This is a great 6"x6" firing board, which is a heat reflective surface. For metal clay this is perfect as your piece is getting the heat twice! Use for torch firing metal clay! I don't recommend using them as a kiln shelf as they tend to thermal shock easily decreasing the life of the shelf. If you do use at a kiln shelf, allow the kiln to cool down before opening the door. Do not use as kiln shelf above 1700F.
I use this shelf to demonstrate at every show and prefer it above all others. The surface is reflective so you can place it on anything except acrylic. The small amount of moisture that accumulates under the acrylic makes the acrylic sheet pucker. Place directly on any other surface with no worries. If you are looking to pin down items for soldering, then look at the Kaiser Lee Board
. It's a great soldering block!