OED2 gives it as a Physics term, more in use in the 18th and 19th Cs.

1. trans. To cause to burn away with sudden evolution of flame and rapid, sharp combustion (e.g. a mixture of charcoal and nitre thrown into a red-hot crucible).
1727 Bailey vol. II, Deflagrate, to inkindle and burn off in a Crucible a Mixture of Salt or some mineral Body with a Sulphureous one. 1794 J. Hutton Philos. Light, etc. 208 When coal is deflagrated with nitre. 1876 S. Kens. Mus. Catal. No. 1369 The spark from this battery deflagrates a platinum wire a foot long.

2. intr. To burst into flame and burn away rapidly.