Another use for alginate is for shrinking 3D objects. When left undisturbed, the soft, gelled alginate will slowly lose its water to evaporation, leaving behind a dry, hard, smaller form. These two alginate castings took four days to completely dry.
The middle piece is after two days of drying. The smaller one is after four days. For the middle one I molded the alginate while it was still soft and moist, with silicone rubber.
Usually you will get some distortion as the alginate shrinks, so there are limits to how well this technique works. As a rule the more spherical the object, the more uniform the shrinkage, whereas long and thin objects will typically bend as they dry. The face on the left is a life-casting, and the one on the right an alginate reduction. You could mold the reduced alginate and do the process all over again in order to shrink it further.