I'm also having similar troubles which makes testing a pain in the butt. I'm trying to test how resistant each kiln tier is, how much of an effect fuel types have on firing, and whether opening or closing hatches mid process affects anything, but between kilns just not breaking (a normal fire clay kiln with 92 hours running with no broken blocks, can't tell if bug or luck) and things instantly firing from raw to fired ceramic with no discernible reason, my 20 kiln testing area produces nothing but mixed results. I don't know whether the bug is because I made them in creative, made them with the worldedit copy paste tools, or maybe there's a seemingly unrelated mod is causing trouble in some way.
It seems to work properly when you see a "Fired for X hours" tooltip on the ceramic, which you can only check by looking directly at it in the kiln.
I'm also not sure whether the "Firing: for XX hours" is just a measure of the kiln's lifetime or if there is an actual 'finish' state like a cementation furnace where block damage takes effect. Is a "cycle" as mentioned in the handbook defined by the fuel burning out or the pottery becoming fired?
So far I have a few observations that may or may not be based on bugged behavior.
1) Never use fuels other than what is most plentiful or convenient since the clay finishes firing long before the fuel burns through.
2) It takes 9 hours (according to aforementioned tooltip) to fire the whole batch. It doesn't matter if it's bricks stacked to the ceiling or a few storage vessels.
3) Fired ceramics start cooling down when they're done, regardless if there is fuel burning underneath or not, bug or feature I couldn't tell you.
4) Ceramics begin their "Fired for X" timer once they hit around 950 degrees.
5) All fuel types heat the interior to 1200 degrees.
6) The color of the fired bricks is determined at the moment it finishes firing. e.g. If every hatch was closed for 8.99 hours of the process and you opened two in the last minute, your red clay bricks would be red instead of the expected tan. Be careful of trolls turning all your blue clay bricks into clinkers I guess.
I agree.