Pretty As A Picture
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Pretty As A Picture Part 1: Materials for Black-and-White Photography T h e advent of photography and photographic materials profoundly affected not only scientific research but our view of history, law, and daily life. For science, photographic materials capture permanent images of experiments and phenomena. Photographic materials can integrate incoming light over time and record events too faint or too rapid to be seen by the human eye, or at wavelengths beyond the visible range. This year, 1989, is the 150th anniversary of the rise of the daguerrotype process, the first widespread black-and-white photographic technique that created "pictures" of the world without requiring the skill and patience of a painter. This year is also the centennial of George Eastman's introduction of rolls of flexible photographic film plus the portable cameras to use them, which revolutionized photography and made it available to the general public. This month's Historical Note will look at the beginnings of photographic chemistry and early materials for capturing black-andwhite images on metal or glass plates or on sensitized paper. A s early as 1565 some chemists had noticed the blackening of silver salts on exposure to light or heat. In 1725 German physician Johann Heinrich Schulze placed stenciled letters over a mixture of silver nitrate and chalk, showing that the darkening was caused by light alone. Schulze, however, merely wanted to investigate an unusual chemical phenomenon and did not attempt to develop it into a technique for producing images. In 1777 Swedish chemist Karl Wilhelm Scheele exposed silver nitrate to a spectrum from the suri, and saw that blue and violet light produced a more pronounced effect than did red. Scheele also investigated the light-sensitive properties of other silver salts; he determined the chemical basis for the phenomenon in silver chloride, showing that light reduced the solution to metallic silver and free chlorine. At the turn of the 19th century Thomas Wedgewood published the results of his experiments treating leather and paper with silver nitrate. Working with his partner Humphrey Davy (see the Historical Note in the January 1989 issue of the MRS
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BULLETIN), Wedgewood made silhouette images by placing leaves and other natural objects on top of the sensitized paper—but he had no way to stop the exposure or "fix" the image by dissolving away the unexposed silver salts, and the image turned completely dark in a short time. The demand for inexpensive, quick; but realistic portraits caused 19th century artists and entrepreneurs to look for ways to solve the problem. Some artists created silhouettes, projecting a shadow of their subject onto a drawing surface and then filling in the outline. Lithography, the technique of drawing an image by hand onto a prepared stone surface, etching the stone with acid to allow the "undrawn" areas to absorb ink, and then printing the image onto paper, became popular around 1813.
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