Unidentified group of men having a meal, Nebraska, 19__, George Condra, photographer
Close-up of the iodide intensification causing the yellowing of the glass plate negative, scanned to show the plate as it would appear if you held it in your hands.
Close-up of the same area of the plate scanned as a positive image.
Close-up of table in negative.
Close-up of the table in positive. Notice the Heinz Horseradish bottle with the cork and the stuffing and french fries.
The most likely explanation for the deterioration of this glass plate is mercury iodide intensification. Glass plate negatives were notoriously difficult to expose. Photographers did not have light meters to determine the correct exposures for the plates, so they learned from experience. If a plate was thin after development or underexposed, it was possible to put the plate through a bath of mercury iodide solution to intensify or add density to the image. Often, the photographer forgot to not run the plate back through the developer, although manufacturers recommended this. Over time, the mercury began to deteriorate, causing the iodide to react with the silver, creating silver iodide and a yellow glass plate.
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