In one of the musical films,probably in the forties or fifties there is a line on one song refering to riding in an open carriage ' and Isignlasss curtains to pull right down in case there is a change in the weather', So it seems to be able to be made into a transparent, waterproof sheet,
See;
Isinglass is a substance obtained from the dried swim bladders of fish. It is a form of collagen used mainly for the clarification or fining of some beer and wine. It can also be cooked into a paste for specialised gluing purposes. The English word origin is from the obsolete Dutch huizenblaas – huizen is a kind of sturgeon, and blaas is a bladder, or German Hausenblase, meaning essentially the same