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’d be amazed if this could generate temperatures hotter than about 40 °C. So how else could ignition take place?
• It was interesting to read earlier letters discussing how damp hay leads to internal heating in haystacks. As children growing up on a smallholding in west Wales, we had to build hay ricks using either small bales or loose hay. Hay was always carted and baled after the dew had evaporated from the fields, but occasionally rain, or the threat of rain, meant we had to build the ricks sooner than desirable. So occasionally there were some damp bales, and occasionally one got into a rick.
If there were any worries about damp hay, the rick was tested every few days for any heating in its centre. This was done by pushing a pole between the bales or through the loose hay, and then someone would stick their arm in to take the temperature of the interior of the rick.
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Only once did we have cause for concern, when the interior was very hot. All hands immediately pulled the rick apart and removed what appeared to be a smoking bale. It was also damp and had inadvertently been tightly packed in. We discarded the offending bale, allowed the others to cool, and rebuilt the rick more loosely.
I recall that another mishap involving fire and hay occurred in an outlying field by a railway line. The hay had been formed into a continuous row to make baling easier, when a spark from a steam train set it alight. Fortunately it was spotted, the burning hay was isolated and neighbouring householders brought water to damp it down, so only a small amount was lost.
Mary Sinclair, Narberth, Pembrokeshire, UK
• An earlier reply stated that insurance companies in the 18th century were willing to cover farmers’ haystacks, implying that the insurers clearly believed hay could catch fire spontaneously. Actually, it doesn’t imply spontaneous combustion of hay is possible or that insurers believed it to be.
Instead, the companies may have offered this insurance in the belief that it was not at all possible for hay to catch fire like this. They would thereby attain insurance-company nirvana: collecting a premium to cover a non-existent risk.
Ian Cargill, Leatherhead, Surrey, UK