This mug has thin walls and was bisque fired to cone 04 (so it had a fairly porosity). As a result the glaze went on thinner when it was dipped. This was not evident at the time of glazing but at firing the thinner sections produced the brown areas.
GA6-C - Alberta Slip Rutile Blue Cone 6
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
A type of ceramic glaze in which the surface variegates and crystallizes (on cooling) from the presence of rutile mineral in the recipe.