Boric acid is a white powder which is used as an additive to compositions containing aluminium and a nitrate. The metal powder can reduce the nitrate to an amide, which will react with the metal powder in a very exothermic reaction that can lead to spontaneous ignition of the composition. This process is often accompanied by a smell of ammonia and is most likely to occur with wet compositions. Addition of a few percent boric acid can often prevent this reaction from taking place since it neutralizes the very basic amides forming ammonia and a borate. It is also advisable to avoid using a water-soluble binder for these compositions. Using red gum or shellac with alcohol or nitrocellulose lacquer is safer. Boric acid is hygroscopic. The use of boric acid with magnesium or magnalium is not advised, as it actively attacks and corrodes these metals.