|Monthly Tech-Tip |
The top two terra cotta clay test bars were fired at cone 01 and cone 02. Notice how they puff up inside and eventually split open the outer layer revealing an "Aero chocolate bar" interior. Why? The fine-particled clay at the surface has vitrified and oxidized enough to become an almost porcelain-like surface, sealing it. But terra cotta clays have particulates of many minerals, inside the bars where oxygen is lacking some of them are decomposing and melting (and releasing CO2) at the very same temperature. Guess what happened when I mixed this clay 50:50 with Redart: This effect was gone, it fired to a stable and strong red stoneware. Redart, although also a terra cotta, raises the temperature at which the surface seals, beyond when the gas escape is happening. Some people actually seek this effect. The secret of making it happen is finding a native clay that vitrifies completely at the same temperature as mineral particles are decomposing to create gases.
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization
Bloating occurs when the off-gassing of decomposing particles in a body has not completed by the onset of density and impermeability associated with the vitrification process.