When I wrote in a previous post that John Carey had reported that "green mildew grew on [Tolkien's academic] gown," but I queried whether this was really likely, Chaz Brenchley referred me to Tolkien's Gown & Other Stories of Great Authors and Rare Books by Rick Gekoski (Constable, 2004) for the definitive word on this vital topic.
Gekoski became the unlikely owner of this relic in 1972. The newly-widowed Tolkien, on moving into a residence owned by Merton College, asked his scout, Charlie Carr, "to help clear away a lot of unwanted rubbish," including his gown and some old shoes and jackets. (14) Carr thought of Gekoski, whom he had scouted for when Gekoski was a grad student a few years earlier, as a possible customer. Gekoski, by then teaching at Warwick University, whimsically decided to take the gown, and stored it in his attic for some ten years, until he sold it through his catalogue when he began a new career as a rare-book dealer.
How does Gekoski describe the gown? As a "raggedy old scrap of black cloth" with "many DNA-rich stains," and, in book-dealer language in his catalogue, as "slightly frayed and with a little soiling." (14-15)
At the question of whether Carey, a bored undergraduate sitting some distance away in a lecture, might have mistaken these stains for green mildew, I find I have reached the limit of my devotion to Tolkienian arcana.