The vast majority of 20th century books and pamphlets will be printed via offset press or as letterpress. What’s the difference? Offset transfer the ink to a rubber blanket before its put to a surface. It provides a more even print on less even papers. Letterpress directly applies the plate to paper which also produces a slight indent around the lines. It can make for slightly higher contrast in illustrations due to the faint indent.
Then there’s the tiny fraction of other things that were done using other duplication processes. Many items that were produced in very small batches for special events , local groups, etc were produced using various other methods that may require special handling. If you handle lots of ephemera, you may encounter alternate printing processes. Many fanzines, chapbooks, pamphlets, and APAs may use this methods. Also, if there’s no date on the item, but you CAN ID the process used, it will give you a better idea of the date and how many copies were probably produced.
Spirit Duplicator- 1930s to about 1970s, though some schools and churches used them into the 1990s. They’re still in use in developing countries. These are also called Ditto machines in the US, or Banda machines in the UK. This is actually an inkless process using colored wax. Draw, write, or type on them to make the master and then run it through a drum to produce the image. Spirit duplicators can produce multiple colors, but were generally pastel hues (purple being the most common) They often looks vaguely similar to block prints or very large stamps, with uneven coverage. This used alcohol as a solvent, so when direct off the press had a distinctive odor. The scent has probably faded by now, but if you have a booklet where the type is in pastel colors or appears to have duplicated handwriting, its probably done with a spirit duplicator. A given sheet usually gave out after 500 copies, so there won’t be more copies than that.
Mimeograph- 1870s to around 1970s, though its still around. They are also common in developing countries as they don’t require electricity to use. This is done using a stencil that you typed onto. It was possible to put the stencils in upside down, so you might find a “mirrored” page in a pamphlet. If not kept in repair, or if the stencils were wearing out, these often had ink spots or letters that bled around the edge. Sometimes you had a blurry shadow, as in the example above. Since the master was typed, it was also possible to have errors with doublestrikes. Mimeographs could produce multiple colors, but required a drum change each time and if they weren’t lined up precisely, might be color shifted. As home printers became available, some dot matrix types were used to cut mimeograph stencils. The upper limit on how many times the stencil could be used really depended on how well the machine was maintained and how careful the operator was. Sloppy handling could cut short the life. Careful handling could extend it greatly. However you were unlikely to have more than a thousand copies before it became unreadable and fuzzy.
Hectograph- 1800s- 2000s. Uncommon, but still used. Also known as a jellygraph or gelatin duplicator. Write or draw a master using some type of ink. Lay it over a thin layer of gelatin. Remove, then lay a paper on top of it. It transfers the ink. Depending on how long it was left to sit, you may have anywhere from 20-80 copies. This is very low tech and was used during WWII by various prisoners and resistance groups. Stephen King produced a small newsletter with his brother using this method. Today they’re primarily used to make temporary tattoos or to do art monoprints. These are perhaps most difficult to identify (thus the popularity for clandestine work) but look for handwriting or images that appear to be printed but appear slightly smudgey. These generally won’t be typed.
Thermofax- 1950-1980s- This used a specialized paper rather than ink. The text or image is burned onto the heat sensitive paper. Unfortunately, the paper is heat sensitive, so can curl and discolor afterward. Due to the way is reads the master, what looks black to the human eye might reflect differently to the machine and produce strange, off-color copies. The machines also heat up during use, so often became darker as they ran. They might also produce hot spots or change print color across a single sheet. Look for sheets that are browned, have an odd feel to them, and have apparent burn spots. Thermofax is rarely used with paper, but is still widely used to produce transparencies for silk screening, tattooing, and other art applications.
In the rare cases where you DO find an item on Thermofax paper, be careful to keep it somewhere cool! They were never intended to be archival, so these are rare to find and heat can totally destroy them.
Xerography-1960s to present-This uses electrostatic and toner to deposit the print on paper. The photocopier and the the laser printer both use this same technique. Some all digital presses uses this as well, for smaller book runs. Early photocopied items may be prone to grittiness and spotting. You can also get “burn” around the edges where the lid wasn’t properly shut, or the pages may be crooked since the original can be put in off square.
Electrofax- 1950s -1980s. Like xerography, but utilizing a roll system and requiring special paper. It also generally could only produce blacks as grays. It individually trimmed each page from a roll, so you may have a nonstandard size, or bad trim jobs. The chemicals used gave it a kerosene like odor, though that should have faded by now. Look for things that look like photocopies that are too light, may smell strange, and are on a nonstandard papersize.
There are many more types of duplicating processes, but weren’t used as much for printing documents and pamphlets. Other methods for duplicating art, blueprints, or printing on non-paper surfaces may rarely have been used by individuals that had access to the machinery for them, but aren’t going to turn up very often.
While the various processes have largely been displaced by xerography, all bets are off if you’re dealing with something from a developing country. Many places sold off (or donated) their older machines to places in developing countries. Thermofax and Electrofax require electricity and special no longer available products to make, so are unlikely to turn up in modern small runs from developing countries… but mimeographs, spirit duplicators, and hectographs ARE still in use in areas with no or unreliable electricity. They’re not totally dead technologies, they’re just confined to specific applications.