Blancmange: In cookery, a name of different preparations of the consistency of jelly, variously composed of dissolved isinglass, arrowroot, corn-starch, etc., with milk and flavoring substances. It is frequently made from a marine alga, Chondrus crispus, called Irish moss, which is common on the coasts of Europe and North America. The blancmanger mentioned by Chaucer in the General Prologue to the Canterbury Tales, 1. 387, was apparently a compound made of capon minced with flour, sugar, and cream.One of the commenters, Gil, didn’t see how the isinglass he knew could be edible:
I dunno where that cockamamie reference book got isinglass and such. Who today knows what isinglass is? I have seen some as a kid -- it's a transparent sort of quartz that can be split into thin sheets (and used for windows on the surrey with the fringe on top). … I don't think the FDA would approve of an isinglass pudding nowadays.Like Gil, I knew the isinglass that was translucent (and a feature of the pretty little surrey in the song from “Oklahoma!”). But I also vaguely knew it was an animal product. So what is it really?
It’s both. The original isinglass (first OED cite 1545) is "a firm whitish semitransparent substance (being a comparatively pure form of gelatin) obtained from the sounds or air-bladders of some fresh-water fishes, esp. the sturgeon; used in cookery for making jellies, etc., also for clarifying liquors, in the manufacture of glue, and for other purposes." The word may be “a corruption or imperfect imitation of an obsolete Dutch huisenblas (Kilian huysenblase, huysblas), German hausenblase isinglass, lit. ‘sturgeon's bladder.’”
Two centuries later, the second sense of isinglass appears: “A name given to mica, from its resembling in appearance some kinds of isinglass.”
Whether this isinglass could be used in curtains that would "roll right down, in case there's a change in the weather" is still being debated. Luckily for me (and you), Joel Segal, a bookseller in England, looked into the matter quite thoroughly in January at his blog. The fish-based isinglass, he reports, "was a versatile and expensive commercial product, used as a gum, a food gelling agent, as the sticking medium for surgical plasters, as stiffener for cloth, as a sealant for preserving eggs, and for making mock pearls."
The other isinglass, he says, is
the transparent variety -- otherwise called muscovite -- of the mineral mica. In some parts of world, notably Russia (hence the name muscovite -- i.e. pertaining to Moscow), it's found in large enough sheets to make small window panes, so it was historically used for applications where tough, slightly flexible, heat-resistant, transparent material was needed, such as furnace or lantern windows.
But in actual usage, he finds, the two substances -- both now unfamiliar -- have often been conflated or confused. Thanks for all your research, Joel!