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I mould the sand / clay mix around the outside of a polystyrene (Styrofoam) cup, then allow to dry for a few days. When set, just run your propane torch around the inside to burn off the polystyrene. You get a nice smooth inner surface.
Actually it's YOU who seems clueless. You anneal copper by heating to a dull reddish black and QUENCH in cold water. The same process is used for brass and aluminium. To anneal steel, you heat and cool down as slowly as possible. With steel quenching hardens it. So annealing copper uses the exact opposite process that you'd use for annealing steel.
3d scan anything using just a camera