I’ve been working on this for a while, and finally assembled all the primary sources and material I needed to write it. I explain in some great detail about flongs and stereotypes, a method of effectively duplicating typeset material and engraved images and halftones photographs that was required for rotary presses to make any sense.
A flong is a paper mold made from something that would otherwise be printed on a letterpress (in relief printing). The stereotype is the solid metal cast plate (flat or curved) cast from it. For newspapers and high-volume printing, the curved plate paired with the rotary press, allowing super-fast continuous printing fed from large rolls of continuously made paper.
Making paper in rolls, rotary presses, the Linotype, and the largely forgotten flong/stereotype process together allowed periodicals to expand in size (number of pages) and volume (number of copies per volume). Newspapers benefitted most, but the plate-making technique was used for standard commercial work (job printing), book publishing, magazines, and other specialized purposes.
I wrote this article in part because there was no authoritative modern source that tracked the scope of flongs and stereotypes. Now there is!
Chunks of this research will feed into the Six Centuries of Type & Printing book. I have a far better and deeper understanding from those primary, contemporary sources of how printing took an incredible leap across the 1800s right at the start of the century and all the way through the end. While printing and typesetting changed multiple times in the 20th century, the 19th century had just as many revolutions, if not more.