Originally Posted by
Fan of Bronze
You have asked such a broad question!
There is no one answer that covers all of comics history. There have in fact been, broadly, three means of reprinting comics since the periodical comic as we know it was introduced in 1935: 1) plates, 2) film, and 3) digital.
The age of reprinting by re-use of printing plates began at least as early as 1939 (Superman #1 had three printings). There are tales of other comics that were reprinted, not always with strict adherence to copyright laws, after plates were found, abandoned, in some storage facility. Of course, printing plates are big, bulky things, and most were not preserved. Thus, for many years it was not practical to reprint most comics, and for many years, most comics were never reprinted.
This changed, at least at DC Comics, in the early 1950s. My best guess is that this is when production of a film negative became part of the pre-press process, and someone got the bright idea of saving all the negatives so that, down the road, they could publish comics filled with reprints without having to pay writers or artists! (It's possible that film negatives were being produced and discarded prior to the early '50s; I just don't know printing history well enough to say for sure.) DC started publishing all-reprint giants ca. 1960, but they couldn't go back any earlier (at first) than the earliest negatives they retained. Film remained part of the pre-press process until about the turn of the current century.
Comics today are produced digitally, of course. We have all heard that digital production promises perfect reproduction, preserved forever. Well, that's the claim. In fact, fulfilling that promise depends on rather expensive techniques of preservation of data carriers (tapes, CDs, etc.) that may not have been designed for long-term storage of data; also on updating of digital files to keep pace with development of new versions of software. There is ample evidence that publishers, blind to these problems inherent in digital production, have converted thousands of pages of classic comics to digital storage and discarded the film negatives (I weep for the loss).
Digital production has also permitted "restoration" to publishability of old, pre–film age comics. If you ever get the chance, compare a modern reprint of any golden age comic to the original printing. To say that something gets lost in the process is being charitably understated.
This is only the briefest gloss of a very complex subject (that I can tend to get passionate about). There are enough subtleties and special cases that this thread could become very, very long if there is interest in discussing it all. Let me know if you want more.