Here Robert Heinecken created portraits of the American writer Susan Sontag using images of pages from her widely read book On Photography and snapshots from his own everyday life. A few fragments in the collage have turned brown due to insufficient application of a protective coating that stops oxidation (a type of deterioration caused by oxygen). Indeed, the type of black-and-white Polaroid film Heinecken used here required the immediate application of a protective coating to prevent oxidation of the silver in the image. The photographer Ansel Adams, who tested products for the Polaroid company, noted that in downtown San Francisco, pollution caused uncoated prints to begin discoloring within an hour.