Pressure must not be employed, but the coating must be obtained by renewed working out of the rollers on the color-slab, and repeated coating. When at last the desired strength is obtained, dust with the finest asphaltum powder, blow the traces of dust out of the hollows, and warm till the layer becomes slightly brilliant. Next, etch in a new bath of 4000 c.cm. of water and 40 c.cm. of acid for 7 minutes, wash, and dry well; then, with more ink on the rollers, coat the dried plate, dust once with resin, heat gently, then apply the asphaltum and burn in strongly. Thus the first relief is protected laterally, and the first grade can be etched down completely, without endangering the drawing. To remove the second grade, we repeat washing, and put on etching ink mixed with one-third wax. Rolling must continue till the narrowest spaces of the etching are completely closed. Then follow dusting with resin, warming, and placing the warm plate in the asphalt, dusting, removing the excess, and finally burning in.
With this coating a further rounding of the grades can take place (in a bath of 10° Baumé). Continue this process until all the grades are removed and the etching appears as perfect as a cast plate.
Since hitherto all publications on chemigraphy have duly described this method, I believe that I should not enter into further details, improvements, etc. Suffice it to call attention to weak points. In view of the new method later to be described, the reader may make a comparison between the two.
A very weak point is the method of strengthening based on the lithographic process of superimposing new color upon the old color. In consequence of the repeated melting in of resin, this latter acquires an uneven surface, which can be blackened in only with great difficulty. Equally untrustworthy is the lateral protection by melting off, which is only attained with safe uniformity by skilled workers. And what is more, with the weak coating at the beginning; even weak acid often works its way through to the etching, so that the finished etching appears slightly porous, and gives no dense black print. Also shaking, which is to promote the removal of the metallic mud and the gas bubbles from the lines of the etching, has an unsatisfactory effect, inasmuch as the single lines are often thus under-washed.
But the principal defect is that no accurate determination of the degree of each operation can be made, everything depending on the delicate execution of the process.
The time that the etching of a correct, deep cliché requires by this method is from eight to ten hours. This procedure also demands better trained hands {84} than the Austrian, which, in large establishments, is based on a division of labor, and in which men trained in a single manipulation, under good supervision, accomplish the maximum work surely and excellently.
Furthermore, the expenses of the manufacture on a large scale are far greater, since each etcher draws a greater salary than a workman skilled in a few manipulations, such as the following method requires.
THE AUSTRIAN METHOD—THE ETCHING ON.
I have already shown how a transferred plate should be prepared for etching by this method. Before proceeding to the details, however, we will prepare the etching baths, which, by the way, may always be employed for several plates. Corresponding to the three steps of etching, we require three baths of different concentration to be determined by means of an areometer with a Baumé scale. The bath for etching on is prepared in strength of 2° Baumé at a temperature of 18° C. There must be a quantity sufficient to cover the plate to a depth of 4 to 5 centimetres. The bath for middle etching requires 10° Baumé and a temperature of 18° C.; the bath for deep etching, 18° Baumé. These baths are prepared at the beginning, and are used for the five successive stages of etching. If, after continued use, the bath for deep etching should grow weak, it may be used for middle etching. For protecting the trays, I recommend a coat of gutta-percha over a well-coated inner surface of asphaltum. Everything being ready, immerse the prepared plate in the bath, and with an etching-brush brush the surface of the plate uniformly for two minutes.
The hairs of the brush must only rest lightly on the metallic surface. For determining time, use sand-glasses of two, three, and five minutes. After two minutes, take the plate out of the acid, wash it well with water, and examine it carefully. If the effect has not been uniform, see that two minutes more can be safely given in the etching bath. If by any sort of careless handling the plate has been injured, dry it with blotting-paper, vaporize the remaining dampness, and cover the soiled spot with very thick solution of asphaltum in turpentine. Cool, and continue brushing lightly for two minutes in the bath.