What material did Joseph Nicéphore Niépce use to coat his pewter plate for the Heliography process?

Answer

Bitumen (a form of petroleum asphalt).

Niépce's Heliography method used bitumen, a form of petroleum asphalt dissolved in lavender oil, which was coated onto a polished pewter plate.

What material did Joseph Nicéphore Niépce use to coat his pewter plate for the Heliography process?
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