The Romans were very good at keeping written records, which is an essential part of why we know so much about them.
Millions of Roman writings have survived, from private letters written on soft wax and stone inscriptions on great monuments to elegant poems and histories carefully written on papyrus scrolls.
While there was no paper in the Roman world, they had other materials on which they wrote.
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What Did the Romans Write On?
In place of paper, the Romans used:
- Wooden tablets covered with wax
- Parchment made of animal skins
- The thin rind of the Egyptian papyrus
The Egyptian Papyrus
The papyrus plant or tree, found in swamps of tropical countries, especially the Nile valley, had its stems and stalks cut, wetted, pressed together, and then sun-dried.  These individual sheets were between 3-12 inches wide and 8-14 inches high.
The ancients would write on these sheets and paste them together at the sides to make a book. Authors could continue this pasting process when writing books, with the occupied sheets stretching at least 50 yards when laid out. 
However, Roman authors usually divided any long work into several rolls, as a large book would mean sheets pasted to make one large roll (at least 90 yards).
The papyrus rolls would be placed in a parchment case stained yellow or purple, which the poet Martial referred to as a purple toga.
Interesting Fact: Papyrus is stable in dry climates like Egypt. In European conditions, it would last only a few decades. Imported papyrus, once common in ancient Greece and Italy, has degraded beyond repair. 
Wooden Tablets Covered With Wax
In Ancient Rome, they used tabulae, meaning tablets of any kind (wood, metal, or stone), but mostly wood. Mostly made of fir or beech, occasionally citron-wood or even ivory, they were oblong-shaped and covered with wax.
These wax tablets had an outer wooden side and wax on the inner sides. Using wires for hinges, two pieces of wood would be fastened to open and shut like a book. A raised margin around the wax on each tablet would prevent them from rubbing against each other.
Certain tablets were tiny and could be held in hand. These were mainly used for writing letters, love letters, wills, and other legal documents and keeping accounts of sums received and disbursed.
The Ancient Romans developed the codex form (plural – codices) from these wax tablets. The gradual replacement of the papyrus scroll with the codex was one of the important advances in bookmaking.
Codex, the historic ancestor of the modern book, used sheets of papyrus, vellum, or other materials. 
Animal Skin Parchments
Among the Romans, papyrus and parchment sheets seem to have been the only materials used to write books.
As a writing surface, papyrus gained a rival in the first centuries BCE and CE – parchment made of animal skins. Parchment sheets were pasted together and folded, forming quires, used to fashion book-form codices like those made from the papyrus plant.
Parchment was better than papyrus since it was thicker, more durable, and reusable, and both sides could be used for writing, although its back wasn’t used and was stained a saffron color.
With the codex form adopted by the early Christian writers, codices would be formed by cutting sheets from papyrus rolls in the Greco-Roman world. An improvement over the papyrus scrolls, codices were better, especially for creating large-volume texts.
What Other Writing Materials Did They Use?
The Romans wrote with metallic ink, mainly lead-laced ink. Important manuscripts or holy works were written with red ink, symbolic of the noble Romans. This ink was made from red lead or red ochre.
However, the more common black ink, or atramentum, used ingredients like soot or lampblack suspension in a glue or gum arabic solution.
Metal or reed pens were widely used, and there were quill pens around medieval times.
The Romans also had an invisible or sympathetic ink, possibly used for love letters, magic, and espionage. It could be brought out only by heat or the application of some chemical preparation.
There are records of invisible ink made with myrrh. Also, text written using milk was made visible by scattering ash over it.
Inkwells of pottery or metal were used to contain the ink.
How Did Paper Become Commonplace?
While papyrus scrolls used in Egypt around the 4th century BC form evidence of the first plant-based paper-like writing sheet, it wasn’t until 25-220 AD, during the Eastern Han period in China, that true paper-making came about.
Initially, the Chinese used cloth sheets for writing and drawing until a Chinese court official made a paper prototype using mulberry bark.
The Chinese paper-making secret spread to the Middle East (replaced papyrus) in the 8th century and finally to Europe (replaced wood panels and animal-skin parchment) in the 11th century.
Around the 13th century, Spain had paper mills using waterwheels for papermaking.
The papermaking process improved in the 19th century, and wood from trees was used to make paper in Europe. This made paper commonplace.
The oldest document in Europe, dating back to before 1080 AD, is the Mozarab Missal of Silos. Containing 157 folios, only the first 37 are on paper, with the rest on parchment.
Romans used Egyptian papyrus, animal skin parchments, and wax tablets in ancient times as they didn’t have paper till long after the fall of the Roman empire, like most of the Western world. It might seem unbelievable, but it’s only around ten centuries that paper has been around, while it has been commonplace for an even shorter period.