Unless your balsa is quarter grain (in which case you should save it for som other application) or if you want to bend it around a very small radius ammonia is not usually needed. The trick is not to soak the balsa, but rather to wet it on the "outside" surface. Because the wet side will swell, the balsa sheet will bend in a radius. In some severe cases I helped the process allong by heating the wet side with an iron. Wing torsion boxes and rear-fuselage turtledecks are a piece of cake using this very simple method.