Hi. I fired underglazed earthenware bisqueware in my kiln to 04 hotness. Unfortunately, after my kiln had cooled and I looked inside, my 04 witness cone had bent only to a clock-angle of 3:30. It should have bent to a clock-angle of 4:00 at minimum, according to Ortonceramic.com.
It typically takes 15 to 25 minutes for a cone to bend, depending upon the cone number. The cone bends slowly at first but once it reaches the halfway point, it bends quickly. When the cone tip reaches a point level with the base, it is considered properly fired; however, the difference between a cone touching the shelf and a cone at the 4 o'clock position is small and rarely affects the fired results.
To make sure my kiln heats to 04, should I start using an 03
pyrometric bar under the kiln sitter's actuating rod, like it mentions below (also from Ortonceramic.com)? Is there a downside to that solution?
The Kiln Sitter® shuts off the kiln when a small cone (or bar) placed under the sensing rod receives enough heat for it to fully bend. Bending is caused by the weight of the sensing rod. Because the cone in the Kiln Sitter® is located at the kiln wall (closer to the heating elements), it frequently receives more heat than witness cones, causing the kiln to shut off early. Using the next hotter cone/bar may be necessary.
One more thing: my kiln has no timer, just a kiln sitter, so I can't add extra minutes to a firing. I'm assuming adding extra minutes would solve this sort of problem.
Thanks for any help!