Richard I of England's heart was preserved with mercury and tar before being "sweetened" with herbs to make it smell nice after his death, researchers have found.
Forensic scientists analysed the organ of the king - called The Lionheart - after removing his remains from a church in France where part of his body was laid to rest.
They found that embalmers preparing him for burial after death had used mercury and tar-like creosote to preserve the heart.
The organ had then been daubed with frankincense, myrtle, daisy and mint to make it smell sweet, before it was wrapped in linen and placed in a lead-lined box.
Forensic expert Philippe Charlier, who led the team that carried out the research, said there may have been an attempt to make the body part holier.
He said: "We found things that we didn't expect.
"The frankincense is something we have never seen until now. It is a substance whose use comes directly from divine inspiration.
"It was one of the three gifts brought by the Wise Men at Jesus' birth, and it was used by Joseph of Arimathea to help preserve Jesus's body at his death. So using it is a direct reference to Christ."
Richard I came to be buried in what is now France as it was part of the English king's territory at the time.
The researchers reached their conclusions after doing an analysis of the compounds surrounding the heart using a technique known as gas chromatography.
The organ had been buried in Rouen Cathedral after Richard I was killed at the rebellious French town of Chalus while laying siege to the castle there in 1199.
At the time, a king's organs were often separated from the rest of his body before being placed in the ground.
His body is said to have been buried elsewhere, in Fontevraud Abbey in the western Loire Valley, and his entrails interred in Chalus.
The final resting place of the heart was discovered in 1838 when Rouen historian Achille Deville discovered the lead box that contained it in the cathedral's crypt.
By the time Mr Charlier's team were allowed to analyse it, the contents of the box had reduced to dust and they were only permitted to take two grammes (0.07 ounces) from the 80 grammes inside.
As well as the materials used to preserve and sweeten it, they found no clearly identifiable tissue, but did find an antibody reaction that suggested a protein found in human muscle.
Several bacteria and fungi species were also seen, but none confirming how Richard died.
The scrutiny revealed a tiny scrap of linen and ancient pollen from poplar, oak and pine that probably came from airborne contamination before the box was sealed.
King Richard I died in 1199 at 41 while fighting the French in Chalus, central France, where he was shot in the left shoulder by a crossbow arrow, reputedly fired by a boy.
He died 12 days later, presumably from septicaemia or gangrene, although some folk tales suggest the arrow was deliberately poisoned.
He is said to have been king at the time of Robin Hood and he was succeeded by his flawed and much less popular brother John.
DNA and carbon dating were impossible because the lead and creosote had caused organic matter to decay.
Mr Charlier said 12th Century embalmers were usually cooks and butchers, who were used to cutting meat and removing offal and had access to herbs, spices and other aromatic substances.
Contrary to a popular image of the Middle Ages as being barbarous, he said the people who worked on Richard's heart "were extremely skilled," combining complex metals, including liquid mercury, with vegetable residues.