I have always assumed that the belief that haystacks can burst into flames spontaneously was a convenient myth to cover for careless farm workers having a crafty cigarette break while forgetting their surroundings, but a friend insists that it can happen. Surely, the only way hay can warm up significantly is if it is wet and bacteria begin to heat the stack as part of the process of biodegradation. But I鈥檇 be amazed if this could generate temperatures hotter than about 40 掳C. So how else could ignition take place?
鈥� Cut hay may look dry, but it is considered inert only once its moisture content falls below about 15 per cent. When moisture levels are above 30 per cent, the tissues continue to respire, generating heat and water, which emerges as vapour through the leaf pores. Within the confines of the bale, the water condenses and spreads by capillary action, promoting bacterial and fungal growth, which adds respiring biomass.
In recently harvested hay, the result can be a single temperature peak of up to 60 掳C between five and seven days from the day the reaction begins. This is self-limiting because the temperature kills most microorganisms and drives off the moisture. Sometimes a few weeks of warming cycles can follow as colonies of fungi wax and wane, but the successive temperature peaks likewise dwindle and the bale cools to match its surroundings.
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However, if the hay has just been harvested, and the weather is humid, say, or wet with rain or dew, then a stack may sustain these warm conditions long enough to promote heat-loving microbes. These kick in at around 45 掳C and die by 80 掳C. They are not in themselves dangerous but as they raise the temperatures, they can trigger exothermic chemical reactions that accelerate with rising temperature. As chemistry takes over from biology, temperatures can rise to a blistering 280 掳C. Deep inside the bale this may stop at charring, but when the temperature rises above 231 掳C the hay can auto-ignite on contact with air 鈥� it does not need a spark or a flame. The sequence varies with bale shape and size, porosity to air, and whether it is stacked or confined (in a barn, for example).
鈥淲hen the temperature in a hay bale rises above 231 掳C, it can auto-ignite on contact with air鈥�
A dry outdoor bale of hay (which is grass and mingled herbs) or straw (which is the cereal stalks after the grain has been removed) could also conceivably catch fire from a discarded half-empty glass drinks bottle focusing the sun鈥檚 heat onto it. I recall reading a news item about a house fire caused by a bowl of water on a windowsill. Forensics would show such a hay fire to have started on the outside of the stack. Spontaneous fires, on the other hand, originate internally. Farmers know to monitor their stacks regularly.
Similar warming can occur in domestic compost heaps, which makes them popular egg-laying sites for . If the eggs aren鈥檛 actually poached, a two-headed snake often emerges, reflecting an unusual sequencing of critical biological processes. A fuller explanation of the twin-head snakes can be found in Mark S. Blumberg鈥檚 book, Freaks of Nature (Oxford University Press, 2009).
Len Winokur, Leeds, UK
鈥� I grew up on a farm in Suffolk and we used to make a considerable amount of hay each year. Hay is nothing more than air-dried grass. It is made during the late spring and early summer, and before it is mowed the grass is often quite thick and the ground can remain very wet even after long periods without rain. The mower leaves the hay in swathes, so that they start to dry and to allow the ground in between them to dry as well. The drying grass is then turned a number of times using a hay bob, and when it is judged ready, or nearly ready but rain is forecast, the hay is rowed up and baled.
Baling hay, especially with a traditional baler, is an art. When you start baling a field it can often be mid-morning and the dew may only just have lifted. As the day wears on the hay gets drier and so the bale density has to be continually adjusted to keep the bales tight. It is this tightness that can cause issues with combustion.
Hay is an incredibly good insulator and the denser, and hence wetter, a bale is, the less air gets into it. Anybody who spends time in a hay shed in the middle of winter will find out how warm it can be. The wet hay starts to rot and gives off heat, which is kept inside the bale. Eventually, the hay gets so hot that the dry hay in the bale starts to smoulder and if not dealt with will burst into flames. Hay burns very well.
I can remember occasions during my childhood when I would go into the barn months after the hay had been stacked and find it full of smoke. I would then have to pull apart a stack to find the offending bales. This can be very dangerous because the sudden inrush of oxygen into the stack can cause flames to leap out.
More than once I have found bales that, when opened, were too hot to touch in the middle yet the outside remained at ambient temperature.
Joel Woolf, Cullompton, Devon, UK