How to make an Autochrome The Plate 1. The original Glass Plates were slightly concave. Today this would not be an issue today but originally the emulsion was applied on the concave side. The plates varied from 0.9 mm to 1.8 mm thick. 2. The Plate was washed and a coating was spread to enable the starch grains to adhere. There are two possibilities for this coating. There is reference to a beeswax and pitch mixture and also a latex coating that remained tacky after drying. The starch The starch grains were extracted from potatoes. They are ovoid and clear. In there normal shape the walls create a shadow around the perimeter of the cell and the shape of the cells polarize white light such that a red and blue hue is seen at either ends of the cell as viewed through a microscope. When flattened on the Microscope slide with my fingernail - as Lumiere had discovered by accident, the cells completely flatten and lose both the border shadow and their polarizing property. This was beneficial to the quality of the Autochrome plates. The starch grains were then sorted to be between 10 to 15 microns. Many references quote approximately 7000 grains per square mm. I believe this number is arrived by the following formula: A micron is .001 of a Millimetre. The most grains per square mm would be the smallest size .010 mm This gives 100 grains per linear mm. Square it for a total area of = 10000 Grains The least possible grains would be the largest size at .015 mm This gives 66.6 grains per linear mm. Square it for a total area of 4435.56 Grains If you average the two numbers you get a result of 7217.78 Or approx. 7000 grains per square mm. The grains were then dyed. The colours were green, violet, orange-red. After drying the grains were sorted to appear neutral grey when view by reflected light. The approximate proportions were 40/35/25 G/V/OR. After thorough mixing the grains were dusted on the tacky glass plate. Next the starch was flattened by a machine that had a needle that swept back and forth across the plate while rotating. In fact, it is possible to see these lines in the historic autochromes, usually in the light or sky areas of the picture. Although many articles state that the effective pressure was about five tons per square centimetre, it is possible to manually flatten the grains with much less pressure. Next a dusting of Carbon black was used to fill the spaces between the flattened starch grains. This was to prevent white light from passing through the gaps in either direction - both for exposure and viewing. The freshly applied grains were then sealed with varnish. Varnish protected the starch and enhanced the optical characteristics of the starch screen. The Emulsion: The next step in the process is to apply a pan-chromatic bromo-iodide silver gelatin emulsion on top of the varnish. The key to this step is a good pan-chromatic formula. Pan-chromatic means a black and white emulsion that is sensitive to all the colours of the visible spectrum. This is important so all colours could be acurately reproduced after development. The plate was then cooled and dryed and boxed for sale. The Exposure: The plate was placed into a view camera that accepted glass plates. A yellow filter was fitted in front or behind the lense. This filter compensated for the emulsion's sensitivity to violet and blue rays. The emulsion had an effective sensitivity of 4 ASA. The emulsion was actually faster but the starch screen and yellow filter reduced the plate's overall sensitivity. The Development. The first developer was either a Metoquinone or Pyro solution. Next the image was reversed by immersing in a Potassium permanganate and sulphuric acid solution - and then re-exposed to white light, and in hot weather hardened. A Dianol solution was used next to re-develop the reversed image. After rinsing and drying varnish was applied and dried to protect and enhance the image. Finally a protective glass cover plate was afixed to the protect the emulsion. The finnished slide was viewed in a diascope or latern projector. Easy!