Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone / Also Collotype and Heliotype

by W. T. Wilkinson

MIDDLE ETCHING.

In order to give to the middle tints of the drawing the depth necessary for printing, we begin with a new blacking-in. We need for this purpose excellent, smooth, and hard rollers, which are immersed in minium and linseed oil, and are then left to dry for a short time. Contrary to the French method, we need here a thin composition for the etching ink, obtained by concocting equal parts of paraffine, suet, beeswax, and ordinary ink, prepared in an excess of good linseed oil. (Ready-made color of right composition may be had of E. Liesegang, in Dusseldorf, and other dealers.)

This color, when of right composition, possesses the power of absorbing resin-powder, and changes in combination with the same, when heated, to a compact, very solid, yet brittle mass.

Of this color, which is soft as butter, take a portion on a knife-point, and spread it upon the smooth rollers, forming a perfectly uniform layer. The complete covering is done by rolling, changing the adjustment at intervals. When a perfectly uniform, oily coat is spread on the color-slab and the rollers, begin rolling-in the washed plate.

Do not exert any pressure, but roll, carrying the handles so as to black the relief only from the surface of the rollers. Of course, it cannot be prevented, and is of no consequence, if ink attaches itself to the large white spots. Rolling continues till the oily brown coating has deposited itself uniformly. Exercise care in order not to get more ink on some spots than on others. The plate, now completely rolled, is placed in the finely powdered resin, with which it must be completely covered. For the space of two minutes the oily color absorbs the powder, which should, therefore, be strewn on it and rubbed off repeatedly. If saturation in the cold has taken place, dust off lightly with brush and cotton, blow off well, and heat uniformly to 35° C. The resin should not melt now, but should only grow sticky and combine intimately with the color, which forms the binding material. This change is observed immediately in the change resulting in the gray resin coat. As soon as the change occurs (beware of unequal heating), again place the warm plate into the resin, saturate as before, dust out, blow strongly on the bright, metallic spaces, to remove remaining traces of resin, and heat to 50°–55° C. At this temperature cover the film with fine asphalt powder, dust off as before, and heat to 65° or 70° C. In this condition the plate is again saturated with asphalt, and the layer, after careful dusting and blowing, is heated until it grows slightly {88} brilliant. Thus a complete coating is formed, and the plate has the appearance of Fig. 6. The black spots represent the dirt which has clung to the intensest lights, and must be removed. But before attempting this, we varnish the back of our plate with a solution of shellac, and cool it completely in water.

Fig. 6.

Now take a very coarse scraper, ground with a sharp edge; hold it between the index and middle fingers, and scratch away the dirt from the light spots, making the edge of the scratcher draw the strokes close together. The brittle backing breaks off finely, and in a few minutes the dirt is removed and the plate is ready for etching. If (which will only happen to beginners) the drawing should be scratched, the exposed spot must be covered, by means of a brush, with a thick, warm solution of asphalt in turpentine; the dirt which was scratched loose is brushed off, and the plate goes into the bath for middle etching for about three minutes. Brushing and washing follow in the same way as in the previous operation. At the expiration of three minutes inspect (with or without a magnifying glass); places that have been uncovered must instantly be covered with asphaltum, as already directed. Etching is continued till the lateral coating no longer protects the first relief, which generally takes place after the third period (in about nine minutes) in a new bath, but requires twelve to fifteen minutes in an old one. It is proved by the existence of only a narrow thread alongside of the relief. The moment can be well ascertained. If we wish to be sure of it we may do so by exposing the edge of the black rim at different places; the width of the thread is then apparent, and a safe conclusion may be drawn from the single spots as to the whole plate. If not enough middle etching has taken place, the thread remains wide and requires an extraordinary clean etching, which may lead to the picture growing thin and endanger the finest lines. Care, then, is necessary to ascertain the right moment of suspending the middle etching, and of continuing in the above-described manner with oil of turpentine, brushing, sawdusting, and washing. After drying and brushing off the sawdust with a strong brush, we have the clean plate ready for examination. We observe the grade of etching produced by {89} middle etching parallel to the contours of the different parts, and can safely assert that the process has taken its natural course. Etchings without real deep places (as high lights) might here be etched clean—that is, might undergo removal of the formed etching grade. Yet it is preferable to place the edge of the drawing as low as possible, to prevent a tendency to crookedness, so-called facetting, and to get a deep edge for nailing the plate on wood. The outward appearance of the etching improves considerably, and the cliché can be fastened much more securely. The chemigraphist must endeavor to satisfy the demands of the printer in all respects, and should omit nothing that serves to perfect his work. Thus only can he gain a good reputation both for himself and for his work.

DEEP ETCHING.

Having been successfully treated in the middle etching process, the plate needs only depression of the larger light spots. Some simplify this part of the work by cutting out the highest lights by means of a chisel, reverse ground. This can easily be done after a little practice, and often saves time and acid. Care should be taken with the chisel; the plate should lie on a flat surface and the cutting twist be executed lightly and regularly. If this course is decided upon and is applied to the highest lights, proceed to the execution of the deep etching. Such chiselling does not happen in my laboratory, since I prefer to etch all plates alike to their normal depth.