A conversation in the Globe a couple of days ago turned from a discussion of beer production (for reasons that'll became apparent) to a Rodgers and Hammerstein song. The question arising: what are the "isinglass curtains" mentioned in the Oklahama! song embedded above?
With isinglass curtains you can roll right down,
In case there's a change in the weather.
- "Surrey with The Fringe on Top", Oklahama!, Rodgers and Hammerstein
The object in question is evidently the storm top or storm curtain that attached to the roof of carriages (this is a surrey) and early cars as rainproofing. This is a relatively well-trodden topic, but it's worth retracking as an example of the confusion that can arise if you try to brain out historical/etymological stories purely from the words present (at the level of assuming "plum pudding" contains plums).
The phrase "isinglass curtains" is actually an anachronism anyway. Oklahoma! is set in 1906, but a look at Google Books Ngram Viewer for isinglass curtains, and at Google Books, finds the earliest print occurrence to be 1927, after which the term was used for the storm curtains of early popular models of motor vehicle. The explanation for this at least is that the 1943 Oklahoma! was based on a play by Lynn Riggs, the 1931, Green Grow the Lilacs, and Hammerstein's adaptation of the play text altered the character Curly's words. In the play he refers to isinglass windows:
And this yere rig has got four fine side-curtains, case of a rain. And isinglass winders to look out of!
- The Cherokee night and other plays, Lynn Riggs, University of Oklahoma Press, 2003
As to "isinglass", it has two main meanings. One is a clear gelatine derived from fish swim-bladders, used in fining of beer and wine. Historically, it 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 1, and for making mock pearls. The classic source was Russian: the swim-bladder of the "Huso", aka the Isinglass Fish aka the beluga or cavier sturgeon. The etymology of the word "isinglass" isn't 100% certain, but the most likely (according to the OED) is derivation from Dutch "huisenblas" = sturgeon bladder.
The other main meaning of "isinglass": 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. Why it's also called "isinglass" isn't clear, but the OED says "from its resembling in appearance some kinds of [the gelatine] isinglass".
Neither of these sound promising curtain material, and the twin meanings are already a recipe for confusion. Some sources opt for the first, despite the unlikeliness of making rain curtains or windows from a water-soluble gelatine:
In the stove window trade, mica acquired the name isinglass, after its resemblance to a gelatin made from fish bladders that was made into windows for carriages and early cars ...
- Roadside geology of Maine, Dabney W. Caldwell, 1998
Some sources mistakenly conflate the two ...
Sturgeon — regarded as a trash fish until about 1870, prized thereafter as sources of smoked meat, oil and the isinglass that was once used as windows in stoves and lanterns ...
- Biologists tackle fishery questions, Milwaukee Journal, March 24, 1991
Isinglass is a mica or clear gelatin made from certain fish bladders, principally the bladders of sturgeon. Isinglass was once used in glues and jellies, as well as (get this) the manufacture of drop-windows on buggies.
- Isinglass: this wordsmith correct in use of language, Richmond Times, Sep 16, 1994
... but the predominant assumption is that "isinglass curtains" refer to curtains with inserts of mica, and some sources explicitly confirm this, such as
... there were the Isinglass curtains — sheets of transparent mica sewn into black leatherette frames ...
- Saturday Review, Volume 50, 1967
But is any of this terminology reliable? A 1937 Q&A in Hospitals: the journal of the American Hospital Association, mentions that the "isinglass windows" of an oxygen tent were actually cellulose acetate, and before that Edwin E Slosson summed up the terminological lag in the area of these substances:
Fish glue films were called isinglass as though they were a kind of glass. When mica took its place it too was called isinglass, even in stove windows, and what is now called mica is mostly celluloid, and what is called celluloid, may be something else. Language lingers and lags behind the advance of science.
- The human side of chemistry, Edwin E Slosson, Am. Journ. Pharm, Nov 1922
This process continues. "isinglass curtains" and "isinglass windows" are names still used for the polymer canopies of powerboats and yachts.
Ultimately the best approach to finding what these isinglass curtains/windows seems to be to avoid the anecdotal, and look in contemporary hardware publications, which have technical descriptions of vehicles. The majority of hits (see "side curtains" celluloid) solidly confirm the existence of vehicle side curtains with celluloid windows; a significant minority (see "side curtains" mica) refer to mica windows. Given the stiffness of mica, however, I'm inclined to think that an isinglass (probably sic) curtain that "rolls" must refer to one of celluloid or of canvas with a celluloid light. But I guess
With curtains of either celluloid por canvas with celluloid (or possibly mica) curtains you can roll right down,
In case there's a change in the weather
lacks a certain something, and is reminiscent of Charles Babbage's comment on Tennyson's The Vision of Sin.
Another appearance of isinglass curtains in literature: the poem Parent by Josephine Miles.
Letting down the isinglass curtains
Between the wet rain and the back seat
Where new plaster flattened me, my father said,
Wait here. I will get you the sherbet
Of your eight-year-old dreams.
There were no drive-ins then.
The rain roared. Don't go away,
I said to him? He said to me.
Her poetry is worth checking out: minimal yet poignant, often stream-of-consciousness. Parent comes from the 1979 autobiographical collection Coming to Terms, which initially tells of her childhood when she had to endure a full-body plaster cast (hence the "new plaster flattened me") due to disabling arthritis (see Josephine Miles: mentor to a revolution). The isinglass curtains in this case, as told in another poem in the collection, Doll, in the family's Mitchell.
1. Although I suspect many of the references are confusing it with the far more widespread use of water glass, a syrupy solution of sodium silicate, for this purpose.