I think they’re referring to the other kind of “isinglass”—thin sheets of transparent mineral, like mica. (Not from fish, not related to glue or food/brewing…same word, spelled the same, but has two different meanings.)
The old gas stove in the house I grew up in had a small round window in front of the pilot light. I remember my dad said it was made of mica, and he called it isinglass. They used to use it for windows in things like stoves and furnaces because it could withstand heat.
I don’t think it would be practical in a flexible item like an umbrella, but I think we’ve collectively determined that this umbrella is a fiction…
