Dating rotograph postcards

Poitevin discovered that a bichromated gelatin-covered plate could be used to produce prints after being exposed to light through a negative. First, a plate was rendered light-sensitive by coating it with warm potassium or ammonium bichromated gelatin and heating it at a steady temperature in an oven until dry.

The hardening of the gelatin resulted in less absorption in the areas which received the most light (those which will appear the darkest in the print).

The plate was soaked in cold water, dried, and before printing it was wet again with a glycerine and water solution.

One variation of these cards were printed as novelties in a 6 by 8 inch format.

They also printed postcards in duotones and tinted monochromes of various colors.

In addition to using its own archive of original negatives, the Albertype Company also reproduced photographic images taken by other companies or individuals.

The Albertype Company was created to take advantage of the commercial applications of the collotype, a type of printing which used photographic negatives.

Collotypes were important to the industry of photographic reproductions because they were fairly cheap to produce, and their range of tones permitted exact reproductions of photographs through a photomechanical process.

They are also noted for their ability to accurately reproduce drawings, prints, and watercolors, and are still in limited use to this day.

Towards the end of the nineteenth and in the early twentieth century, improvements in mechanical presses and a switch to the rotary collotype, a high-speed process using an aluminum plate, meant that up to five thousand collotype prints could be produced daily.

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