This 1 gallon heavy crock was fired to cone 6 (at 108F/hr during the final 200 degrees) and held 20 minutes (in a electric kiln). The bare clay base should be the color of the top test bar (which has gone to cone 6). Yet, it is the color of the bottom bar (which has gone to cone 4)! That means the base only made it to cone 4. The vertical walls, which we exposed to the direct radiant head of the elements, are the right color (so they made cone 6). It may seem that this problem could be solved by simply firing with a longer hold at cone 6. But again, electric kilns heat by radiation, that shielded and heatsunk base will never get the same thermal treatment as the sidewalls!
The process of holding a kiln at the final temperature (or at other temperatures) to enable the heat to penetrate the ware or to effect or complete a glaze or body reaction
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose.
Electric Hobby Kilns: What You Need to Know
Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, being aware of the limitations and keeping them in good repair is very important.