A 100 speed emulsion in those 1920's books are not ISO speed. They are most likely H&D speed. 100 H&D speed is comparable to or slower than today's multigrade paper emulsion used as a negative emulsion. (There is no exact conversion factor to speak of, because they are measured with different conditions.)
Originally Posted by Donald QuallsI'm with Ole on shutters -- simple mechanical devices like these, that don't run continuously, can reasonably be expected to last centuries if cleaned and adjusted periodically. I have three from the late 1920s that are within 1/2 stop at all speeds from 1 to 1/200, and I expect them to be still usable after I'm long gone, unless they're destroyed by environmental insults. FWIW, I also own a mechanical clock from the same era, one that wasn't expensive when new, which does run continuously -- and which I've recently managed to adjust to the point of gaining or losing less than a minute a month, the same accuracy that used to be advertised for quartz watches. Same for glass -- American Civil War era lenses can still make fine images, 140 years later, and there's no reason to believe non-exotic glasses will deteriorate in normal storage and use for millennia; they'll be destroyed by rough cleaning or physically broken first.
I have an electronic copy of a 1920s book on making emulsions, which includes the (1920s style) chemical names of the sensitizing dyes and very detailed process information. I can't see this as being beyond the ability of the kind of people who used to perform the experiments that got written up in Scientific American's, "Amateur Scientist" features; it's certainly simpler in many ways to make a gelatin-halide emulsion and coat it on glass, acetate, or polyester than it is to, say, extract and amplify DNA from plant cells (as I recall being done in one such article) or build a basement fusion reactor (as has also been done -- no, the rate of fusion is well below break-even, but they're working on it). The modern ISO of the emulsions covered in the book I have would range up to 100, possibly even 400 with the right ripening process (though it would be as grainy as old Royal X or 4275 Recording Film -- might not matter, if it were coated on 8x10 plates).
What this won't be is cheap. Now, someone like me (with very limited disposable income) can pursue photography fairly seriously, as long as he's patient and mechanically astute, without spending a bunch of money (I probably spent less than $1000 in 2004 including all equipment purchases, film, and processing/chemistry). Once large volume manufacture of film ends, unless we have something akin to Star Trek replicators we'll be forced to spend lots of time and money just to create the medium to record the image. Our hobby will become somewhat akin to fireworks making -- dealing with chemicals that, though reasonably common on an industrial basis, are expensive and hard to get in small quantities, might be hazardous to handle, and will involve enough work for a single use that most won't bother. The difference is, you can still buy fireworks, most places (even if they're illegal) if you're not inclined to make your own. By the time most photographers are making their own materials, you'll only be able to buy them from someone who makes them by hand or in very small volume.
Look at what Bostick & Sullivan get for carbon printing tissue that's not even presensitized -- and think what that would cost if it incorporated five times as many manufacturing steps, in the dark, and included silver as an ingredient instead of soot. That's what film will cost once it's made in runs of 100 sheets of 8x10.