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Australasian Biotechnology (backfiles)
ISSN: 1036-7128
Vol. 7, Num. 2, 1997
Australasian Biotechnology,
Volume 7 Number 2, March/April 1997, pp.104-107

The Case of the Exploding Haystacks: Spontaneous Combustion of Natural Products in New Zealand

by Max Kennedy,
Industrial Research Ltd, Box 31-310, Lower Hutt, New Zealand ph 64-4-5690000, fax 64-4-5690132, email

Code Number: AU97017
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Microbial activities are generally slow in nature. Fermentations for example take days or weeks, mould growth on the bathroom wall can take months. There is however one exception; the case of the exploding haystacks. From time to time on hot summer days haystacks will explode into flames without warning. What is intriguing is that there is no apparent human intervention in starting the fire, nor is there any source of ignition involved, and such explosions of hay are difficult to reproduce in the laboratory. The danger to farmers is substantial. Not only is the hay and hayshed lost in the fire, but workers can be caught in a potentially lethal trap. Sometimes such explosions occur when farmers are dispersing the overheated hay to avoid the approaching explosion. Even if a fire does not result, or is small in extent, the economic loss can be substantial as cows will often not eat charred or smoky hay. The culprit is microbial activity which, in the presence of moisture, heats the hay to autoignition temperatures. This is the simple explanation. However, the details are much more complex.

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." (Walker, 1966). Old microbiology books often contain anecdotal evidence of haystack explosions, e.g. Nicol, 1939 states "The stack that sets itself on fire does so in a curious way dependent at first upon both moisture and microorganisms. A really dry stack of hay wont heat spontaneously; a really damp stack can't be set fire to". Even today the threat of exploding haystacks is taken seriously as the New Zealand Fire Service warned farmers in January to watch for spontaneous combustion in stored hay; "We have already had one fire in hay on a property at Temuka and another report of a haystack starting to heat" (Anon, 1996, Collins, 1996)

Hay is not the only natural product to be affected by spontaneous combustion, although not all involve microorganisms. On 25 January 1966, a spontaneous fire in dried spent brewers grains occurred in Christchurch. This fire was stated to be one of the most difficult that the Christchurch Fire Brigade had ever experienced (Walker and Harrison, 1983). In 1980 lucerne pellets used for stock feed exploded in a Timaru port storage silo.

The resulting explosion blew the top off the silo and deposited it in the harbour between moored fishing boats. An overhead gantry used to load the top of the silo crashed to the ground. The shock wave from the explosion blew out several shop front windows a distance of approximately 500 meters from the silo. At the time of the explosion several witnesses saw the silo roof lifting above roof lines of central city buildings before sinking into the harbour (Anon, 1980; Collins, 1996).

Linseed oil offers a particular hazard. One example in 1995 related by the NZ Fire Service was where the owner of a property had been using linseed oil based stain on interior timber. At the end of the day the cotton waste rag used to apply the stain was placed in a four litre tin containing shavings. Several hours later the rag ignited, spread to the shavings and by the time the local volunteer fire brigade arrived the house was totally engulfed by the fire (Collins, 1996). Wool bales exploding on ships was a historic problem for NZ wool exporters. After a long history of fires at sea on boats carrying New Zealand wool bales, research showed that one particular grade of fellmongered wool (pie wool) was prone to spontaneous combustion when dry. The pie process is now no longer in use so the problem of wool explosions has now almost disappeared. The spontaneous combustion of coal is well known in New Zealand mines; and the coal used in the Meremere power station has to be stored under water to prevent ignition. Heaps of wet sawdust and wood waste from sawmills are subject to spontaneous combustion. These sawdust heaps can be large and burn for years and if they are in the middle of valuable forests the problem is serious (Walker, 1966).

Data on how big a problem spontaneous combustion is worldwide are hard to come by. 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 (Wolk and Sarkar, 1993). In New Zealand 500 fires were blamed on spontaneous combustion for the ten years up to 1966. In the UK the equivalent figure was 1,000 fires (Walker, 1966). Table 1 shows that between 1991 and 1995 an average of 198 cases of spontaneous combustion fires occurred in New Zealand, with one case per year of spontaneous haystack fire. The true figures may be higher than this as these data are for fires reported to the NZ Fire Service, and in isolated farm settings farmers may deal with haystack fires themselves.

The mechanism of spontaneous combustion of haystacks is now well understood thanks to a series of studies by Chemistry Division of DSIR (now Industrial Research Ltd) from the 1960s to the 1980s (Rothbaum, 1963a,b; Walker, 1966, Walker and Harrison, 1983). This research explains why silage that gets as hot as some haystacks never explodes, and why the phenomena is so difficult to duplicate in the laboratory. The following discussion is taken from this work.

Table 1: The number of fires caused by spontaneous combustion in New Zealand. Data from the FIRS incident database of the New Zealand Fire Service Whakaratonga Iwi.

Year       Total Fires Caused by       Haystack Fires Caused by 
          Spontaneous Combustion        Spontaneous Combustion 
              in New Zealand               in New Zealand   
1,991             215                            2   
1,992             238                            1   
1,993             190                            1
1,994             183                            1
1,995             162                            0
Average           198                            1

Spontaneous combustion of hay is simply the reaction of large surface areas of hay with oxygen. This reaction becomes spontaneous/explosive if the temperature is above about 76 C. This temperature is initially attained from microbial metabolism and then later increases because the heat from the chemical reaction of hay with oxygen does not dissipate fast enough.

A hay explosion occurs as a result of the following situation. Hay is stored at a relative humidity above 75% (all relative humidity figures in this article refer to the relative humidity inside the stack, not the relative humidity of the air outside the haystack). This initiates microbial action. This microbial action produces heat which raises the temperature of the stack to 76 C. If the relative humidity in the middle of the stack is below 95% then the microorganisms become inactive and the temperature of the stack drops. If the relative humidity in the middle of the stack is above 97% then the resultant heat of vaporisation of the water dissipates the heat rapidly and the temperature of the stack drops. This explains why very wet silage does not explode. However if the narrow window of 95%-97% relative humidity is obtained then the microorganisms continue to produce heat, which cannot escape, which raises the temperature. This temperature rise accelerates the chemical oxidation of the hay releasing more heat. An ever increasing rate of temperature rise is obtained, i.e. bang - one haystack fire. It is indeed an unfortunate fact that the microbial tolerance of temperature and the start of the chemical oxidation of hay overlap at around 76 C when the relative humidity is 95%-97%. A small window of opportunity (see figure 1) but one which never-the-less can occur.

    Figure 1. A temperature versus humidity chart which shows conditions that lead to spontaneous combustion.

    Figure 2. The state of hay bales immediately before combustion. Smoke and charred hay can be seen. Photograph courtesy of the N Z Fire Service.

    Figure 3: Caught in the act of combustion. The commencement of flaming combustion is the brighter glow towards the top right of the exposed bale. This series of photographs is the only known example that shows a microbially induced spontaneous combustion starting. Photograph courtesy of the NZ Fire Service.

    Figure 4: A haystack engulfed in flames. Although this fire was established as arson, the result depicted would be similar if the fire was the result of spontaneous heating. Photograph courtesy of Tony Smith, NZ Fire Service.

Figures 2 and 3 show an actual fire starting in a haybale removed from a haystack. When the Fire Brigade arrived the farmer was frantically removing overheating bales which he had been monitoring. Figure 3 shows the commencement of flaming combustion towards the top right of the exposed bale. Shortly after these photos were taken the stack burst into flames. This series of photographs is unique in that they reveal the state of the hay and stack immediately pre-fire and this is the first known instance of this event being captured on film. Figure 4 shows a large barn full of round bales totally involved in fire. The air space surrounding stacked round bales assists rapid fire spread across the exposed surface of the bales. Although in this instance the cause of the fire was established as arson, the end result as depicted in the photograph would be similar if the fire was the result of spontaneous heating. (Collins, 1996).

The obvious solution to the problem is to only stack thoroughly dry hay. However, fire prevention procedures for suspect stacks involve temperature monitoring. Francis, 1968 recommends thrusting a crowbar into the side of the stack and leaving it for two hours, then to feel the end of the bar on removal as a gauge of temperature. This technique has been improved by the New Zealand Fire Service who recommend pushing a 18mm galvanised pipe into the stack and lowering a thermometer down the pipe. If the temperature is lower than 60°C then the hay should not cause any concern. If the temperature is between 60 C and 70 C then a ventilation hole should be cut to allow air to circulate. If the temperature is above 70 C then a stack or stored area should be dismantled and fire fighting equipment held in readiness as well as ensuring an adequate water supply.


The author would like to thank the New Zealand Fire Service Whakaratonga Iwi, especially K.J. Collins of the Canterbury Area Fire Safety Department, Timaru for their help, information used in compiling this article and for supplying the photographs.


Anon (4 Jan, 1980) Waterfront Silo Blows its Top: Roof Cone Ends in Sea, Timaru Herald.

Anon (4 Jan, 1996) Watch Stored Hay Fire Service Warns, Timaru Herald, p3.

Collins, K.J. (20 June 1996) Canterbury Area Fire Safety Department, NZ Fire Service, Haystack Fires - Spontaneous Combustion, personal communication.

Francis, B.J. (1968) Spontaneous Combustion of Hay, Journ al of Agriculture South Australia, 166-169

Nicol, H. (1939) Microbes by the Million, Penguin Books Ltd.

Rothbaum, H.P. (1963a) Spontaneous Combustion of Hay, Journal of the New Zealand Institute of Chemistry, 27(4), 122.

Rothbaum, H.P. (1963b) Spontaneous Combustion of Hay, Journal of Applied Chemistry, 13, 291-302.

Walker, I.K. (1966) Spontaneous Combustion :Research at Chemistry Division, Industrial Bulletin, 22(4): 23-25.

Walker, I.K. and Harrison, W.J. (1983) Spontaneous Ignition of Spent Brewers Grains 3 Exothermic Oxidation at a Low Oxygen Concentration, New Zealand Journal of Science, 26, 33-38.

Wolk, M. and Sarkar, S. (1993) Hay Self-Ignitions Between 1970 and 1990 in Wurttemberg: Causes, Dimensions of Damage and Methods of Prevention, Zeitschrift Das Wirtschaftseigene Futter, 39(3), 228-235.

Copyright 1997 Australian Biotechnology Association Ltd.

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