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

by W. T. Wilkinson

(c) A peculiar method of strengthening, nowhere described, as far as I know, is that of drying the plate after coating it with gum, then washing the gum with water; then, after drying, of covering the plate with a thin solution of wax, asphaltum, rosin, and shellac in ether, and of carefully coating the quickly drying film with gum. The film remains on the lines of the copy, but easily detaches itself from the bright metallic spaces, which have an insoluble combination with the gallic acid and gum, and are thus protected from the fat.

Faults in strengthening are caused by the following:

a. By ink which is too thick or too thin, or which has not been well ground. Stripes are caused by the dissolving of the color which is already present.

b. By excessive dampness on the plate (it does not take the color; the color in the sponge changes to a slimy emulsion).

c. By too great pressure with the color sponge the fine parts are lost. The same is the case with careless washing of the color of hand-drawings.

d. In rolling on excessive moisture on the plate spoils the work. The surface of the rollers becomes glossy and must be again inked.

e. In pouring out the ink a very thick layer prevents the strengthening of the finest lines.

Dirt on the plate often comes from handling the drawings; it must be scratched off and the plate must be perfect before proceeding.

Now comes the changing of the ink into an etching varnish. But before beginning we must cover the free edges of the zinc for the distance of half a centimetre around the drawing with a solution of ink in turpentine; the same applies to large white surfaces in the drawing, which it is more convenient to cut out with a scroll saw. Then subject the plate to the finest asphaltum {79} powder, passed through a hair-sieve, and cover it by the dusting brush copiously with powdered rosin. Continue the rubbing on of fresh powder for a minute, until the ink is saturated, then dust out the superfluous powder with the dusting brush and fine cotton, so that the metal is bright (no film should be perceptible on looking over it).

The plate should now be heated to about 60° C., till the color of the asphaltum changes from a coffee tint to a dead (matt) black. Place the plate in this condition a second time with the asphaltum, which is again absorbed rapidly. Dust off carefully as before and burn in again till the color changes to a somewhat shiny black; the yellow tint must be avoided, because this degree of burning exposes the lights in the closest hatchings to the danger of running together.