The Canon and his Yeoman are not mentioned in the General Prologue of The Canterbury Tales, where most of the other pilgrims are described, but they arrive later after riding fast to catch up with the group. The tale the Canon's Yeoman tells is in two parts. The first part is an expose of the shady business of his master the Canon as an alchemist. The second part is about another canon who is also an alchemist who is even more devious than the first.
It is not known if the introduction of these characters was an afterthought by Chaucer or if they were part of the design of the Tales from the start. It is believed it was one of the last tales to be written and it seems to many scholars such a lively attack on alchemists that Chaucer must have had a real person in mind. In 1374 a chaplain called William de Brumley confessed to making counterfeit gold coins after being taught by William Shuchirch. Shuchirch was a canon at King's Chapel, Windsor and in 1390 Chaucer supervised repairs of the chapel so he may have known Shuchirch.
There is no source for the tale although it has similarities to one by Ramon Llull. Chaucer probably got much of the technical detail from Speculum Naturale (Mirror of Nature) by Vincent of Beauvais and Arnold of Villanova is mentioned within the tale itself although he may have read many other alchemical texts. Chaucer's grasp of alchemy seems very accurate and in the 17th century the tale was cited by Elias Ashmole as proof that Chaucer was master of the science. Chaucer did have a great interest in science and technology, writing a Treatise on the Astrolabe.
The Yeoman seems much the more talkative of the two arrivals. When Harry Bailly, the host, asks the Canon for a tale, his yeoman chips in to announce how clever his master is, saying:
The host then asks why the Canon is dressed so poorly if he is so clever and the Yeoman admits that he may have wit but he misuses it. He then explains his master is an alchemist:
The Canon tries unsuccessfully to silence his Yeoman but ends up fleeing in shame; after which the Yeoman feels free to tell the history of the Canon. He describes how the Canon works to discover the philosopher's stone and many of the processes he goes through but how in the end the pot breaks and they lose most of the metal they had. He then continues with a story of a second canon who sells to a priest an alchemical 'crap' for producing silver after tricking him into believing that he can produce the metal spontaneously.
After each of the tales the Yeoman adds a moral such as:
He also explains that we should not try to discover things God keeps secret as it will not succeed and be like picking a fight with God.