The tracing, made with very black ink, is placed in the printing frame, the drawing in direct contact with the plate; then place over it the sensitised paper, the prepared side in contact with the back of the tracing. There is no necessity to make use of photometric bands as the progress of insolation is sufficiently seen on the sensitised paper during the exposure. From yellow that it was it should become perfectly white in the clear portions, that is to say, upon which there is no drawing of the transfer or positive cliche that is to be copied; this is ascertained by raising from time to time the shutter of the frame. The exposure lasts 10-12 minutes in the sun; in summer less, in winter more. When the exposure is ended remove the print from the frame, and it should show a yellow drawing upon a white ground. If in the sensitising bath a few cubic centimetres of a rather highly concentrated solution of sulphocyanide of potassium have been added, this bath becomes blood-red and colours the paper the same: in this case the print also whitens during exposure, but then the image, instead of being yellow, is red on a white ground.
This substance, however, is, if we may so speak, inert, or without any other action; it is very fugitive, and even disappears in a short time in obscurity; it has no other use, therefore, than to render the drawing or the image more visible after exposure.
When the print has been sufficiently exposed it is taken from the pressure-frame and floated for a minute in the following solution, so that the side upon which is the image should alone be in contact with the surface of the liquid, avoiding air bubbles between the two surfaces. Otherwise defects would be found in the print; to ascertain this, raise in succession the four corners. The developing bath is composed as follows:-
Gallic acid (or tannin) .31-46 gr.
Oxalic acid ...1 1/4gr
Water ....34 oz.
In this bath the orange yellow or red lines are changed into gal late or tannate of iron, and form, consequently, a veritable black writing ink, as permanent as it. The print is then plunged into ordinary water, well rinsed, dried, and the print is now finished. The violet-black lines becomes darker in drying, but unfortunately the ground which appears of a pure white often acquires, in drying, a light violet tint. For prints with half tones this is of no importance; but for the reproduction of plans, for example, it is very ob-jectionable. By this process we have the satisfaction of obtaining a drawing in black lines similar to the original, and in most cases this is sufficient.
(41) The Papier Zeitung gives the fol-lowingdirections for making an improved "graph ": - Soak 4 parts of best clear glue in a mixture of 5 parts pure water and 3 parts ammonia (presumably liquor ammonia) until the glue is thoroughly softened. Warm it until the glue is dissolved, and add 3 parts of granulated sugar and 8 parts of glycerine, stirring well and letting it come to the boiling point. While hot, paint it upon clean white blotting paper, with a broad brush, until the blotting paper is thoroughly soaked and a thin coating remains on the surface. Allow it to dry for 2-3 days, and it is then ready for use. The writing or drawing to be copied is done with the usual aniline ink upon writing paper. Before transferring to the blotting paper, wet the latter with a sponge or brush and clean water, and allow it to stand one or two minutes. Place the written side down and stroke out any air bubbles, and submit the whole to gentle pressure for a few moments, remove the written paper, and a number of impressions can then be taken in the ordinary way. When the impressions begin to grow weak, wet the surface of the "graph " again.
This "graph " does not require washing off, but simply laying away for 24-36 hours, when the surface will be ready for a new impression.