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Writing with Sunlight: Recreating a Historic Experiment

This demonstration uses sunlight to write letters on a slurry of silver nitrate and calcium carbonate, recreating an experiment dating to the early 18th century. The letters appear as light silhouettes against a dark-gray background, with the gray due to elemental silver. The demonstration is a striking illustration of the photoreduction of silver ions, the fundamental process in conventional film-based photography. This demonstration kindles the interest of beginning students in oxidation–reduction reactions.