No. 76 Special Incendiary Grenade

The No. 76 Special Incendiary Grenade also commonly known as the A.W. bomb or SIP Grenade (Self Igniting Phosphorus) was an incendiary grenade based on white phosphorus used during World War II.

On 29 July 1940, manufacturers Albright & Wilson of Oldbury demonstrated to the Royal Air Force how their white phosphorus could be used to ignite incendiary bombs. The demonstration involved throwing glass bottles containing a mixture of petrol and phosphorus at pieces of wood and into a hut. On breaking, the phosphorus was exposed to the air and spontaneously ignited; the petrol also burned resulting in a fierce fire. 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. Initially known as an A.W. bomb, it was officially named the No 76 Grenade, but more commonly known as the SIP (Self Igniting Phosphorus) grenade. The perfected list of ingredients was yellow phosphorus, benzene, water and a two inch strip of raw rubber; all in a half-pint bottle sealed with a crown stopper. Over time, the rubber would slowly dissolve making the contents slightly sticky and the mixture would separate into two layers – this was intentional and the grenade should not be shaken to mix the layers as this would only delay ignition. 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.

Strict instructions were issued to store the grenades safely, preferably underwater and certainly never in a house. Mainly issued to the Home Guard as an anti-tank weapon, it was produced in vast numbers; by August 1941 well over 6,000,000 had been manufactured.

The grenade could either be thrown by hand, or fired from the Northover projector, a simple mortar; a stronger container was needed for the latter and the two types were colour-coded. As any breakage of the flask would be dangerous, storage under water was recommended.

There were many who were sceptical about the efficacy of Molotov Cocktails and SIP grenades against the more modern German tanks. Weapon designer Stuart Macrae witnessed a trial of the SIP grenade at Farnborough: "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." The drivers were proved right, trials on modern British tanks confirmed that Molotov and SIP grenades caused the occupants of the tanks "no inconvenience whatsoever"

The Home Guard hid caches of these grenades during the war for use in the event of an invasion. Not all locations were officially recorded and some caches were lost. Occasionally, the caches are discovered by builders digging foundations. In all cases, the grenades are still found to be dangerous and typically are destroyed via a controlled explosion.