This is Ravenscrag Slip Oatmeal over a 5% Mason 6666 stained glossy clear at cone 6. You have to be careful not to get the overglaze on too thick, I did a complete dip using dipping tongs, maybe 2 seconds. Have to get it thinner so a quick upside-down plunge glazing only the outside is the the best way I think. You may have to use a calcined:raw mix of Ravenscrag for this double layer effect to work without cracking on drying.
A mottled glaze created by double-layering glazes of different melt fluidity and color
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
This is an overview of the various mechanisms you can employ to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.