Technical Details
How does it work?
Autochrome plates are covered in microscopic red, green and blue-violet coloured potato starch grains (about four million per square inch). When the photograph is taken, light passes through these colour filters to the photographic emulsion. The plate is processed to produce a positive transparency. Light, passing through the coloured starch grains, combines to recreate a full colour image of the original subject.
Making Autochromes
The manufacture of Autochrome plates, undertaken at the Lumiere factory in Lyon, was a complex industrial process. First, transparent starch grains were passed through a series of sieves to isolate grains between ten and fifteen microns (thousandths of a millimetre) in diameter. Many different types of starch were tried, but the humble potato was found to give the best results. These microscopic starch grains were separated into batches, dyed red, green and violet, mixed together and then spread over a glass plate coated with a sticky varnish. Next, carbon black (charcoal powder) was spread over the plate to fill in any gaps between the coloured starch grains. A roller submitted the plate to a pressure of five tons per square centimetre in order to spread the grains and flatten them out. On every square inch of the surface of an Autochrome plate there are about four million transparent starch grains, each one of which acts as a tiny coloured filter. Finally, the plate was coated with a panchromatic photographic emulsion. Following exposure, the plate underwent development to produce a positive transparency. In the finished plate, transmitted light, passing through the millions of tiny red, green and violet transparent starch grains, combines to give a full colour image.
For a detailed account of the manufacture of Autochrome plates, see: Bertrand Lavedrine and Jean-Paul Gandolfo, 'The Autochrome Process - From Concept to Prototype', History of Photography, Summer 1994, pp 120-128.
Autochromes - Dawn of Colour - essay.pdf
Taking Autochromes
Although complicated to make, Autochrome plates were comparatively simple to use - a fact that greatly enhanced their appeal to amateur photographers. Moreover, they did not require any special apparatus. Photographers could use their existing cameras. However, they did have to remember to place the Autochrome plate in the camera with the plain glass side nearest the lens so that light passed through the filter screen before reaching the sensitive emulsion. Exposures were made through a yellow filter which corrected the excessive blue sensitivity of the emulsion and gave a more accurate colour rendering. This, combined with the light-filtering effect of the dyed starch grains, meant that exposure times were very long - about thirty times that of monochrome plates. A summer landscape, for example, taken in the midday sun, still required at least a one second exposure. In cloudy weather, this could be increased to as much as ten seconds or more. Spontaneous 'snapshot' photography was out of the question, and the use of a tripod was essential
Viewing Autochromes
For private viewing they could, of course, simply be held up to the light. However, for ease and comfort, Autochromes were usually viewed using special stands, called diascopes, which incorporated a mirror. These gave a brighter image and allowed several people to look at the plate at the same time. For public exhibition, Autochromes were also projected using a magic lantern. Stereoscopic Autochromes, viewed in stereoscopes, were particularly effective: As The Photographic News noted in 1908: '...when the effect of relief is joined to a life-like presentation in colour the effect is quite startling in its reality. It is not easy to imagine what the effect of anything of this kind would have been on our ancestors. Witchcraft would have been but a feeble, almost complimentary term, for anything so realistic and startling.'