Complex sentence: The resulting marks would adhere firmly to the vellum or parchment, and (unlike india ink or other formulas) could not be erased by rubbing or washing--only by actually scraping a thin layer off the writing surface.
Split into simpler sentences: The resulting marks would adhere firmly to the vellum or parchment, and (unlike india ink or other formulas) could not be erased by rubbing or washing. The marks could only by erased by actually scraping a thin layer off the writing surface.