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.