I made this pot a coupla years ago, posted on clayart at the time that I got red from wfl brown by refiring to ^04. The red areas on this pot are just wfl brown, and then I dipped over the rim with clear liner, also in "the book"- the overlapping of the 2 glazes gives alot of color, purple included.
However, I think that it has to do with the thickness of the wfl brown, and the cooling, or holding somewhere in the 1800 degree neighborhood. The reason I say this is because I have refired wfl brown cups in my bisque kiln, and they come out red, for the most part- but, where the glaze is on thick enough to run slightly, I get hints of purple- as in the mug at top- sorry, not a great pic.
I have not been able to repeat this with any regularity, either. I can always get red on refire, but not purple- or, if I get purple, it blisters because the glaze is on too thickly.