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 to cone 6, on the right to cone 4. Thus, at some point between cone 4 and 6, the umber in the body is generating gases of decomposition. Glazes can bubble it through but this underglaze cannot pass it. To continue working at cone 6 I made my own body by mixing Mason 6600 black stain into a porcelain.