These plates and platters are purely decorative and are not food safe.
The designs are created using painters tape as a resist. I remove portions of the taped design as I apply different colors of glaze.
The glazed pieces are fired in a gas kiln that is fired to 1850F-1900F.
I remove the piece while it is red hot and place it in a container with sawdust,newspaper or other combustible and seal the container. Color is influenced by the type and thickness of the glaze and by the amount of oxidation or reduction that takes place in the container.
Sometimes it's just the whim of the kiln goddess.