What is isinglass is as in “isinglass curtains you can roll right down in case there’s a change in the weather”? It’s a collagen obtained from dried fish bladders. It’s semi-transparent and was used as window in the earliest cars since they didn’t shatter.
There is no record, however, of them being used on fringed surreys in Oklahoma in the 1890s.
Isinglass is used today as a clarifier for beer and wine. Some vegetarians consider isinglass unsuitable for their diets.