Thank you High Bridge Pottery, it's just commercial underglaze; I draw the image onto lightweight paper and then transfer the drawing onto leather-hard clay.
All of my forms are made with either Georgie's G-mix 6 or Georgie's Trillium Porcelain. Image is carved into a layer of black-brown underglaze. After the bisque I typically stain the decorated area with a dark glaze. I'm experimenting with clear glaze thickness, some pieces have glaze barely brushed on while others have several layers of glaze. I use commercial glazes.