So, I haven't done this just yet, but I'm preparing to within the next few days. I have a few pieces waiting to be fired at a paint-your-own pottery shop, but the person I've talked to told me that creating a slip mold from fired goods would not be possible. Is this true, or does it depend on the nature of it? For example, one is vase-like, and I had planned on laying the bottom half in clay, casting the top, and repeating the process for the bottom half, so it would be a two. Does it have to be greenware for this (she also said after casting, the piece within would be destroyed), or can it be ran through the kiln and cast afterwards? If so, does it need a release compound?