Were they completely dry when you fired em?
The best test is to weigh them, and when they don't lose weight after a couple days, you're good.
Of course, pit fires can heat them up too fast. A handheld IR therm can help you keep them hovering around 200 for a while, before picking up the board they are on, and sliding it into the fire further.
I've been able to "pitfire" the local without breaking it, so you'll get it.
My 102 YO Potter friend said she was able to take clay from the bottom of a lake, and fire it all the way to cone 6, this may have been Michigan, but it could have been Wisconsin. Of course, that was so long ago the composition could've changed! Lol!
We used to visit a lake in SoWi that was clay underfoot, by Jellystone Park?
So even if it was Mi..it could be the same deposit I reckon.
This gal Ginger is active and I've messaged her on Glazy, she has some good knowledge of Wisco materials. This is a cone 10 glaze. Which means, these materials can also increase the temp and change the composition of your tests if needed.
View attachment 305995
Since the "window of vitreous" with earthenware is so small, between solid and melted, you can add "openers", some kind of organics, to make it so porous it won't crack in the freeze.
So far, 1850F doesn't melt our local.
Thanks for sharing!
Sorce