So as mentioned in an earlier post, I had a poorly packaged A25 crucible arrive with a crack in it. As I'm trying to zero in on tuning the furnace for reliable iron melts, the cracked crucible went in the furnace as a "Dummy load" for fifty minutes of run time. Roughly 15 minutes into the session I heard some loud pops and assumed it was the crack lengthening from the heat. After the session was over and there were some blisters on the outside only of the crucible in the coating. They come from the factory with a gloss black coating that looks brushed on and in previous uses the coating had peeled off like stripped paint when hot. The fired coating is rough and glittery, almost like a silicon carbide rich coating and is the layer popped off by escaping gasses, probably steam. When the blister is disturbed you can see the shiny factory glaze underneath it. You can see a painted on coat in this video here at the 11:36 minute mark and I suspect this is how the glossy black coating is applied with it's brush marks and so the factory wouldn't fire it for a third time just for a protective coat. So if the crucible is dried before use and fired empty, the painted on layer may just be a carbide rich protective coat to extend the crucible life...at least that's my theory and would explain the longer life of quality crucibles. There's a PDF about drying wet crucibles at the bottom: Unfired A25 Salamander Super with nasty crack and the glossy brushed on coating: Same crucible after 50 minutes in an oil fired furnace reaching molten iron temps, note the multiple blisters and the rough glittery surface finish now. The same blister "popped" in a very thin layer exposing the fired glaze of the crucible from the second factory firing.