Pop go the hay bales

Last month there were enormous wildfires in Australia, but as that continent bakes in the summer sun, there are other hazards. Like hay bales and entire sheds of hay exploding. Last year (2008) was "the worst season ever for spontaneous hayshed fires." [1]

It's not as sensational or mysterious as spontaneous human combustion, but it has the benefit of being verifiably real. The first article I found estimates some 400 cases of sudden hayshed fires in New South Wales alone, though it's not clear whether this is only for 2008 or for some longer period. [1] "One German study using data from an insurance company reported 304 haystack fires between 1970 and 1980, dropping to 118 between 1980 and 1990 due to the insurance company distributing 2500 hay thermometers to fire brigades since 1980." [3, quoting Wolk and Sarkar, 1993]

Oddly, it's not the driest bales that burst into flame, but ones with a moisture content around 20%. The moisture allows bacteria to thrive, and my initial impression is that the bacteria could be producing methane. However, the explosions are the result of bacteria raising the core temperature of the hay bale to 76 degrees C, and the ignition is just a reaction with oxygen. [1,2]

"The phenomena of exploding haystacks has been with mankind for as long as he has been making hay. Pliny, the Roman Philosopher wrote in 60BC, 'When the grass is cut it should be turned towards the sun and must never be stacked until it is quite dry. If this last precaution is not carefully taken a kind of vapour will be seen arising from the rick in the morning, and as soon as the sun is up it will ignite to a certainty, and so be consumed.'" [3]

It's odd that something which has been known and documented for over 2,000 years still sounds absurd and unlikely, but a lot of farming procedures and parameters are largely unknown to the general public.

1. Bizarre weather sends hay bales up in flames (News.au.com, Jan 21 08)

2. Spontaneous Combustion of Hay (PDF) (Dep't of Primary Resources, South Australia - PIRSA)

3. The Case of the Exploding Haystacks: Spontaneous Combustion of Natural Products in New Zealand (Australian Biotechnology, March/April 1997)