This cone 6 mug is made from a black clay (containing 10% burnt umber). The engobe on the inside is covered by a clear glaze. The color is the same as if the engobe were on a white or buff firing stoneware. Engobes get their covering power from the fact that they do not melt. If you see an engobe with lots of frit it will likely melt too much, be suspicious.
This is Odyssey slip, a engobe recipe that is trafficked on the web. It is recommended for low, medium and high fire ware. It is 30% Ferro Frit 3110 and 70% ball clay. This is a bi-clay strip, a sandwich of two plastic clays rolled into a thin slab and cut into a bar (to make the bar the Odyssey slip was dewatered to typical pottery clay stiffness). We are looking at the engobe side of an EBCT test (the other side is Plainsman M390). During the latter stages of the firing the engobe has begun to melt and blister and darken in color (which it should not be doing). During earlier stages of firing this engobe would certainly have had a higher shrinkage and would have bent the bar its way. But it is now bent the other way. That means the engobe could well be under compression (having a lower thermal expansion than the body). Or the body could simply have pulled it the other way when the engobe lost its rigidity. Either way, the engobe does not fit this body at this temperature.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.