Why a Porcelain Plate Rings and an Earthenware One Thuds

Strike a porcelain plate with a fingernail and it rings, holding a clear note for a moment after the tap. Strike an earthenware one and it thuds, the sound dying almost before it begins. The difference lies not in the glaze that both may carry but in the body underneath it. Clay fired hot enough undergoes a change that runs right through the object rather than sitting on its surface. Part of the material softens and flows into the spaces between the particles, and on cooling it locks them into one continuous mass with almost no pore space left. Earthenware, fired several hundred degrees cooler, never reaches that point. Its particles are joined only where they happen to touch, and air remains between them throughout. That structural difference decides what becomes of a vibration. In a continuous body the energy of the tap travels through uninterrupted material and keeps moving, which is what a sustained note is. In a porous one the vibration meets a boundary at every pore, and each boundary scatters a little of the energy and absorbs a little more. The note is not so much silenced as spent. Potters and dealers have long put this to use. A plate is held loosely at the rim and tapped, and the sound is listened to rather than the surface examined. A plate that rings is sound. One that answers dully is set aside for a closer look.