Manufacturer and builder 9, 1881
Dr J. M. Eder describes in the Photographic News an improved process of cyanotype printing for copying drawings, designs, tracings and the like, which is said to give excellent results. We give below the essential parts of the description. Thirty volumes of a solution of gum arabic (5 parts of water to 1 part of gum) are mixed with 8 volumes of an aqueous solution of citrate of iron and ammonia (water 2, salt 1), and to this is added 5 volumes of an aqueous solution of perchloride of iron (water 2, iron 1). This mixture rapidly thickens, and should therefore be applied with a brush quickly after preparing it, to well sized paper. The paper is dried in the dark, and then exposed to the light under the tracing or drawing to be copied. A few minutes exposure to good light is sufficient, and the print should then be developed by brushing over the surface with an aqueous solution of the ferro-cyanide of potassium. The picture appears almost instantly in dark blue. As soon as it appears distinct in all its details, it is quickly rinsed in water, then immersed in a bath of very dilute hydrochloric acid, which strengthens the image, whitens the ground, and removes the gum-iron film. The washing of the print now in water completes the process.
A simple and satisfactory printing process for copying tracings, etc., is often very desirable to architects, machinists, engineers and others, and the one here described would appear to meet the case very satisfactorily. The whole operation, including the preparation of the paper, is said not to take up more than an hour or two in fair weather. The older blue printing processes were unsatisfactory, as the ground was invariably left more or less blue by the running of the color. The use of gum in the present process avoids this objection.