Mysterious dark patch on the bottoms of pots after glaze firing

[tokati]

Recently in our glaze firings to 1250 deg C in an electric kiln, the bottoms of our pots have been making this darker toasty patch, sometimes completely covering the bottom and other times with this kind of pattern.

One of my studio mates seems to think its from the glaze of other pots, but I doubt it, since there would be a darkening on the walls of pots. Especially considering that the attached photo was unglazed on the outside and there was no effect on that part.

Has anyone experienced and diagnosed this issue? Any information would be appreciated.

[Callie Beller Diesel]

Hi and welcome!

if it’s only appearing on the bottom of the pots it’s possible  that there’ something on the kiln shelves or kiln furniture that’s fuming. If the shelves are old and have glaze drips that have been kiln washed over, there’s still things that can come through.  If that’s the case, grinding shelves to remove all old bits and pieces, and apply fresh kiln wash ought to fix it. 

[Babs]

Any manganese drips in your kiln?

Any contamination when sponging or drying pots.

Change when you glaze and fire again? 

[High Bridge Pottery]

Could be soluble salts in the clay - https://digitalfire.com/glossary/soluble+salts