The umber is bubbling the underglaze in these clay rocks
The body is Plainsman Coffee clay, it is stained black with 10% raw umber. I painted Amaco Velvet white underglaze over the black clay (in the leather hard state), then over-painted the colors. When they were dry enough to handle, I cut the black lines using a Kemper WS sgraffito tool. The rock on the left is fired at cone 6, on the right to cone 4. Thus, at some point between cone 4 and 6, the body is starting to generate gases of decomposition (likely from the raw umber). Glazes can bubble the gases through but this underglaze cannot. To continue working at cone 6 I made my own body by mixing Mason 6600 black stain into a porcelain.