AUTOCHROME was a photographic transparency film patented in America, June 5,1906 (No.822,532) by Auguste and Louis Lumiére of Lyons, France (FR.Pat.No. 339,223, 1903).
Like other techniques of the time, it employed the additive method, recording a scene as separate black and white images representing red, green and blue, and then reconstituting color with the help of filters. To do this on a single plate, the Lumiéres dusted it with millions of microscopic (avg. size 10 to 15 microns) transparent grains of potato starch that they had dyed red (orange), green and blue (violet). ( 1 ) This screen of grains worked as a light filter to interpret the scene when the light passed through them exposing a panchromatic B&W emulsion. The exposed plate was then processed reversal resulting in a transparency. The illustration below is from their American patent application.
( 2 )
Like film today, their's was a composite construction, comprised of four distinct layers in addition to the glass support.
Glass was coated with liquid pitch (def.1) mixed with a small percentage of beeswax (to help keep it "tacky") then the prepared grain was dusted on. By this very action, the resultant screen was stochastic (or random) in nature. In order to comply with the first black condition (def.2) it was necessary to fill the spaces between the irregularly shaped grains. Lampblack was used as a filler, applied by way of a special machine. The result is shown in the "enlarged" illustration below
The starch was probably (facts are a bit sketchy) dyed using triphenylmethane dyes (note 1) to achieve a color wavelength of between 550 to 670 for the red, 470 to 570 for green, and from 430 to 520for the blue.
Later, the Lumiéres discovered the transmission quality of the plates could be improved by applying pressure (5000Lbs. per sq. inch (3) to the composite prior to the addition of lampblack. Potato starch grains are not flat, but somewhat rounded, and in my opinion, their method of elutriation (def.3) contributed to the puffy condition of the starch.
The next stage in construction was to coat the composite with liquid shellac to totally encapsulate the grain layer (in essence, forming an envelope of amber around the grain).
After drying, the panchromatic B&W emulsion was then coated on the composite plate and the final plate was soon ready for market.
The plate was exposed in a glass plate type view camera by placing it in the holder with the coated side away from the lens, so that when exposed, the light traversed the glass, through the grain and exposed the light/color sensitive emulsion from the back. After exposure, the plate was processed to reversal in an acid dichromate type process.
The final photograph has a beautiful look with wide tonal gradation and if you could see an original well preserved Autochrome today, you would be amazed at the extrodinary way they age, and can in fact appear as though they were processed only yesterday. The image below is an original Autochrome, photographer unknown.
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