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Screen-Plates — Historical and Theoretical Data 467
E. Ventujol43 gave some very interesting details as to the manufacture of the Autochrome plate. In the first place the starch grains were sifted by means of a sieve made from sheet copper 0.01 mm. thick, which was sensitized with asphalt and exposed under a negative of a black and white screen with clear spaces of 16 microns. The back of the copper plate was varnished and the plate etched with ferric chloride till perforation took place at the holes protected from light. Unfortunately it was found that these holes clogged up, and a system of centrifugal separation was tried without satisfactory results. Finally a system of elutriation was adopted.
The particular method of dyeing was not stated, though the triphenylamin dyes were found to be the best. A colorimeter was designed in which a sector shutter with 60 degree sectors, was used and the sectors were covered with glasses coated with each of the colored grains. By comparison of the resultant light on revolution, with a neutral grey, obtained by various thicknesses of black gauze, the sectors were altered till a complete match was obtained and the angles of the sectors gave the respective quantities of the colored grains. Very finely powdered charcoal was found to be the best filling for the interstices between the grains, and a special machine had to be devised, which would apply this to the plates, without filling the air with powder, which might give rise to spots on the plates. A machine was made which applied the charcoal to the interstices without attaching it to the grains.
The plates were examined daily with the microscope and one day great astonishment was created by the greater transparency. This was found to be due to the crushing of the grains by the roller. Therefore, this crushing was adopted as part of the daily routine, and at first the plates were compressed between a thin sheet of steel and a rubber blanket with 5,000 kilos pressure per square centimeter; but this did not prove a success, and finally a roller of 1.5 mm. diameter with tangential lines was used, and with this the pressure might be adjusted up to the crushing point of the glass. Considerable trouble was found in selecting a suitable insulating medium for the colored stratum, as it must have a refractive index akin to the starch, be not readily fusible because of the heat in projection and the solvents must not attack the dyes. A special machine was built to apply this varnish. The grains run about 6,000 to 7,000 per qmm, or about 140,000,000 to a 5x7 inch plate. The thickness of the tri-color elements is about 15 microns. The emulsion had to have a special character and be of very fine grain, approximately 6 microns, and it must be rich in silver bromide and poor in gelatin.
As regards the sensitiveness of the Autochrome plate and other screenplate emulsions, Mees and Pledge stated that it seemed characteristic that they showed a deep gap in the blue-green, probably due to the fact that