TIMING AND EXPOSURE.
The time of exposure to light is measured by means of an actinometer, which is simply an instrument in which is a strip of sensitive paper, exposed under a graduated series of different thicknesses of translucent paper, each division or gradation being distinguished by a number, thus: No. 1 has only one thickness of paper over it; No. 2, two thicknesses, and so on. But on account of the ever varying intensity of the light, a screen actinometer that is reliable is almost an impossibility. The best must be closely watched in a bright light, so that the tints are changed evenly.
Johnson’s actinometer is very simple and handy; it consists of a cubical box with two lids, the inner one serving to press the paper in contact with the glass of the outer lid; the outer lid has a circular opening with a narrow rectangular strip in the centre transparent, the rest of the glass being covered with pigment the color assumed by the sensitive paper after exposure to the light. When the strip of sensitive paper inside has assumed this color, it is called one tint; the strip of paper is then pulled forward, and another portion, quite white is brought under the transparent portion of glass, and when the light has turned that the color of the pigment, that counts two tints, and so on. The paper used in the actinometer is ordinary albumenized paper.
The exposure of zinc in the printing-frame under a good line negative, will be about six or eight tints in diffused light, but in direct sunlight the exposure may be timed by the watch, a suitable line negative never requiring more than from three to five minutes. {44}
INKING THE EXPOSED ZINC.
The sensitive zinc having been exposed to light under the negative, next cover the surface with a thin coating of litho-transfer ink, stone to stone.
For this purpose we require a type-printer’s composition roller, mounted upon a litho stock. This roller must be perfectly smooth and cast in a solid mould, as the line along the side, formed by a split mould would render it useless for inking up a zinc plate. A roller eight inches long by five or six inches in diameter is a useful size. We also require an inking slab, a palette knife, a bottle of turpentine, with the cork cut so that the turpentine can be sprinkled out without having to remove the cork, a tin of transfer ink, and a linen cloth or two.
The inking slab may be made of smooth iron, or a suitable lithographic stone is good.
CHAPTER V. PRINTING ON THE ZINC IN ALBUMEN.
The negative being ready for printing, select a piece of zinc a little larger each way than the picture, polish, then grain, and after well washing it under the tap and gently rubbing it with a fine sponge, or a pledget of cotton-wool, place it in the whirler; again rinse under the tap, now invert the whirler and set it revolving; then examine the surface of the zinc and see that there is no dirt on the surface, but that it is quite smooth. If it is not, rinse it again under the tap, and rub it gently with the sponge, and again rinse. This should quite free the zinc from any deposit of dirt. Now whirl it so as to get rid of as much as possible of the water, then pour over the surface of the zinc sufficient to cover it, of the following solution (carefully filtered through cotton-wool):