How easily can a medieval granary explode?

Question

The villainous gremlins are attempting to cause as much damage to the lands of the Empire as possible with as little effort as practical. They have numerous plans to this end. One such plan is to blow up the granaries/silos which store all the recently harvested grain (mostly wheat and barley), depriving the Empire of food and possibly killing people as an added bonus. They don't have explosives, but if conditions are right all they need is a flame.

It is well known in the modern era that granaries and silos are an explosion hazard, due to dust explosions. However, from my investigations, it would appear that such explosions were only of considerable risk after industrialisation. The list of notable dust explosions starts in the 19th century. The smaller scale of pre-industrial grain and flour handling seems to have reduced the risk of explosions. Food storage was typically in the form of whole grain rather than finely powdered flour, which would further reduce the risk of explosions.

In favour of explosions, grain is still combustible (although I don't know how combustible). And our saboteurs can deliberately stir up grain and dust to improve the chances of an explosion. But it is unclear if these factors will be adequate.

The climate of the region is comparable to Great Britain or some other parts of Europe (with cold, wet and sometimes snowy winters). The technology is roughly comparable to the late medieval era. Sufficient grain is produced to warrant bulk storage over winter.

I'm not entirely sure on what layout a practical granary or silo for this climate might look like. Great Britain preferred to have shacks raised above the ground as granaries. The pictures don't show how the grain is stored, but it seems to be at the same level as you enter, which seems to suggest that it is confined to boxes or sacks or something which might inhibit explosions (but if you have information which says otherwise, do share).

Another way of storing grain is in a silo, which is essentially a pit in the ground. This is the method implied in the answers to this question. Having a large pile of grain sounds like a prime candidate for an explosion. However, a source I have found seems to suggest that pre-industrial silos were preferred in dry lands, and some wet lands depending on their soil (such as France) were not suitable for silos. (I don't have full access to that source, so can only read the first page.)

If there was need for additional grain storage, would the people of the Empire build silos or granaries, or could they build either? If these structures can explode with different measures of ease, then the relative abundance of these structures would affect how easy it is for the saboteurs to damage the Empire's food supplies.

In these circumstances, would it be relatively easy to cause a catastrophic dust explosion in a medieval grain storage building? I am aware that it should be possible to contrive a scenario where such an explosion can occur, but if the scenario is too contrived then the saboteurs would look for an easier method to destroy the Empire's food supplies. On the other hand, if blowing up a granary is as simple as kicking up some dust and chucking in a match then they could use the strategy with abandon.

Any examples of pre-industrial grain explosions would be appreciated. Otherwise a solid argument on the plausibility or otherwise of grain explosions in the medieval era will be good. Bonus points for being applicable to my setting's climate.

Answer 1

Yes they are possible, we have records of them happening.

Although grain mills were a much bigger risk, since they could generate both far more dust and a spark. They even occasionally developed safety systems to warn when the conditions were right, mostly when the stones could easily generate a spark. This tells you they must have been common since safety measures tended to be few in antiquity.

There were many ways grain was stored, it varied quite a lot from place to place and there was surprising little standardization. In bulk storage shoveling and stirring grain was common and could generate a lot of dust in dry climates. Grain was handled constantly for a multitude of reasons and people were regularly employed to sweep up the dust it generated.

J. V. Van Leuven, 'Prehistoric grain explosions', Antiquity

This source indicates the following factors contributed to grain explosions in early history (and indeed today):

• Dry climate

• Large-scale storage

• Lack of ventilation to remove dust

• Regular handling of the grain

• Long-term storage (on the order of a year), which increases the amount of dust

The source does not specify how forceful these explosions were, whether they could be comparable to modern grain explosions. Nor do the specify how much damage was done by the explosion vs the subsequent fire. Note also that the bulk of the source is behind a paywall, like so many papers.

Note however grain FIRES were far more common, so if you just want to destroy the grain a fire will work just as well if not better. Fire are possible in a much wider set of circumstances. They could even occur to spontaneous ignition of damp grain. Grain like any dry starch burns very vigorously.

Burning grain stores was a known way of attacking an enemy, and was very effective.

James A. Thorne, Warfare and Agriculture:The Economic Impact of Devastation in Classical Greece, Greek, Roman, and Byzantine Studies 42 (2001)

Answer 2

A medieval granary is missing the main source of grain dust that could form an explosive mixture: high volume mechanical transport of the grain.

Augurs (for lifting grain), belts, and so forth jostle the grains against one another, rubbing off tiny bits of the bran. This is what forms the dust that can then collect in non-circulated air to form an ignitable/explosive mixture. In a medieval granary, the grain is moved by people with scoops (think of a shovel with moderately deep spoon-shaped blade), or by gravity, in far smaller quantities and at far lower rates than in a modern grain elevator. Because of this, the level of dust in the air will tend to be much lower.

Where grain dust explosions did occur (as far back as Roman times) is in mills. Here, the grinding of the grain produced dust which, in the relatively enclosed space of the mill itself, could easily collect to ignitable levels. In an age when artificial light meant an open flame, this could and did lead to explosions that killed people and destroyed mills. This was common enough that torches, lamps, and candles were banned from mills for centuries.

Because static electricity and friction can still produce ignition, such a ban wasn't a universal solution to the problem, but it made mills safe enough to work in that they at least weren't banned from populated areas the way gunpowder mills were.

Answer 3

The key is pressure

If rapid combustion occurs in a confined space, enormous overpressure can build up, causing major structural damage and flying debris. The sudden release of energy from a "detonation" can produce a shockwave, either in open air or in a confined space. If the spread of flame is at subsonic speed, the phenomenon is sometimes called a "deflagration", although looser usage calls both phenomena "explosions".

https://en.wikipedia.org/wiki/Dust_explosion

While grain dust is certainly the hazard in modern silos that normally starts the combustion, it is more likely the silo and not the grain that has meaningfully changed.

The issue with industrial era granaries is that they are much more airtight systems (typically made out of welded plate steel or sealed concrete) than these older buildings were which means that pressure caused by things burning on the inside can build up much faster than the system is designed to vent gases. This pressure is the difference between grain that burns and grain that explodes.

In contrast, older silos were generally much smaller and made out wood. This meant that you had more gaps for air to flow out through, and that the size of the chute to the volume of the container was much larger. So when things went up in flames, you'd have much more of a burn instead of a blast.

How to get a boom

This is a two step sabotage.

First you need to make the grainery airtight. If the gremlins sneak into it when it is first done being built, they could paint the walls with a thick layer of pine resin or hide glue to make sure airflow can't get in and out through the walls.

Then when the silo is done being filled, you throw in a few buckets of water, and glue the door shut.

When you add water to the grain, it will begin growing yeast. The yeast will then start turning the starches in the grain into alcohol which as James Cook pointed will eventually combust due to exothermic decomposition. With the grainery now being a closed system, you will get the exact same sort of issue you see when you don't properly vent the stills where people make alcohol on purpose: you get an explosion.

Answer 4

As noted, medieval granaries did not explode, because of the lack of large quantities of fine powder found in modern grain mills and silos.

As also noted, spontaneous combustion does occur in all sorts of materials, including hay, sawdust and anything else flammable with a large enough surface area. However, people are usually aware of this and take precautions (such as farmers checking internal temperature of haystacks).

If you have coal mines these might be a better bet. It was once thought that explosions were caused only by gas, it turns out many were caused simply by coal dust. One test with a few hundred kilos of powdered coal blew out windows several miles away.

Fuel-air explosions with liquid fuel can be equally dramatic. If your culture uses oil, with a bit of heat and confinement and mixing with air your gremlins might create a satisfactory boom.

Answer 5

Add water. Wet grain decomposes generating heat. The heat in turn increases the speed of decomposition. Grain and for that matter hay, always needs to be stored dry.

Many farmers have lost barns due to wet hay causing fires. I knew several farmers that had this happen to them.

I also grew up next to a family owned feed mill that ground grain to make animal feed. The owner went on vacation leaving his adult sons to run it for a couple of weeks. They go a load of wet grain. Instead of throwing it out they decided to wait until their dad returned from vacation. Instead the grain exploded in to flames and the mill was burned down.