This change happened gradually during the 3rd and 4th centuries, and the reasons for adopting the codex form of the book are several: the format is more economical, as both sides of the writing material can be used; and it is portable, searchable, and easy to conceal. A book is much easier to read, to find a page that you want, and to flip through. A scroll is more awkward to use. The Christian authors may also have wanted to distinguish their writings from the pagan and Judaic texts written on scrolls. In addition, some metal books were made, that required smaller pages of metal, instead of an impossibly long, unbending scroll of metal. A book can also be easily stored in more compact places, or side by side in a tight library or shelf space.The fall of the Roman Empire in the 5th century A.D. saw the decline of the culture of ancient Rome. Papyrus became difficult to obtain due to lack of contact with Egypt, and parchment, which had been used for centuries, became the main writing material. Parchment is a material made from processed animal skin and used—mainly in the past—for writing on. Parchment is most commonly made of calfskin, sheepskin, or goatskin. It was historically used for writing documents, notes, or the pages of a book. Parchment is limed, scraped and dried under tension. It is not tanned, and is thus different from leather. This makes it more suitable for writing on, but leaves it very reactive to changes in relative humidity and makes it revert to rawhide if overly wet.