Why St. Joan dressed in men's armor in prison...
Heresy was a capital crime only for a repeat offense. Joan agreed to wear feminine clothing when she abjured. This created a problem. According to the later descriptions of some of the tribunal members, she had previously been wearing male (i.e. military) clothing in prison because it gave her the ability to fasten her hosen and tunic together into one piece, which deterred rape by making it difficult to pull her hosen off. A woman's dress offered no such protection. A few days after adopting a dress, she told a tribunal member that "a great English lord had entered her prison and tried to take her by force. [i.e. rape her]" She resumed male attire either as a defense against molestation or, in the testimony of Jean Massieu, because her dress had been taken by the guards and she was left with nothing else to wear.
Her resumption of male military clothing was labeled a relapse into heresy, although this would later be disputed by the inquisitor who presided over the appeals court which examined the case after the war. Medieval Catholic doctrine held that cross-dressing should be evaluated based on context, as stated in the "Summa Theologica" by St. Thomas Aquinas, which says that necessity would be a permissible reason for cross-dressing. This would include the use of clothing as protection against rape if the clothing would offer protection. In terms of doctrine, she had been justified in disguising herself as a pageboy during her journey through enemy territory and she was justified in wearing armor during battle and protective clothing in camp and then in prison. The Chronique de la Pucelle states that it deterred molestation while she was camped in the field. When her soldiers' clothing wasn't needed while on campaign, she was said to have gone back to wearing a dress. Clergy who later testified at the posthumous appellate trial affirmed that she continued to wear male clothing in prison to deter molestation and rape.
She referred the court to the Poitiers inquiry when questioned on the matter. The Poitiers record no longer survives but circumstances indicate the Poitiers clerics had approved her practice. She also kept her hair cut short through her military campaigns and while in prison. Her supporters, such as the theologian Jean Gerson, defended her hairstyle for practical reasons, as did Inquisitor Brehal later during the appellate trial. Nonetheless, at the trial in 1431 she was condemned and sentenced to die. - SourceArt: Eugene Thirion, 1876, Jeanne d' Arc
Chatou, Church Our-Lady
The technical reason for her execution had been a Biblical clothing law. The nullification trial reversed the conviction in part because the condemnation proceeding had failed to consider the doctrinal exceptions to that stricture. The appellate court declared her innocent on 7 July 1456. - Source