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Screen-Plate Patents 483
or chemically stained fluxes and fuse on to glass. A. Montagna and G. Ascoli35 also patented the use of glass which contained a trace of arsenic acid, and this was powdered with a mixture of blue and crimson, or violet or green, low-smelting grains, then fired in with a yellow mass. The previously prepared side was then coated with a silver chloride emulsion, the plate exposed through the back and the unreduced silver chloride dissolved and the plate again heated. Obviously the silver emulsion was to give the familiar yellow-flashed glass. Wieland, Hamm and Mohr36 also proposed glass globules having different melting points so as to form an even surface without interspaces, or mixed colors when fired. It was pointed out37 that McDonough and Palmer had proposed to use glass elements.
F. M. Duncan38 patented the use of particles of gum tragacanth or allied gums and also the use of dichromated fish glue. A. Borrel and P. E. Pinoy39 would use microbes, staphylococcus, etc., killed by heat and stained. Later40 they claimed yeast cells, though this had been patented by the Lumieres. The Chemische Fabrik B. T. Silbermann41 proposed the use of stained particles of calcined magnesia. The Farbenfabrik vorm. Bayer42 suggested a multi-color screen of irregular pattern, in which a bottom layer was covered with particles, and in any interstices the layer was stained, this last color depending on the tint which the screen as a whole should have.
A. Wiebking43 proposed to spray dissolved resinous or colloid substances into a liquid that should coagulate them; for instance, in the case of gelatin, ferric chloride, chrome alum, tannin, etc., might be used; and in the case of alcoholic solutions of resins, water. E. Fenske44 introduced the "Aurora" plate, which was prepared by sifting fine particles of dyed materials on to a tacky surface. The particles were rather unequal in size and there was a deficiency in green. A black filling was used to fill interstices. P. H. Uhlmann45 provided a negative with a grainless matt layer of slightly adhesive material, to which color particles would adhere, and the use of a tacky matter was also claimed.40
Dichromate Processes. — The hardening action of light on dichromated gelatin and the ready absorption of dyes by the latter would naturally lead to its use for making screen-plates, particularly as extremely thin films can thus be obtained. Ducos du Hauron (see p. 456) was the first to suggest this and C. L. A. BrasseUr47 seems to have been the first to copy him. A suitable support was substratumed, flowed with colored gelatin, or this might be colored afterwards, sensitized with dichromate and exposed with a black and white screen as suggested by du Hauron. After printing an insulating varnish or coat of colorless gelatin might be applied and a second coat of colored gelatin be applied, sensitized, printed and again insulated and the third applied. It was stated that screens having over 500 lines to the inch had been thus prepared. The necessity of using a compensating