When W. H. F. Talbot began experimenting with silver chloride papers in the mid-1830s his first images were no more advanced than those made by T. Wedgwood in 1802. Talbot’s first contribution to the evolution of negative/positive photography was alternate applications of silver nitrate and sodium chloride solutions to the paper. More important was the stabilization of silver chloride images with a strong solution of sodium chloride, potassium iodide, or potassium bromide.
Stabilization did not actually remove the unexposed silver halides, but made them much less sensitive to light. Extended exposure to light eventually turned the highlights of chloride stabilized prints blue and iodide prints yellow. By making the negatives less sensitive to light Talbot was able to superimpose them with a second sheet of sensitive paper, expose them to sunlight, and made a positive image that was also stabilized. Both negatives and positives were called photogenic drawings when stabilized.
The photogenic drawing negative was superseded by Talbot’s calotype process patented in 1841. Talbot continued to make stabilized photogenic drawing prints from calotype negatives despite their questionable permanence. It is assumed that he did this because the colors of the prints were more attractive than hypo fixed salted paper prints which were shades of brown.