The description is given by Lucian of Samosata (Λουκιανὸς ὁ Σαμοσατεύς, Latin: Lucianus Samosatensis) who lived ca. AD 125–180.
"Listen, therefore, in order to be able to show up such impostors.
The first, my dear Celsus, was a well-known method; heating a needle, he removed the seal by melting through the wax underneath it, and after reading the contents he warmed the wax once more with the needle, both that which was under the thread and that which contained the seal, and so stuck it together without difficulty.
Another method was by using what they call plaster; this is a compound of Bruttian pitch, asphalt, pulverized gypsum, wax, and gum Arabic. Making his plaster out of all these materials and warming it over the fire, he applied it to the seal, which he had previously wetted with saliva, and took a mould of the impression. Then, since the plaster hardened at once, after easily opening and reading the scrolls, he applied the wax and made an impression upon it precisely like the original, just as one would with a gem.
1. Lucian of Samosata, Anacharsis or Athletics. Menippus or The Descent into Hades. On Funerals. A Professor of Public Speaking. Alexander the False Prophet. Essays in Portraiture. Essays in Portraiture Defended. The Goddesse of Surrye, trans. A. M. Harmon, vol. 4, 8 vols., LCL 162 (Cambridge, Mass.: Harvard University Press, 1925), 234–235.Let me tell you a third method, in addition to these. Putting marble-dust into the glue with which they glue books and making a paste of it, he applied that to the seal while it was still soft, and then, as it grows hard at once, more solid than horn or even iron, he removed it and used it for the impression. There are many other devices to this end, but they need not all be mentioned, for fear that we might seem to be wanting in taste." (Lucian Alexander the False Prophet, 21–22.)1