I want to say that the alum foundry I programmed for used potatoes for a degasser. I do know that they put the potatoes in their melts for something.
Two cheap tips for aluminum casting:
(1) Use canning or rock salt as a flux when melting. It will float on top of the melt protecting the aluminum and if the charge is salvage metal it seems to cause the contaminates to separate so these can be skimmed or sink to the bottom. Learned this from watching an aluminum salvage operation.
(2) For degasser, fill a piece of ½ inch black iron pipe about 4 inches long with swimming pool chlorine powder. (much cheaper than the tablets). Use a wad of aluminum foil or brown paper in each end of the pipe to keep the powder in. Use tongs to plunge the pipe to the bottom of your crucible and stir it around. Be careful not to breath the fumes!
If you are getting porosity in your castings, check the pigs/ingots that you pour when the molds are full. If these are sound most likely the metal is too hot.
Try painting the bottom and sides of your foam patterns with water glass (solution of sodium silicate) and let dry. Use plaster or sand to support the coated foam pattern. I can get very smooth surfaces this way.