You don't actually fire the marbles in the clay - you drop one into your glazed bowl. It melts in the kiln and leaves a puddle of colored glass on the bottom of the bowl. The solid colored marbles are kind of gross, but the clear ones are gorgeous. I did find that you need to glaze the entire bowl, and not leave the inside bottom of the bowl without glaze, or there will be a seperation of melted marble/glaze. I have been doing this for years and the kids love it. I love the idea of the flat marbles - I have a ton of those! :-)