Okay, here goes.
First, type metal. You’ve got your metal pieces, with your letters and whatnot to be printed on their surface, raised and mirror-reversed, which gets inked and pressed to the paper, yes? And these were made by carving your letters, or your illustrations, or whatever, into a material that forms the end of a mold in which said metal is cast. Casting metal with molds was a fairly well-known technology in Europe at that time — particularly for a goldsmith like Gutenberg.
Now, you want this metal to have several properties:
Forming a sharp and accurate impression of the mold, and it’s nooks and crannies.
Relatively easy to cast at comparatively low temperatures.
Not too brittle, so it doesn’t crack or chip or shatter under the stresses of the press
Not too soft, so it doesn’t deform and wear down quickly under the stresses of the press
Not too expensive.
Now, lead was cheap, easy to cast, and not at all brittle. But I don’t know if any of you have ever used lead fishing weights, or done any lead casting, but I’ve done a fair bit of fishing, and my dad used to cast his own fishing weights. And lead is soft. And it doesn’t hold sharp edges. And then there’s thermal expansion — most materials expand when heated… and thus contract when cooled, particularly from molten to solid. Lead is particularly bad in this regard. From the moment you take the cooled metal from the mold, you’ll find a less-than-accurate impression.
Well, people of the day, particularly the pewterers, knew you could toughen up lead, and let it flow more fluidly into the crevices of a mold while molten, if you alloyed it with some tin — which also has an even lower melting point than lead. But this doesn’t solve the contraction problem. What does is adding antimony. Now, the sources I’ve read seem to disagree as to whether or not European alchemists figured out how to isolate antimony from stibnite (antimony sulfide) ore before or after Gutenberg’s time; but they agree that early typefounders mostly just added stibnite to the alloy along with some scrap iron (horseshoe nails), causing a reaction between the stibnite and iron, wherein both the iron and the sulfur are rejected from the resulting alloy while the antimony is left behind.
Now, while type metal alloys vary somewhat in their proportions, they’re generally in the ranges of 54‒86% lead, 3‒18% tin, and 11‒28% antimony.
Second, there’s the ink.
Inks are traditionally a pigment or dye in a liquid solution, occasionally with binders. Most early inks used carbon-based black pigment — “lampblack” soot, charcoal, “bone black,” graphite, etc. “India ink” — invented in China — consisted mostly of these bound together with animal glue into an “inkstick,” which is ground with water on an inkstone to form the ink. Or then there’s iron gall ink, mixing iron (ii) sulfate with tannic acid from oak galls into a solution, with gum arabic as an added binder (the soluble ferrous tannate formed penetrates the paper, then darkens and becomes insoluble by oxidizing via exposure to air into a ferric tannate; however, excess tannic acid generally meant that it could slowly eat through the paper over time). Or then there was cephalopod ink — see the origin of the word “sepia.”
But what all of these have is that the solvent for the solution is water. And all these water-based inks will generally run right off your metal type. To adhere to the type metal, the inks used by Gutenberg and his successors weren’t just thicker than those used with pen or brush, they were oil-based — closer to an oil paint or varnish than a traditional ink. (Hence, why they’re applied to, spread across, and smoothed over the type via ink balls and rollers) And while his inks used carbon soot as their pigment, they also contained high levels of metal, including copper, lead, and titanium (and thus his printed works have text which is actually rather shiny). Later printers mostly converged on a black ink recipe combining walnut oil, turpentine, and soot.
(And while I said earlier it wasn’t about the paper, but Gutenberg also figured out how to moisten the paper to allow the type to better “bite” into it.)
So you see, there was a lot more to the printing revolution than just making the mechanical press.