Printing and bookmaking gruntle, part 1
Sep. 20th, 2021 07:17 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
glaurungEurocentric history tends to be very self-congratulatory. "(European guy) invented (arguably very important technology)," and "(historical change of import) happened because of (thing european or Greco-Roman people did)." It gets very tiresome, especially when the technology in question was actually invented hundreds or thousands of years earlier, far outside of the Euro-Greco-Roman sphere.
Today's example: a thread by Incunabula on Twitter. In James Burke-ian Connections style, he says that "Cheese 🧀 is one of the 5 things the Western book as we know it depends on. The other four are snails 🐌, Jesus ✝️, underwear 🩲 and spectacles 👓." Sigh. Burke's "Connections" blew my mind when I was eight. Because I was too young to notice how a shallow, facile and simplified, all white, mostly male narrative was being constructed from a far richer and larger history.
Today I'm tackling two of Incunabula's five things: cheese (parchment) and Christianity (the adoption of the codex). My primary source for most of what follows is an online version of "The Birth of the Codex" by Colin Roberts and T C Skeat, supplemented by Wikipedia and lots of blog posts/articles found via google. The online version of Birth of the Codex includes (in green and red text) incomplete edits and updates of the original book
1. "Cheese" (aka, parchment/vellum):
First, in the ancient Mediterranean world, the default writing material for making book scrolls was papyrus. Made from a reed that grew only on the banks of the Nile, it was one of Egypt's main exports. An alternative (in places too far from the Mediterranean coast to make imported papyrus affordable) was parchment, a material with vague origins in the middle east, perhaps invented around 1500 BCE.
Nobody seems able to agree on the difference between parchment and vellum, and modern archivists, who have to deal with books made from the stuff, often say "animal membrane" to avoid the whole issue. Just a taste of the contradictions and ambiguities in the two terms: on the one hand, vellum is supposedly thinner and of higher quality than parchment. On the other hand, there seem to be two different manufacturing technologies: parchment involves splitting the skin onto inner and outer layers. The outer layer goes to the tannery to become thin, soft leather, while the inner layer gets made into parchment by a process of soaking, stretching, and scraping. Vellum uses the whole, unsplit skin, which, after being treated with caustic solutions to remove the hair, gets soaked, stretched and scraped. So the allegedly thinner, higher quality material is the one that sometimes exhibits blemishes like hair follicles on one side and is made from the whole skin, while the allegedly thicker, lower quality material is made from only the inner half of the skin, thus starting out thinner and avoiding follicles and other surface blemishes. And then there are those, like Incunabula on Twitter, who regard vellum as being made from calves instead of from grown cows, while other sources insist that the difference, whatever it was, had nothing to do with the ages of the animals used. I can see why the archivists have given up. Vellum seems to be a subtype of parchment, so I'll just say parchment.
Egypt's massive papyrus export business made parchment uneconomical in the Mediterranean. The fact that animal skins were always in demand for making leather probably didn't help anyone looking to undercut Egypt (there's no historical records that let us compare costs, so we have to go with inferences like this). But parchment got a boost around 170 BCE, when Antiochus IV Epiphanes, ruler of the huge Seleucid empire (which stretched from Palestine through Babylonia to Afghanistan), invaded Egypt during the sixth Syrian War. This disrupted papyrus exports for a few years, and in Pergamon (a Greek city in what is now Western Turkey), plentiful writing materials were desperately needed because the massive 200,000 volume library of Pergamon was being being stocked at the time. Pergamon set up parchment manufacturing, presumably to deal with the wartime shortages of papyrus, and parchment became so synonymous with Pergamon, it got its name from the city.
The transition from papyrus to parchment took centuries, and again, there are no records that let us say how and why, but somewhere in the first few centuries CE, parchment became the default in the Roman world and papyrus became the less common alternative. This did not just happen in the West where the fall of Rome disrupted trade, but in the Eastern empire as well. On the one hand, Egypt's papyrus export business was a massive operation, and getting a viable parchment factory up and running had to fight against Big Papyrus. It took a very long time for Big Papyrus to die out: Egypt continued making and exporting papyrus into the 1100's CE, long after the world had mostly switched to parchment. On the other hand, parchment was stronger than papyrus and had a smoother surface that was more pleasant to write on. Also, unlike papyrus, it was possible to scrub the ink off of parchment without damaging it, giving you a clean surface for re-use. Finally, there are some offhand references that seem to say that papyrus was rougher on the back side, whether intrinsically or just because the factory in Egypt only sanded one side of each sheet to be nice and smooth. Parchment could be written on both sides more or less equally. Eventually, the smoother, stronger, more re-usable, double sided writing material won out despite the economic barriers thrown up by Big Papyrus.
But, contrary to Incunabula, the earliest codexes that have survived, from Egypt, are nearly all made of papyrus rather than parchment - and every early Christian codex is made of papyrus.
2 "Christianity" (AKA, the rise of the codex). Parchment won out, but as best anyone can tell, that has nothing whatsoever to do with the transition from rolled up scrolls to flat sheets bound in codexes for the physical format of books. So now we turn to the codex.
Pre-imperial Romans used wax covered wooden tablets for jotting down notes and lists, composing rough drafts, and generally for any kind of temporary writing. One wrote on the wax with the pointed end of a stylus, and, when the writing was no longer needed, rubbed the wax smooth again with the blunt end. Two (or more) tablets sewn together, with a spacer or rim around the waxed writing area to protect the wax from getting smushed, made it possible to fold your notes into a portable bundle, with the writing protected on the inside. Cicero used the words tabulae and codex interchangeably for these objects.
At some point in between 50 BCE and 60 CE, Romans began transitioning away from wooden waxed tablets, switching to washable parchment for the same temporary writing. But they kept the name, and, it seems, the familiar format, sewing together multiple leaves of parchment instead of multiple wooden tablets into a new object that they continued to call a codex.
There's a vague reference in Suetonius's biography of Julius Caesar, referring to Caesar's reports to the Senate, I think WRT his campaigns in Gaul: "Some letters of his to the senate are also extant, and he seems to have been the first to convert such documents to pages and the format of a memorandum book, whereas previously consuls and generals did not send their reports except (on sheets) written against the papyrus fibers." Some have interpreted the line as a reference to Caesar using a codex, but the Latin is far from clear and it's possible the "memorandum book" bit is talking about how Caesar organized his writing on the page (in vertical columns?), rather than the nature of the pages.
Another vague reference a few decades later occurs in the poems of Horace, which show that he was accustomed to using parchment for drafts and note taking, but it's not clear whether or not the parchments (plural) he speaks of were sewn together in book form or not.
Around 55-60 CE, Persius describes the materials needed by a student: "Now a book, and bicolored parchment devoid of hair and there comes to hand papyri and a knotty reed." The hair side of parchment is noticeably yellower than the flesh side, but in scrolls, the sheets were always sewn together with the same side up, and the back was always blank, so you wouldn't notice the difference much. In a codex, you'd be seeing skin and flesh sides alternate as as you turned the pages, making the shifts in colour noticeable.
By 90 CE, Quintilian was providing advice on what to use for writing rough drafts:
"It is best to write on wax owing to the facility which it offers for erasure, though weak sight may make it desirable to employ parchment by preference. The latter, however, although of assistance to the eye, delays the hand and interrupts the stream of thought owing to the frequency with which the pen has to be supplied with ink (32) But whichever is employed, blank pages (tabellae) must be left in which one is free to make additions at will." (so, wax covered tablets were not extinct, because scrubbing the ink off of parchment was harder and thus wax was better for frequent reuse).
As a thing to jot notes in, codexes were a huge success and they seem to have quickly come to coexist with wax tablets since the two formats were suited to slightly different uses - wax for small amounts of very short term writing, parchment for lots of longer term writing.
As a format for literary works, codexes didn't do as well. "Real" books were scrolls, and remained scrolls for quite some time. We know codex books existed in the first century, because Martial talks about them, but after Martial, the next mention of codex format books in Latin sources appears over a century later.
In the introductory verses to a revised edition of his epigrams, Martial writes: "You who long for my little books to be with you everywhere and want to have companions for a long journey, buy these ones which parchment confines within small pages: give your scroll-cases to the great authors - one hand can hold me" (and then the poem goes on to state where one my buy these little hand-held codexes of Martial's poetry: from "Secundus, the freedman of learned Lucensis.")
Also in Martial, Book 14 lists presents brought for distribution during the Saturnalia, which include (along with many other things) mentions of both notebooks and books. Notebooks are described as made of citrus wood, of ivory, as being three-leaved and five-leaved, and as "Pugillares membranei" - Notebooks of parchment. Fancy notebooks were still assumed to be in wax tablet form, unless someone mentioned that they were "of parchment."
Likewise, the books are either referred to by author/name with no further qualification, like "Menander's Thais", "Homer's Battle of Frogs and Mice"; or they're qualified as being "of parchment": "Vergilius in membranis," "Cicero in membranis." And then you have bits like this:
"Homer in handheld parchments
The Iliad and Ulysses, enemy of Priam's kingdom,
are there together, preserved in many folds of skin"
"Titus Livy on parchment.
Compressed in tiny skins vast Livy,
for whom complete my library has not room."
This was a new compact format of book, designed to take with you, but, like pocket paperbacks in the 20th century, seen as of lesser stature than full sized scrolls if you weren't planning on carrying the book around with you.
Martial's later works don't mention Secundus or the codex format, so it might be that Secundus's venture into codex formatted publication of books did not succeed.
And then, for the next century plus, historical sources have nothing whatsoever to say about books in codex format. But, while not being talked about in the writing that has survived, codex style books continued to be made: there is a discussion in a legal treatise by Ulpian dating from around 210 CE about what does and does not count as "books" for bequests in wills:
[[Under the heading "books" all volumes are included, whether they are made of papyrus, of parchment, or of any other material whatsoever; but even if they are written on wood-slabs (as is sometimes done), or upon any kind of prepared skins, they come under the same appellation. If, however, they are codices of parchment, or papyrus, or even ivory, or any other material, or are composed of wax tablets, let us determine whether they ought to be included? Gaius Cassius writes that where books are bequeathed, the parchments are also included. Hence, it follows that everything relating to them will be included if the intention of the testator was not otherwise]]
And then goes on to say that while blank writing material does not count as "books", anything written on, regardless of whether it had been fastened together yet or not (with a list of fastening/binding methods that clearly includes codexes as well as scrolls), does count as books.
The next bit comes from archaeology in Egypt, the one place in the ancient world where papyrus and parchment had a chance to survive the intervening millennia. Which does present a problem, because Egypt was also papyrus central. For example, examples of the parchment notebook, which gave its name to the codex-style book (in both Latin and Greek), have not been found in Egypt - and a without the benefits of erasability, papyrus style notebooks seem to not have been a thing. So, the one place where we can look at the spread of the codex as a way of building a book, is the one place where you are least likely to find the precursor of the codex book. And digging in Egypt does not help at all with seeing the displacement of papyrus by parchment: the vast majority of finds in Egypt are always papyrus.
That said, Birth of the Codex provides a table: The authors compiled a database of finds of preserved ancient papyrus and parchment found in Egypt. They excluded all Christian material, excluded all non-book items (lists, lecture notes, etc), and then, of the fraction that were definitely books or fragments of books, sorted them by approximate date and categorized them as scroll or codex (I assume this means they also excluded book fragments where it wasn't possible to tell if the book had originally been bound as a scroll or codex).
1st century: 0% codexes (1/252)
2nd century: 2%
3rd century: 19%
4th century: 74%
5th century: 89%
The draft revision to the book updates the table with many newer finds, and those new finds reduced the share of codexes to 11% in the 3rd, 44% in the 4th, and 54% in 5th centuries. The trend is clear either way: Codexes were slowly displacing scrolls.
The surviving books and fragments of Christian scripture from the same time frame are almost exclusively written on papyrus and bound in codex form. The few that are scrolls tend to have started out as some other book, and then had a Christian text written on the back side of the roll. Excluding scripture, but including books that are clearly Christian (including apocryphal scriptures which were distinguishable from now canonical scripture at the time, so this division is kind of arbitrary), 83 out of 118 were codexes.
The early history of Christianity is very obscure, in large part because very little written from a point of view that didn't match up with later orthodoxy survived. Only a fraction of a much larger body of gospels and epistles were allowed into the canon. I don't doubt that an even larger amount of non-scriptural material from the earliest church didn't pass doctrinal muster, failed to get copied, and was lost long ago. So, no one has any idea why Christians latched on so strongly early on to the codex book format. The explanations that I have seen offered are either woefully inaccurate (that they were better able to expound from scripture while holding a codex open in one hand, as suggested by Incunabula, retrojects our post-protestant obsession with exact textual quotations onto a culture that did not have authoritative versions of texts to quote from), or just plain unsatisfactory (the theories proposed in Birth of the Codex).
My crackpot suggestion is to remember that the early Christians were a cult: a very small group that felt, very deeply, that they were definitely not like everyone around them, that they were a special in group surrounded by people who disliked them and who were Wrong About Everything. And that means that a single early church leader could have had an outsized impact on the entire community's attitude towards what a book should be like. Perhaps one influential person in Antioch was a major fan of the codex format (like Secundus the freedman), and had an outsized founder effect on the early Christian community's attitude towards codexes vs scrolls.
Outside the codex-obsessed Christian bubble, codexes clearly had a few things going for them: scrolls were always written on only one side, codexes on both sides of the page (the few examples of double sided scrolls are almost all repurposed books where someone on a limited budget used the back side to copy a completely different book than was written on the front). So a codex needed half the number of sheets, leading to a modest cost savings (the labour of copying remained the same, of course). A comfortably sized codex could also hold more sheets than would fit on a comfortably sized scroll, so you could have the same text as one object instead of several (This was a significant advantage even for the earliest codexes which rarely had more than about 100 sheets. The Chester Beatty codex of Numbers and Deuteronomy, with 108 sheets, would have required a scroll 28 meters long. Codexes became thicker over time, with monster 700 sheet and larger tomes appearing in the 4th and later centuries, providing an even greater advantage over scrolls). A closed codex was always the same size: you didn't have to worry about loose re-rolling resulting in a book that no longer fit in its place. Codexes were rectangular instead of round, using storage space more efficiently. Codexes were more durable, especially if they had protective covers (the earliest ones did not). And finally, of course, unattended codexes were less prone to being unrolled by cats than a scroll. These advantages had to work against a cultural prejudice of what books *ought* to look like, which made for tough going at first, but codexes, like parchment, eventually caught on and largely displaced scrolls for books (scrolls continued to be used for government records well into the medieval and early modern period, mainly because it was much easier to add and subtract pages from them: various government records are still called "rolls" because of this).
How much did Christianity's extremely early and wholehearted embrace of codexes have to do with the switch from scrolls to codexes? Not a lot. Christians were a tiny minority in a vast empire until well into the 3rd century. By the time Christianity became a major force in the empire (~300 AD), the switch to codexes was already underway. Clearly the rise of Christianity to the empire's official religion in the 4th century greatly accelerated the transition, but the evidence doesn't point to Christians being the starter of the trend.
Today's example: a thread by Incunabula on Twitter. In James Burke-ian Connections style, he says that "Cheese 🧀 is one of the 5 things the Western book as we know it depends on. The other four are snails 🐌, Jesus ✝️, underwear 🩲 and spectacles 👓." Sigh. Burke's "Connections" blew my mind when I was eight. Because I was too young to notice how a shallow, facile and simplified, all white, mostly male narrative was being constructed from a far richer and larger history.
Today I'm tackling two of Incunabula's five things: cheese (parchment) and Christianity (the adoption of the codex). My primary source for most of what follows is an online version of "The Birth of the Codex" by Colin Roberts and T C Skeat, supplemented by Wikipedia and lots of blog posts/articles found via google. The online version of Birth of the Codex includes (in green and red text) incomplete edits and updates of the original book
1. "Cheese" (aka, parchment/vellum):
First, in the ancient Mediterranean world, the default writing material for making book scrolls was papyrus. Made from a reed that grew only on the banks of the Nile, it was one of Egypt's main exports. An alternative (in places too far from the Mediterranean coast to make imported papyrus affordable) was parchment, a material with vague origins in the middle east, perhaps invented around 1500 BCE.
Nobody seems able to agree on the difference between parchment and vellum, and modern archivists, who have to deal with books made from the stuff, often say "animal membrane" to avoid the whole issue. Just a taste of the contradictions and ambiguities in the two terms: on the one hand, vellum is supposedly thinner and of higher quality than parchment. On the other hand, there seem to be two different manufacturing technologies: parchment involves splitting the skin onto inner and outer layers. The outer layer goes to the tannery to become thin, soft leather, while the inner layer gets made into parchment by a process of soaking, stretching, and scraping. Vellum uses the whole, unsplit skin, which, after being treated with caustic solutions to remove the hair, gets soaked, stretched and scraped. So the allegedly thinner, higher quality material is the one that sometimes exhibits blemishes like hair follicles on one side and is made from the whole skin, while the allegedly thicker, lower quality material is made from only the inner half of the skin, thus starting out thinner and avoiding follicles and other surface blemishes. And then there are those, like Incunabula on Twitter, who regard vellum as being made from calves instead of from grown cows, while other sources insist that the difference, whatever it was, had nothing to do with the ages of the animals used. I can see why the archivists have given up. Vellum seems to be a subtype of parchment, so I'll just say parchment.
Egypt's massive papyrus export business made parchment uneconomical in the Mediterranean. The fact that animal skins were always in demand for making leather probably didn't help anyone looking to undercut Egypt (there's no historical records that let us compare costs, so we have to go with inferences like this). But parchment got a boost around 170 BCE, when Antiochus IV Epiphanes, ruler of the huge Seleucid empire (which stretched from Palestine through Babylonia to Afghanistan), invaded Egypt during the sixth Syrian War. This disrupted papyrus exports for a few years, and in Pergamon (a Greek city in what is now Western Turkey), plentiful writing materials were desperately needed because the massive 200,000 volume library of Pergamon was being being stocked at the time. Pergamon set up parchment manufacturing, presumably to deal with the wartime shortages of papyrus, and parchment became so synonymous with Pergamon, it got its name from the city.
The transition from papyrus to parchment took centuries, and again, there are no records that let us say how and why, but somewhere in the first few centuries CE, parchment became the default in the Roman world and papyrus became the less common alternative. This did not just happen in the West where the fall of Rome disrupted trade, but in the Eastern empire as well. On the one hand, Egypt's papyrus export business was a massive operation, and getting a viable parchment factory up and running had to fight against Big Papyrus. It took a very long time for Big Papyrus to die out: Egypt continued making and exporting papyrus into the 1100's CE, long after the world had mostly switched to parchment. On the other hand, parchment was stronger than papyrus and had a smoother surface that was more pleasant to write on. Also, unlike papyrus, it was possible to scrub the ink off of parchment without damaging it, giving you a clean surface for re-use. Finally, there are some offhand references that seem to say that papyrus was rougher on the back side, whether intrinsically or just because the factory in Egypt only sanded one side of each sheet to be nice and smooth. Parchment could be written on both sides more or less equally. Eventually, the smoother, stronger, more re-usable, double sided writing material won out despite the economic barriers thrown up by Big Papyrus.
But, contrary to Incunabula, the earliest codexes that have survived, from Egypt, are nearly all made of papyrus rather than parchment - and every early Christian codex is made of papyrus.
2 "Christianity" (AKA, the rise of the codex). Parchment won out, but as best anyone can tell, that has nothing whatsoever to do with the transition from rolled up scrolls to flat sheets bound in codexes for the physical format of books. So now we turn to the codex.
Pre-imperial Romans used wax covered wooden tablets for jotting down notes and lists, composing rough drafts, and generally for any kind of temporary writing. One wrote on the wax with the pointed end of a stylus, and, when the writing was no longer needed, rubbed the wax smooth again with the blunt end. Two (or more) tablets sewn together, with a spacer or rim around the waxed writing area to protect the wax from getting smushed, made it possible to fold your notes into a portable bundle, with the writing protected on the inside. Cicero used the words tabulae and codex interchangeably for these objects.
At some point in between 50 BCE and 60 CE, Romans began transitioning away from wooden waxed tablets, switching to washable parchment for the same temporary writing. But they kept the name, and, it seems, the familiar format, sewing together multiple leaves of parchment instead of multiple wooden tablets into a new object that they continued to call a codex.
There's a vague reference in Suetonius's biography of Julius Caesar, referring to Caesar's reports to the Senate, I think WRT his campaigns in Gaul: "Some letters of his to the senate are also extant, and he seems to have been the first to convert such documents to pages and the format of a memorandum book, whereas previously consuls and generals did not send their reports except (on sheets) written against the papyrus fibers." Some have interpreted the line as a reference to Caesar using a codex, but the Latin is far from clear and it's possible the "memorandum book" bit is talking about how Caesar organized his writing on the page (in vertical columns?), rather than the nature of the pages.
Another vague reference a few decades later occurs in the poems of Horace, which show that he was accustomed to using parchment for drafts and note taking, but it's not clear whether or not the parchments (plural) he speaks of were sewn together in book form or not.
Around 55-60 CE, Persius describes the materials needed by a student: "Now a book, and bicolored parchment devoid of hair and there comes to hand papyri and a knotty reed." The hair side of parchment is noticeably yellower than the flesh side, but in scrolls, the sheets were always sewn together with the same side up, and the back was always blank, so you wouldn't notice the difference much. In a codex, you'd be seeing skin and flesh sides alternate as as you turned the pages, making the shifts in colour noticeable.
By 90 CE, Quintilian was providing advice on what to use for writing rough drafts:
"It is best to write on wax owing to the facility which it offers for erasure, though weak sight may make it desirable to employ parchment by preference. The latter, however, although of assistance to the eye, delays the hand and interrupts the stream of thought owing to the frequency with which the pen has to be supplied with ink (32) But whichever is employed, blank pages (tabellae) must be left in which one is free to make additions at will." (so, wax covered tablets were not extinct, because scrubbing the ink off of parchment was harder and thus wax was better for frequent reuse).
As a thing to jot notes in, codexes were a huge success and they seem to have quickly come to coexist with wax tablets since the two formats were suited to slightly different uses - wax for small amounts of very short term writing, parchment for lots of longer term writing.
As a format for literary works, codexes didn't do as well. "Real" books were scrolls, and remained scrolls for quite some time. We know codex books existed in the first century, because Martial talks about them, but after Martial, the next mention of codex format books in Latin sources appears over a century later.
In the introductory verses to a revised edition of his epigrams, Martial writes: "You who long for my little books to be with you everywhere and want to have companions for a long journey, buy these ones which parchment confines within small pages: give your scroll-cases to the great authors - one hand can hold me" (and then the poem goes on to state where one my buy these little hand-held codexes of Martial's poetry: from "Secundus, the freedman of learned Lucensis.")
Also in Martial, Book 14 lists presents brought for distribution during the Saturnalia, which include (along with many other things) mentions of both notebooks and books. Notebooks are described as made of citrus wood, of ivory, as being three-leaved and five-leaved, and as "Pugillares membranei" - Notebooks of parchment. Fancy notebooks were still assumed to be in wax tablet form, unless someone mentioned that they were "of parchment."
Likewise, the books are either referred to by author/name with no further qualification, like "Menander's Thais", "Homer's Battle of Frogs and Mice"; or they're qualified as being "of parchment": "Vergilius in membranis," "Cicero in membranis." And then you have bits like this:
"Homer in handheld parchments
The Iliad and Ulysses, enemy of Priam's kingdom,
are there together, preserved in many folds of skin"
"Titus Livy on parchment.
Compressed in tiny skins vast Livy,
for whom complete my library has not room."
This was a new compact format of book, designed to take with you, but, like pocket paperbacks in the 20th century, seen as of lesser stature than full sized scrolls if you weren't planning on carrying the book around with you.
Martial's later works don't mention Secundus or the codex format, so it might be that Secundus's venture into codex formatted publication of books did not succeed.
And then, for the next century plus, historical sources have nothing whatsoever to say about books in codex format. But, while not being talked about in the writing that has survived, codex style books continued to be made: there is a discussion in a legal treatise by Ulpian dating from around 210 CE about what does and does not count as "books" for bequests in wills:
[[Under the heading "books" all volumes are included, whether they are made of papyrus, of parchment, or of any other material whatsoever; but even if they are written on wood-slabs (as is sometimes done), or upon any kind of prepared skins, they come under the same appellation. If, however, they are codices of parchment, or papyrus, or even ivory, or any other material, or are composed of wax tablets, let us determine whether they ought to be included? Gaius Cassius writes that where books are bequeathed, the parchments are also included. Hence, it follows that everything relating to them will be included if the intention of the testator was not otherwise]]
And then goes on to say that while blank writing material does not count as "books", anything written on, regardless of whether it had been fastened together yet or not (with a list of fastening/binding methods that clearly includes codexes as well as scrolls), does count as books.
The next bit comes from archaeology in Egypt, the one place in the ancient world where papyrus and parchment had a chance to survive the intervening millennia. Which does present a problem, because Egypt was also papyrus central. For example, examples of the parchment notebook, which gave its name to the codex-style book (in both Latin and Greek), have not been found in Egypt - and a without the benefits of erasability, papyrus style notebooks seem to not have been a thing. So, the one place where we can look at the spread of the codex as a way of building a book, is the one place where you are least likely to find the precursor of the codex book. And digging in Egypt does not help at all with seeing the displacement of papyrus by parchment: the vast majority of finds in Egypt are always papyrus.
That said, Birth of the Codex provides a table: The authors compiled a database of finds of preserved ancient papyrus and parchment found in Egypt. They excluded all Christian material, excluded all non-book items (lists, lecture notes, etc), and then, of the fraction that were definitely books or fragments of books, sorted them by approximate date and categorized them as scroll or codex (I assume this means they also excluded book fragments where it wasn't possible to tell if the book had originally been bound as a scroll or codex).
1st century: 0% codexes (1/252)
2nd century: 2%
3rd century: 19%
4th century: 74%
5th century: 89%
The draft revision to the book updates the table with many newer finds, and those new finds reduced the share of codexes to 11% in the 3rd, 44% in the 4th, and 54% in 5th centuries. The trend is clear either way: Codexes were slowly displacing scrolls.
The surviving books and fragments of Christian scripture from the same time frame are almost exclusively written on papyrus and bound in codex form. The few that are scrolls tend to have started out as some other book, and then had a Christian text written on the back side of the roll. Excluding scripture, but including books that are clearly Christian (including apocryphal scriptures which were distinguishable from now canonical scripture at the time, so this division is kind of arbitrary), 83 out of 118 were codexes.
The early history of Christianity is very obscure, in large part because very little written from a point of view that didn't match up with later orthodoxy survived. Only a fraction of a much larger body of gospels and epistles were allowed into the canon. I don't doubt that an even larger amount of non-scriptural material from the earliest church didn't pass doctrinal muster, failed to get copied, and was lost long ago. So, no one has any idea why Christians latched on so strongly early on to the codex book format. The explanations that I have seen offered are either woefully inaccurate (that they were better able to expound from scripture while holding a codex open in one hand, as suggested by Incunabula, retrojects our post-protestant obsession with exact textual quotations onto a culture that did not have authoritative versions of texts to quote from), or just plain unsatisfactory (the theories proposed in Birth of the Codex).
My crackpot suggestion is to remember that the early Christians were a cult: a very small group that felt, very deeply, that they were definitely not like everyone around them, that they were a special in group surrounded by people who disliked them and who were Wrong About Everything. And that means that a single early church leader could have had an outsized impact on the entire community's attitude towards what a book should be like. Perhaps one influential person in Antioch was a major fan of the codex format (like Secundus the freedman), and had an outsized founder effect on the early Christian community's attitude towards codexes vs scrolls.
Outside the codex-obsessed Christian bubble, codexes clearly had a few things going for them: scrolls were always written on only one side, codexes on both sides of the page (the few examples of double sided scrolls are almost all repurposed books where someone on a limited budget used the back side to copy a completely different book than was written on the front). So a codex needed half the number of sheets, leading to a modest cost savings (the labour of copying remained the same, of course). A comfortably sized codex could also hold more sheets than would fit on a comfortably sized scroll, so you could have the same text as one object instead of several (This was a significant advantage even for the earliest codexes which rarely had more than about 100 sheets. The Chester Beatty codex of Numbers and Deuteronomy, with 108 sheets, would have required a scroll 28 meters long. Codexes became thicker over time, with monster 700 sheet and larger tomes appearing in the 4th and later centuries, providing an even greater advantage over scrolls). A closed codex was always the same size: you didn't have to worry about loose re-rolling resulting in a book that no longer fit in its place. Codexes were rectangular instead of round, using storage space more efficiently. Codexes were more durable, especially if they had protective covers (the earliest ones did not). And finally, of course, unattended codexes were less prone to being unrolled by cats than a scroll. These advantages had to work against a cultural prejudice of what books *ought* to look like, which made for tough going at first, but codexes, like parchment, eventually caught on and largely displaced scrolls for books (scrolls continued to be used for government records well into the medieval and early modern period, mainly because it was much easier to add and subtract pages from them: various government records are still called "rolls" because of this).
How much did Christianity's extremely early and wholehearted embrace of codexes have to do with the switch from scrolls to codexes? Not a lot. Christians were a tiny minority in a vast empire until well into the 3rd century. By the time Christianity became a major force in the empire (~300 AD), the switch to codexes was already underway. Clearly the rise of Christianity to the empire's official religion in the 4th century greatly accelerated the transition, but the evidence doesn't point to Christians being the starter of the trend.