Saturday, 30 January 2010
John Cimon Warburg
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.