Niépce turned his attention to other substances that were affected by light, eventually concentrating on Bitumen of Judea, a naturally occurring asphalt that had been used for various purposes since ancient times. In Niépce's time, it was used by artists as an acid-resistant coating on copper plates for making etchings. The artist scratched a drawing through the coating, then bathed the plate in acid to etch the exposed areas, then removed the coating with a solvent and used the plate to print ink copies of the drawing onto paper. What interested Niépce was the fact that the bitumen coating became less soluble after it had been left exposed to light.