for the riser to cool after the casting the riser must cool more slowly than the casting. chvorinov's rule briefly states that the slowest cooling time is achieved with the greatest volume and the least surface area; geometrically speaking, this is a sphere. so, ideally, a riser should be a sphere, but this isn't a very practical shape to insert into a mold, so a cylinder is used instead. the height to diameter ratio of the cylinder varies depending on the material, location of the riser, size of the flask, etc.