| A. | Gum arabic, best quality | 50 parts |
| Water | 170 parts | |
| B. | Tartaric acid | 12 parts |
| Water | 80 parts | |
| C. | Ferric chloride solution at 45 deg. Baumé | 35 parts in volume |
Mix gradually B to C, then C, by small quantities, in agitating briskly. It is important to prepare the solution as directed, for by adding the ferric chloride before tartaric acid, the gum arabic would be at once coagulated. When the ferric chloride is mixed, the solution at first thickens, but [pg 38] becomes sufficiently fluid for use in a certain period. It does not keep, and should be employed the day it is made if possible.
The paper, which should be well sized and calendered, and which, when not giving good results by too much absorbing the sensitive solution, must be starched as before directed, is coated either by brushing or by floating. By the first method a roll of paper five yards long can be prepared without great trouble, and give, perhaps, better results than if prepared by floating; but the latter method is by far the the most convenient: one does not generally prepare by brushing sheets of paper larger than about 30×40 inches.
For brushing, the paper is pinned on a board, then, with a large badger brush dipped in the sensitive solution, the latter is applied as evenly as possible; after which, by lightly passing the brush over, the striae are removed, the coating well equalized, and the paper hung up to dry. The coating should not be very thin, and, above all, not too thick, for then it would require an unusually long exposure to allow the light acting through the whole thickness of the film, which is a sine qua non to obtain a clear ground, i.e., not stained blue.
To prepare by floating, pour the solution in a shallow tray, which needs not to be more than 20×34 inches, 30 inches being the width of the drawing paper usually employed; then roll the paper and place it on the solution. Now, taking hold of it by two corners, draw it out slowly: the paper will unroll by itself. This operation can be done by diffused daylight, but, of course, the paper should be dried in a dark room. It dries rapidly. Endless rolls are prepared by machinery. To expose, the drawing is placed in the printing frame, face downwards, and the sensitive paper laid over it. The whole is then pressed into contact by interposing a cushion between the lid of the frame and the paper, and exposed so that the rays of light fall perpendicularly upon it.
The cyanofer preparation is quite sensitive. From half a minute to two minutes exposure, according to the intensity of the light and the thickness of the coating, is sufficient in sunshine to reproduce a drawing made on the ordinary tracing paper. In the shade, by a clear sky, the exposure is about [pg 39] five times longer, and varies from half an hour to an hour and more in cloudy weather, but then the design is seldom perfectly sharp.
The progresses of the impression is followed by opening one side of the printing frame and examining the proof. The exposure is sufficient when the paper is tinged brown on the parts corresponding to the ground of the design. The image appears then negative, that is, yellowish on a tinged ground.
Another and more safe method of ascertaining the correct time of exposure, which can be employed concurrently with the other, is to place a few strips of the same sheet of sensitive paper between the margin of the design, upon which a few lines have been traced, and the paper, and, without opening the frame, to draw one of them, from time to time, and dip it in the developing solution. If the whole strip be tinted blue, the proof is not sufficiently exposed; but if the lines soon appear with an intense coloration on the yellowish ground of the paper, and the latter do not turn blue in a minute, at the most, the exposure is right. By excess, the lines are with difficulty developed or broken.
For developing, we provide with three wooden trays lined with lead or gutta-percha, or, more economically, coated with yellow wax. The wax is melted, then applied very hot, and, when it is solidified and quite cold, the coating is equalized with a hot iron, whereby the cracks produced by the contraction of the wax when cooling are filled up.
One of these trays should contain a layer, about three-quarters of an inch thick, of an almost saturated solution of potassium ferrocyanate (the developer); the next be filled with water, and the third with water acidified by sulphuric acid in the proportion of three per cent. in volumes.