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Ceramic is a pretty porous material, even when glazed sometimes, and the silicone loves to stick to it for that reason. In my experience, using the soap water bath will not change the unfortunate results very much if at all. I don't know a good method of cleaning a silicone stuck figure besides burning the silicone off and I wouldn't advise using ceramic objects for this process, ceramic just doesn't want to play nice with silicone. If you are still bent on casting it, you could maybe coat the ceramic figure with some sort of resin after it is cleaned to give it a less porous surface...? Something to look into perhaps!