Most of the original N.T. documents were written on papyrus rolls. Later copies of these documents began to be in the form of papyrus codex, or book form. Whereas most O.T. documents had been originially written on leather, papyrus had been used for thousands of years by the time of Christ.
Vellum or parchment came into prominence because of King Eumenes II (197-158 B.C.), of Pergamum. Eumenes was building a "world class" library, to the envy of he Egyptians. The king of Egypt cut off the supply of papyrus to Pergamum, thus forcing Eumenes to perfect another writing medium.
Later copies of N.T. documents began to appear on the vellum or parchment. For more than a thousand years, copies of N.T. documents were made on vellum. Vellum originally referred only to skins of calves and antelopes, (vellum from veal), while parchment referred to skins from sheep and goats. Later, they began to be used interchageably.