In Scene 6, after Joan's execution, the executioner reports to Warwick that the job is done. He seems to be speaking literally, but his report is tinged with dramatic irony and metaphorical meaning:
THE EXECUTIONER. Her heart would not burn, my lord; but everything that was left is at the bottom of the river. You have heard the last of her.
The executioner means that the physical organ of Joan's heart refused to burn. Although organ tissue is not necessarily the part of the body that is most resistant to burning, it is true that it requires extraordinarily hot fire to completely incinerate a body. Part of the executioner's job, if burning people is a form of execution, is disposing of remnants that have not been reduced to ash. He is simply telling Warwick that he has burned Joan and disposed of the remains in the river. He genuinely believes that Warwick has "heard the last of her."
But anyone reading or watching Shaw's play in the 20th century and beyond knows that Joan will only get more famous after her death. Centuries later, people are still talking about her. The whole reason Shaw was inspired to write Saint Joan is because the Catholic Church canonized her as a saint as recently as 1920. This future knowledge allows the audience to enjoy a sense of dramatic irony at the executioner's assertion that Warwick and the world have heard the last of Joan. On the contrary, her execution has ensured that the world will never stop hearing about her. Additionally, the fact that Joan has been made a saint gives metaphorical meaning to the phrase "Her heart would not burn." Saint Joan represents extraordinary faith and commitment to her cause. Whereas the executioner thinks of her as a mere human woman, the audience knows her as a legend. Her fire-resistant "heart" becomes not just an organ, but her strength of character.
