If a bonsai pot is made of stoneware clay, high fired (Cone 10 being an average temp) and free from cracks, it should give a nice clear ‘ping’ when you flick it with your finger. Not as beautiful and clear as crystal but ringing in a similar fashion. To make the pot ‘ping’ you need to hold it in such a way that it can vibrate. Using a light touch, for example if it has two holes then holding the pot with a finger in one hole and a thumb in the other, underneath the pot. Then a flick the pot with a finger on the other hand.
Ceramics pots ‘ping’ as a result of vitrification of the clay in the firing process. Vitrification is from the Latin word vitreum, meaning glass. As the temperature of the clay rises in the kiln the clay hardens and the spaces between the refractory particles (those particles not affected by the kiln heat), are completely filled up with glass, fusing the particles together and making the clay body impervious to water. This also leads to shrinkage of the clay, usually about 12%. The glass elements in the clay melt at peak temperature and solidify as the piece cools, increasing the strength of the finished pot.
Should the temperature of the kiln rise above the maximum temperature for that clay the whole pot can melt and make an expensive mess of the kiln shelf.
If a pot has a crack the piece cannot vibrate adequately and so it will not give a clear ‘ping’ and the sound will be dull and flat.