These are from the same firing, glazed at the same time and are the same thickness. The floating blue effect is a fragile mechanism and affected even by the small color difference in these bodies. The small amount of extra iron in the M370 affects the glaze character more than expected.
GR6-M - Ravenscrag Cone 6 Floating Blue
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe.
A popular cone 6 glaze that employed Gerstley Borate, it was very troublesome to use and to fire. Much work has been done to create alternative recipes.