In 1727 German physicist Johann Heinrich Schulze found that a suspension of chalk and aqueous silver nitrate was blackened by light, forming a negative of the actual image. This primitive work heralded the birth of photography. A photon of light hitting the negative nitrate ion frees an electron, which combines with the positive silver ions to produce stable silver metal, darkening the surface of the material.
In 1835 William Henry Fox Talbot recognised the need for this negative to be transformed into the actual image, which he called a transfer, and for the ability of these transfers to be multiplied. Via trial and error and a little bit of good fortune, he produced his ‘photogenic drawing paper’, containing ~1% sodium chloride and ~20% silver nitrate, which produced an image when exposed to sunlight. To generate a dark enough negative, the exposure time took hours. Talbot believed adding an excess of sodium chloride to his negatives would prevent them darkening further. However, when re-exposed to sunlight, they did gain a thin layer of silver metal, manifested as a violet hue sometimes seen in early photographs. He then experimented with potassium iodide as his fixing agent, but over time his photographs started to fade.
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