Choose your size. These come in 10mm, 15mm, 20mm, and 25mm thickness, and the "average" size of 75mm. At 1mm thick, this means you can add about a 1mm thick piece of clay to the inner side as well, and still maintain the "perfect" 70mm average bracelet size.

They provide a sturdy insert to build your bracelet, and they are not to be removed- that's right, create the inner skin and the outer portion of your bangle, and bake right on the blank. 

Crafted from high-temperature plastic, safe to 350F. DO NOT overbake, be sure to use a thermometer and know your oven hot-spots.