Now I have the 3rd fired panel out of the kiln, I had lowered the final temperature 10º to 2050º and the end “hold” time from 10 minutes to 5 minutes. Also, I placed several kiln posts on the floor of the kiln so the panel could set on top of them on end and be raised about 1″ off the kiln floor and still allow the kiln lid to close.
It looks like this solved both issues I had, one was the red color I wanted had started turning towards the more brown spectrum for this clay at the 2060-2079º temperatures, so lowering the temperature to 2050 solved that, the nice rich red brick color I desired is there.
The other issue was in placing the first two panels in the kiln I had set them in place on end directly on the kiln floor, so what happened was the portion of the panel in direct contact with the kiln floor, and extending about 1-1/2″ across the face of the panel the clay did not reach the hotter 2060 and 2079º temperatures the rest of the panel did, so that narrow area turned the red color I wanted while the rest of the panel started to turn towards the brown tint.
This photo shows panel Nr 2 and Nr 3 side by side, the lighting was not ideal and I also had to correct in photoshop, but the left panel can be seen has an obvious lighter color band on it’s right side, that’s the end that was touching the kiln floor:
The right panel, Nr3 can be seen not only has a brighter red tone than the left half of the left panel, but there is no lighter color band on this one.
Now that I have the red color I want out of this red clay, and I know exactly what temperature it takes to do it, I can repeat the procedures and criteria and keep these pretty consistant.
The thing is, with kilns is that over time the thermocouple that controls the electronic firing sensor on the board tends to lose accuracy with wear and use, “wear” being defined here by X number of firing cycles. I added a digital pyrometer which was used for the first time for this firing, it pretty closely matched the temperature reading of the controller’s display before I went to bed when it was around 1750º. Unfortunately the kiln shut off about 20 minutes before I got out of bed, so I didn’t get to see exactly what temperature the pyrometer reached and how it compared to the controller’s temperature, next time!
I’m very pleased how the hand pressing has gone, none of the three panels cracked, warped or blew out in the kiln under the firing schedule I devised so I know the 36 hours and 57 minutes it took to fire this latest panel is not firing it too quickly. I could probably tweek the schedule a little to shorten the time but the risk there is hitting a critical tipping point and having a pressed piece blow out, also, just because a slightly shortened schedule might work fine, there could be the first firing with a different design and some slight difference in size or mass, or wall thickness could come into play and it’s just enough it blows out during firing.
So I’m going to keep this shedule where it is, shaving an hour or two off the 37 hour time isn’t worth it and saves very little anyway.