The design in the middle is an ancient Celtic design representing the upper, middle and lower worlds, corresponding with the trees that surround it. I'm carving through the white procelain slip revealing the darker clay beneath it, a technique called sgraffito.
I'm using sgraffito tools purchased at ceramics stores.
The tools I use are simply "pencils" with wire loops to write with instead of lead.
I use them to sign the bottom of my pots as well. I've worn out many of the wire loops on these tools, and I decided a while back to replace the wire loops myself.
There's a copper sleeve on the tip that holds the wire loop in place. I've learned heating it up helps pull the sleeve off. The wire loop rests in two grooves on either side of the "pencil." Just pull the old wire out, bend a new one with some kind of wire, cut to size and replace. The photo above shows a tool with the sleeve taken off and the old broken wire.
I've made a couple of loops for the small limbs of my trees. The middle tool in the picture above with three tools is great to pick up a wide mark made by the tool on the left (applying greater pressure) and ending up with a thinner mark (applying less pressure). The tool on the right in the same picture is used for the very thin branches at the end of the limbs.
The longest tool in the picture on right, I use for carving the landscape around the perimeter of the platter.
The following pictures are the completed platter and a completed large bowl.