I think you'd need to ask Eratosthenes in what medium his calculations were performed, and he's unlikely to answer. That sort of information is not preserved, although we know that some of his (now fragmentary) works laid out much of the scientific and mathematical reasoning of various projects. Sand was a common medium of working geometric calculation as late as Eratosthenes' time--Archimedes was calculating in dust when he was killed, according to Livy and Valerius Maximus--but it's anyone's guess precisely on what surface Eratosthenes calculated this specific calculation. Eratosthenes was an Alexandrian scholar and, like most Alexandrian scholars he published scholarship. The calculation of the circumference of the Earth was published in the Περὶ ἀναμετρήσεως τῆς γῆς, "On the measurement of the Earth."