Niépce dissolved bitumen in lavender oil, a solvent often used in varnishes, and thinly coated it onto a lithographic stone or a sheet of metal or glass. After the coating had dried, a test subject, typically an engraving printed on paper, was laid over the surface in close contact and the two were put out in direct sunlight. After sufficient exposure, the solvent could be used to rinse away only the unhardened bitumen that had been shielded from light by lines or dark areas in the test subject. The parts of the surface thus laid bare could then be etched with acid, or the remaining bitumen could serve as the water-repellent material in lithographic printing.