On 29 July 1940, manufacturers Albright and Wilson of Oldbury demonstrated to the Royal Air Force how their white phosphorus could be used to ignite incendiary bombs.
Because of safety concerns, the RAF was not interested in white phosphorus as a source of ignition, but the idea of a self-igniting petrol bomb took hold.
The perfected list of ingredients was white phosphorus, benzene, water, and a two-inch strip of raw rubber, all in a half-pint bottle sealed with a crown stopper.
[3] When thrown against a hard surface, the glass would shatter and the contents would instantly ignite, liberating choking fumes of phosphorus pentoxide and sulphur dioxide, as well as producing a great deal of heat.
Weapon designer Stuart Macrae witnessed a trial of the SIP grenade at Farnborough Airfield: "There was some concern that, if the tank drivers could not pull up quickly enough and hop out, they were likely to be frizzled to death, but after looking at the bottles they said they would be happy to take a chance.