Much is known about life in the Middle East at the time of the Crusades, but one type of vessel still presents something of a mystery.
These ceramic artefacts are called sphero-conical vessels – a term describing their shape – and hundreds have been unearthed across the region.
By studying the residue inside, archaeologists have established they were used to hold beer, medicine, oils and fragrances.
Some believe they were also used as ancient hand grenades.
Now, a team from Griffith University say their latest research provides proof of this theory.
Carney Matheson and his colleagues have been analysing fragments from four sphero-conical vessels found in the 1960s.
They date back to the 11th century, and were unearthed at a site called Armenian Gardens in Jerusalem.
Residue from three of the vessels contain the presence of benign substances such as medicine and fragrances.
Professor Matheson says chemical analysis has revealed that the fourth contains explosive material.
"There's a range of sphero-conicals, from thin-walled ceramics that could be used for drinking vessels, mercury containers, and all different sorts of stuff," says Professor Matheson.
"But these ones are thick-walled ceramics that we call stoneware. Small containers, small content, withstanding high pressures.
"This particular ceramic was found in the destruction layer of the Crusader fortress that the Arabs smashed."
Professor Matheson believes this fourth fragment, or sherd, is from a hand grenade used during the Christian Crusades — the religious wars initiated by Christians intent on reclaiming the Holy Land from Islamic rule.
The evolution of hand grenades and incendiary devices
The earliest weapons resembling incendiary devices date back to the 7th century Byzantine Empire.
In naval warfare the Byzantines famously used flame throwers to project a fluid known as "Greek fire", the precise ingredients of which were kept a military secret and remain a mystery to this day.
This secret technology helped the Byzantines defend Constantinople from the Arab sieges of 674 and 717.
The Griffith University research suggests the Arabs may have adapted Byzantine incendiary knowledge to develop explosive devices for warfare during the Crusades.
Professor Matheson says the findings support both Arab and Crusader texts from the time which refer to Arab defenders using explosive devices.
"These vessels have been reported during the time of the Crusades as grenades thrown against Crusader strongholds producing loud noises and bright flashes of light.
"It is complicated, but one thing I think is certain is that the compositions that have been reported in the Arabic texts are consistent with a subset of the composite that we found in our vessel."
Why this theory is still up for debate
It's not the first time researchers have speculated about the use of grenades in the Middle East during the Crusades.
A 1937 study examined sherds found in 1168 in Cairo, then known as Fustat.
Those researchers also found evidence of explosive ingredients but were unable to prove it conclusively.
"That's when you had the Crusaders defending their fortress in Fustat, and the Arabs were throwing thousands of these weapons against them in the fortress," says Professor Matheson.
"Their research only identified potassium nitrate and sulphur. They were missing the fuel, because the techniques they used couldn't identify the fuel.
"Our research has identified the potassium nitrate, plus other nitrates, the sulphur, plus the fuel.
"We've got the explosive ingredients of what we think to be the first locally developed explosive weapons in the Middle East and Europe."
The findings are also significant because they appear to rule out the use of Chinese black powder in the region at that time.
Previous hypotheses suggested that black powder, the precursor to gunpowder, may have arrived in the Middle East before the 13th century but was kept a military secret.
Professor Matheson says there is no evidence of black powder in the sherd residue.
"We have definitively shown it's not like powder. It's a local invention. And that's really, really important to say that we've got this local invented explosive ingredient."
Debate over the vessels remains unsettled
Professor Matheson's findings are yet to convince some others working in the field, including Stéphane Pradines, an archaeologist at the Aga Khan University in London.
Professor Pradines was the director of the excavations of the Walls of Cairo in Egypt, and is a specialist in Islamic archaeology and military architecture and weapons.
"Unfortunately, it is a sphero-conical vessel, but it is not a grenade," he says.
"The real grenades have thinner clay bodies. The sphero-conicals are stonewares with thick sections.
"Mercury was at the origin of the incendiary grenade's myth. We know what the real incendiary grenades look like; they are not sphero-conicals."
Professor Matheson agrees that previous studies couldn't prove the hand grenade hypothesis but says the Griffith research has applied a more comprehensive analysis of the ingredients.
"In our weapon, we had a very high quantity of magnesium. And if you remember back at school, when you lit the magnesium powder, it gave a bright flash," he says.
"The magnesium constituents that we have in our device may suggest that it is a specific type of grenade, a flash grenade."
Professor Carney says Crusader texts of the time back the findings up.
"The Crusaders' accounts include bright flashes, big bangs, and sticking to things and burning.
"In modern parlance, that would be a flash grenade, a percussion grenade."
He says further research into more sphero-conical vessels and other artefacts will "put this question to rest".
"My next research is to test more of the vessels of the same typology to see if we can confirm maybe that that typology was the grenade typology as opposed to others," he says.