Fractured enamel


I think the key to your problem is that only the last (transparent)
layer is flaking off - not multiple layers, which would be the case
if you were not counter-enameling adequately. And also that it is
only occurring over the opaque red.

As a housekeeping matter, there may be residue from the stoning even
though you rinse in distilled water. Scrubbing with a glass brush is
essential during that rinse or there may be debris that could keep
the flux from fully attaching.

More likely, I think, is a temperature/firing length issue. I’ve
found that the opaques need a longer firing time and/or a higher
temperature than transparents to fully fuse. Is it possible that you
are not firing the piece long enough to get a full melt of the opaque
red and therefore a complete fuse with the overlayer? That would be
my first guess after glass brushing & air drying. I would lean toward
a longer firing time rather than a higher temperature to be sure the
entire thickness of the enamel (all layers) has melted. This is the
only way that stresses can be fully relieved and/or equalized among
the layers.

Good luck, and let us know what happens.