Sometime around the middle of the fourth millennium BCE, the Sumerians in what is now Iraq began to develop a system of marks to record administrative and business transactions. Located near rivers offering copious supplies of alluvial deposits, clay shaped into various sizes of tablets provided a cheap and widely-available writing surface.
Most of these clay tablets would, once the writing had been inscribed on them, be simply dried in the sun. However, some that were designed to be permanent records would have been artificially baked, thus increasing their durability. Archives of clay tablets might later be burnt either accidentally or intentionally. However, unlike most other forms of book, such firing generally increased a clay tablet’s life expectancy.