This happened to me several years ago, in my old kiln. I messed up a cooling cycle and got my Waterfall Brown to turn red. After talking with one of the authors of "Mastering Cone 6 Glazes," I found that just refiring this glaze to bisque temp turned it red.
Not so in my new kiln. Until my most recent firing. In an attempt to save a slow cooling cycle, I turned it back up from 1227 F to 1800 and cooled normally. Red!
Actually, a kind of brick, orange red. Not so impressive. So, I stuck 'em back in the kiln and refired to ^6 to see what would happen. They darkened up and became a much nicer color, with all kinds of surface variation that I kind of like. After flipping out a little that I now have 4 place settings of tableware that don't match anything, I decided that maybe this is worth pursuing. Besides, I've been trying to get my iron red glazes to work out in this kiln with no success until now. Happy accident.