Mountain Glaze unglazed
When you fire cone 6 the clay really melts causing a fusion between the underlying clay and the surface design. If the surface isn’t thick enough it melts away, gets blurred, or gets really dark. Here is one example where the cone 6 glaze really wiped out the pattern and caused mostly a blurred effect. With the glaze applied above and the area below not glazed, but quite melted into the underlying dark clay.