stone surface by hand.
2. Place stones on a clean mesh and dry them in the sunshine (or in shade) and adequate natural breezing.
3. When they are completely dry, wipe stone surface using dry cotton cloth one by one.