|Monthly Tech-Tip |
These differences in color correspond to differences in fired maturity (all of these were in the same kiln load in the same location). It could be due to physical factors like particle size, but more likely differences in the chemistry (e.g. higher levels of K2O and Na2O). The reduction atmosphere is bringing out this grey color as the kaolin reaches maximum density. The whiter ones are less vitreous (e.g. the top one has 3% porosity according to an SHAB test). The grey ones are more vitreous (e.g. the bottom one has zero porosity). These differences can affect the degree of vitrification of clay bodies they are used in. This pattern of maturity development is similar to what happens in buff-burning stoneware bodies as they densify. The higher the percentage of this material in a body the more important it is to monitor the impact on its fired maturity. In extreme cases, adjustment of the percentage of feldspar is needed to compensate for changes in the kaolin.