How Can an Aluminothermic Reaction Help in Breaking into Buildings?
Toward the end of the 19th century, Hans Goldschmidt discovered that elemental aluminum reacts with metal oxides when heated strongly. The method became known as the aluminothermic reaction  for the preparation of metals. The thermite reaction gained practical significance with iron oxide in industrial welding. The formation of aluminum oxide with the simultaneous reduction of the iron creates a strongly exothermic reaction, reaching temperatures of 2400 °C.
2 Al + Fe2O3→Al2O3 + 2 Fe ΔRHØ = –851,5 kJ (per mol Al2O3)
8 Al + 3 Fe3O4→4 Al2O3 + 9 Fe ΔRHØ = –3347,6 kJ (per 4 mol Al2O3)
Thermite was also used in incendiary bombs. It was particularly useful in destroying weaponry made of steel, as Walter explains to Jesse in episode (I-7), mentioning the German super cannon “Heavy Gustaf” as an example.
A thermite reaction is not easy to start as it has a high activation energy – the ignition temperature is around 1500 °C. A burning magnesium strip is normally used to initiate the reaction. Walter and Jesse solved the problem very elegantly by using a gas burner. The lock on the chemical warehouse’s steel door no longer puts up any significant resistance.
 F. W. Hall, Aluminothermic Processes, Ullmann’s Encyclodedia of Industrial Processes, Wiley-VCH, Weinheim, 2012, 471. DOI: 10.1002/14356007.a01_447