Yes and no. Within the context of whatever you're currently molding, yes; any clay removed gets added to the voxel count for the current piece being used, even if that pushes the available count past a full piece. Anything 'leftover', as in anything beyond a full piece of clay, when a mold is finished gets lost. That said, that's balanced by the fact that the game doesn't consume a piece of clay until you use all the voxels for that piece. So, say you have 30 clay in hand and you're finishing up a mold. The game doesn't track that when you're done you have 29 pieces plus 8 voxels left from the 30th piece; you've still just got 30 full pieces when you start the next mold.