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ON 8 NOVEMBER 1605, the king鈥檚 stores at the Tower of London took delivery of an unusual consignment from 鈥渙ut of the vault undernethe the Parliam鈥檛 House鈥�. The records state that it was 鈥淐orne powder xviii c weighte decayed w鈥檆h was there laide and placed for the blowinge up of the said house and destruction of the Kinges Ma鈥檛ie, nobilities and commonaltie there assembled鈥�.

If Guy Fawkes and his fellow Catholic conspirators had succeeded in their plot to blow up the English parliament and the king鈥檚 majesty 400 years ago, 18 hundredweight of gunpowder 鈥� getting on for a tonne 鈥� would have devastated the city of Westminster. 鈥淚t would have been a bloody big bang,鈥� says Robert Smith, an expert in gunpowder and ancient firearms based in Leeds.

The plot was foiled, but even if it had not been, the city might still have been spared 鈥� the gunpowder was 鈥渄ecayed鈥�. 鈥淣o one quite knows what this means,鈥� says Smith, 鈥渢hough it probably means it was damp.鈥� In Stuart England half a kilogram of gunpowder cost about 8 pence 鈥� equivalent in value to 拢60 today. But the high price was no guarantee of effectiveness. Gunpowder was notorious for keeping badly, so the plotters may have found themselves with a job lot of powder that was past its explode-by date.

Hundreds of years on, surprisingly little is known about this enigmatic black powder. From its origins in 11th-century China to the late 19th century, when it was replaced by cordite and high explosives, gunpowder was at the cutting edge of military technology. Yet only now are scientists starting to uncover the secrets of its explosive power. 鈥淲e have only scratched the surface so far,鈥� says Smith.

The main reason for this, he says, is that modern scientific techniques were only being introduced at the end of the 19th century, just when gunpowder was being phased out. These techniques could have answered some of the basic questions about medieval gunpowder: where did the raw materials come from and how were they refined; what was the best chemical composition of the mixture; and what particle size was best for pistols, cannons and blowing up parliament?

Simply applying modern analytical techniques to a sample of old gunpowder is not an option. Unused gunpowder was regularly sent back to the government mills for recycling and none survives.

Even old recipes aren鈥檛 much of a guide to making first-rate powder (see 鈥淒o not try this at home鈥�). 鈥淭hey are more like cooking than chemistry,鈥� says Smith. Many details that would have been known to workers at the time are left unsaid. A lot of the knowledge about gunpowder was handed down through the generations, a tradition that died out more than a hundred yearsago when black powder was relegated from first-rate guns to fiesta-time fireworks.

If one thing is clear, it鈥檚 that gunpowder has always been surrounded by misinformation. More than 200 years ago Robert Coleman, Clerk of Cheque (effectively works manager) of the Royal Gunpowder Mills at Waltham Abbey near London, told the Askesian Society, a Georgian club for the exchange of scientific ideas, that every author he had read described the process of manufacturing gunpowder inaccurately. In many instances the account was 鈥渆xtremely absurd鈥�. And what was true in 1801 is still true today.

Gunpowder is a mixture of saltpetre (potassium nitrate), charcoal and sulphur. The most critical ingredient is the saltpetre, says Smith, not least because it is by far the largest single constituent. By 1801, saltpetre was being supplied from natural nitrate deposits in India and south-east Asia. But from the mid-14th century until the 1620s 鈥� at least 15 years after the plot to blow up parliament 鈥� saltpetre was still made in the traditional medieval way, from manure.

Collectors would scour the country for the waste from stables and dovecotes. The trade was so vital to the crown that it granted a lucrative monopoly to various nobles to organise the collection of manure from their lands. Their agents could force people to work for them or to provide equipment such as carts for less than the going rate. Small wonder they were widely loathed. But getting hold of the dung was just the start. To obtain saltpetre, you need to put it through a lengthy purification process. 鈥淚f you brew up this stable waste and keep it damp with urine, and then extract it with water you get a white powder when you evaporate it,鈥� Smith says. This sounds simple. But unless you add a source of potassium, such as wood ash, the powder you end up with is calcium nitrate, not potassium nitrate.

鈥淏rew up manure and keep it damp with urine, and then extract it with water and you get a white powder 鈥� saltpetre鈥�

Could medieval manufacturers have made gunpowder with calcium nitrate? To find out, a team at the Medieval Gunpowder Research Group at the medieval centre in Nyk酶bing Falster, Denmark, tried to do just that. Smith, a consultant to the group, was present in 2003 when they tested the product of their labours. 鈥淭here were about six or seven of us standing around watching,鈥� he says. 鈥淲e put a match to the powder鈥�and it went out.鈥�

Calcium nitrate is useless for making gunpowder because it is deliquescent 鈥� it sucks water out of the air. Just slight contamination with calcium nitrate or sodium chloride, which also absorbs water, will draw in enough moisture to spoil the gunpowder. It is possible that this is why medieval gunpowder spoiled so quickly.

So the researchers took a different tack, and attempted to make saltpetre from scratch in the more complicated medieval way. They collected chicken manure and created a large saltpetre pit using compost, rubbish and potash. The theory was that the potash (potassium carbonate) would exchange ions with the calcium nitrate to produce potassium nitrate. Two years later, the researchers dug out the pit, extracted some white powder and, with high hopes, sent it off for analysis. It turned out to be mostly potassium sulphate, and decidedly non-explosive. 鈥淭he conversion process from calcium to potassium works,鈥� says Smith. 鈥淲e just did not have much nitrate.鈥� He thinks that a further problem lay in the acidity of chicken dung. Since making saltpetre requires a relatively high pH, horse manure should work better.

Fortunately, these researchers weren鈥檛 the only people inspired to make their own dung heaps. Following a talk by Smith on medieval techniques for making saltpetre, John Edmonds, a retired British engineer, decided to have a go himself. In 2003 he built his own saltpetre bed in his Buckinghamshire garden using layers of garden compost, cowpats and wood ash. 鈥淵ou鈥檝e got to cover it to prevent it getting too wet, you have to keep it damp and you have to get air into it,鈥� he says. After months of turning the ripening heap every week, a whitish efflorescence appeared on the surface. Edmonds removed the thin surface layer and continued turning the heap, collecting more of the substance as time went by. After 18 months he had enough to try and extract saltpetre from it, following instructions laid down in 1862 by Joseph LeConte, a chemist at South Carolina College (see ).

LeConte鈥檚 paper harks back to techniques that were common 300 or 400 years earlier. By LeConte鈥檚 time, saltpetre was produced on a vast scale in Bengal and in Chile, where deposits of sodium nitrate occur naturally. But in 1862 America was in the throes of civil war and the Confederate south鈥檚 ports were blockaded. So LeConte tested and compiled ways for the south to make its own saltpetre.

Buckets of gunge

Following LeConte鈥檚 instructions, Edmonds dissolved the white powder in water, let the solids settle out, and decanted the solution from one bucket into the next until he got a clear liquid. 鈥淵ou end up with a line of buckets, each full of the gunge,鈥� he says. He then heated the liquor to evaporate off some of the water. This reduced the volume of water, concentrating the dissolved salts until they began to precipitate out. Potassium nitrate is about six times as soluble as salts such as sodium chloride, so while these impurities dropped out, the saltpetre stayed dissolved.

According to LeConte, one way of detecting saltpetre in the liquor from the dung heap is to taste it. Edmonds chose a less unappetising technique. 鈥淚f you soak blotting paper in it, dry it and put a match to the paper, it burns like a sparkler if you鈥檝e got saltpetre,鈥� he says. Finally Edmonds evaporated the refined liquid and analysed the crystals. 鈥淭hey were 75 per cent pure potassium nitrate,鈥� he says.

Smith is delighted by Edmonds鈥檚 accomplishment, and remains full of admiration for medieval gunpowder makers. 鈥淭his is some very sophisticated chemistry,鈥� he says. The refining technique depends on the solubility of potassium nitrate increasing faster with rising temperature than the solubility of impurities such as sodium nitrate. 鈥淗ow did they know that?鈥�

But how the medieval world made saltpetre is only part of the puzzle. To make good gunpowder, you need charcoal and sulphur in the right proportions. 鈥淓ven though medieval people knew about guns and gunpowder very early on, it is 200 years before guns become a decisive factor in battles,鈥� says Peter Vemming Hansen, director of the Nyk酶bing Falster centre. Why the delay? That鈥檚 a question the team is still working to answer.

The next step for the group is to look at how sulphur was mined and refined. Most sources suggest medieval sulphur supplies came from Mount Etna in Sicily, but the researchers suspect that in northern Europe, Iceland鈥檚 volcanoes may have been the top source in medieval times. No one knows what kind of refining this material would have required.

These days, the niceties of manufacturing gunpowder are little more than a historical curiosity. But in its day, getting the right recipe and the right ingredients could have changed the course of history.