Public art and religious ceremonies were ways to communicate imperial ideology regardless of ability to read. The Romans had an extensive priestly archive, and inscriptions appear throughout the Empire in connection with statues and small votives dedicated by ordinary people to divinities, as well as on binding tablets and other "magic spells", with hundreds of examples collected in the Greek Magical Papyri. The military produced a vast amount of written reports and service records, and literacy in the army was "strikingly high". Urban graffiti, which include literary quotations, and low-quality inscriptions with misspellings and solecisms indicate casual literacy among non-elites. In addition, numeracy was necessary for any form of commerce. Slaves were numerate and literate in significant numbers, and some were highly educated. Books were expensive, since each copy had to be written out individually on a roll of papyrus (volumen) by scribes who had apprenticed to the trade. The codex—a book with pages bound to a spine—was still a novelty in the time of the poet Martial (1st century AD), but by the end of the 3rd century was replacing the volumen and was the regular form for books with Christian content. Commercial production of books had been established by the late Republic, and by the 1st century AD certain neighbourhoods of Rome were known for their bookshops (tabernae librariae), which were found also in Western provincial cities such as Lugdunum (present-day Lyon, France). The quality of editing varied wildly, and some ancient authors complain about error-ridden copies, as well as plagiarism or forgery, since there was no copyright law. A skilled slave copyist (servus litteratus) could be valued as highly as 100,000 sesterces. Collectors amassed personal libraries, such as that of the Villa of the Papyri in Herculaneum, and a fine library was part of the cultivated leisure (otium) associated with the villa lifestyle. Significant collections might attract "in-house" scholars; Lucian mocked mercenary Greek intellectuals who attached themselves to philistine Roman patrons. An individual benefactor might endow a community with a library: Pliny the Younger gave the city of Comum a library valued at 1 million sesterces, along with another 100,000 to maintain it. Imperial libraries housed in state buildings were open to users as a privilege on a limited basis, and represented a literary canon from which disreputable writers could be excluded. Books considered subversive might be publicly burned, and Domitian crucified copyists for reproducing works deemed