Development and Use in War: (4) Britain: 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. However, there were voices that were more cautious. There were many who were sceptical about the efficacy of Molotov cocktails and SIPs grenades against the more modern German tanks. Weapon designer Stuart Macrae witnessed a trial of the SIPs 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". Wintringham, though enthusiastic about improvised weapons cautioned against a reliance on petrol bombs and repeatedly emphasised the importance of using explosive charges