|Monthly Tech-Tip |
These SHAB test bars show this body fired from cone 8 (top) down to cone 4 (bottom). The test data indicates it reaches maximum firing shrinkage (FSHR column) at cone 7. Its porosity is 2.5% at cone 6 (ABS column), that is where red color is richest. By cone 5 it has reached 3.8%, that is pretty high for strong functional ware. At cone 7 the porosity is lower, 1.4%, but the color change indicates some decomposition is beginning (signifying possible issues with glaze bubbling, pinholes and warping of ware). Clearly, cone 6 is the most realistic temperature, it is still red and dense enough to have good strength and certainly won't leak if glazes are not crazed.
Also called "middle temperature" by potters, cone 5 (~2160F/1180C) refers to the low end of the temperature range at which most hobby and pottery stonewares and porcelains are fired.