The rutile blue variegation effect is fragile. It needs the right melt fluidity, the right chemistry and the right cooling (during firing). This is Alberta Slip GA6C recipe on the right (normal), the glaze melt flows well due to a 20% addition of Ferro Frit 3134 (a very low melting glass). On the left Boraq has been used as the flux (it is a calcium borate and also melts low, but not as low as the frit). It also contains significant MgO. These two factors have destroyed the rutile blue effect!
GA6-C - Alberta Slip Rutile Blue Cone 6
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
P3998 - Boraq 1 Recipe
Gerstley Borate substitute
A type of ceramic glaze in which the surface variegates and crystallizes (on cooling) from the presence of rutile mineral in the recipe.
In ceramics and pottery, colorants are added to glazes as metal oxides, metal-oxide-containing raw materials or as manufactured stains.