One of our life drawing sesssions was to build up a drawing from a slab of clay. This seemed a daunting task to begin with, but in actual fact it was a very relaxing and almost therapeutic activity.
From this I cast a plaster mould and removed the clay once it had set.
I then cut glass to the size of the mould and placed it in the kiln so the glass took on the shape of the mould, so basically having a glass version of the clay relief. I was pleasantly surprised with the result of this, because some of the clay had fused to the glass and given it more depth and distinction, which I was worried it would lack.