Children playing with aerosol cans were blamed for starting a large field blaze.
Fire crews arrived at the remote farmland in Wall Heath near Kingswinford, to find two cans containing the hazardous substance propane in the debris.
Firefighters fought the flames and smoke on foot using rucksack water carriers because they were unable to get close access to the location behind Maidensbridge Primary School off the A449 Wolverhampton Road.
They were called to the scene at 5pm on Sunday.
Watch Commander Mick Clark, of Brierley Hill Fire Station, said: “The field was recently harvested of its crop, with just waste remnants on the ground. When we arrived this waste was well alight and the two cans were in the fire. This is quite dangerous as such cans can heat up to a certain temperature and explode causing a fireball and dispersing bits from the can a fair distance.”