As the time of exposure to light will vary according to the quality of the negative, no precise rule can be made, the best way being to get a small negative as near the same density as the other, cut off a piece of the skin, and placing it in a frame, expose to light at the same time as the big frame, and occasionally examine the progress of the small strip of skin in the dark room. A very good idea of the exposure of the big one can thus be had. A heliotype skin is properly printed as soon as the details in the half-tones show themselves. Of course, it should be understood that the small negative must be exposed to light at the same time and place as the big one. {159}
SUNNING THE BACK OF THE SKIN.
The exposure to light under the negative being completed, remove the skin from the printing frame, and lay it with the side that has just been in contact with the film of the negative, upon a piece of black velvet, covering the margin of the skin outside the picture with strips of brown paper—these strips not quite touching the picture; now cut off a strip of the skin, say a quarter inch wide by one inch long, and double it lengthways, then cut a hole in a piece of brown paper and place the doubled piece of skin under it. Now remove the skin to diffused light, and as soon as the shape of the hole can be seen upon the undermost half of the small strip, the sunning of the skin has gone far enough, and it is ready for mounting upon the pewter plate.
The test here given for timing the sunning of the exposed skin is suitable, when a good ordinary negative has been used, but if a negative somewhat flat, or thin, has been used, then less sunning will be required, and if a very intense negative, the time of sunning must be increased, the object of the sunning being to reduce the relief caused by deep shadows.
MOUNTING ON THE PEWTER PLATE.
The pewter plate upon which the exposed and sunned heliotype skin is mounted for printing from, must be well polished, free from scratches or other marks, the size being 21 by 15 by 1⁄8 of an inch in thickness. This plate is polished with a soft duster, then coated with a solution of India-rubber in benzole (about the consistency of sweet oil), the solution being poured on, and flowed to the corners and edges; then the plate is placed upon a levelling stand, and the India-rubber allowed to dry.
This India-rubber solution is made by dissolving India-rubber solution, as sold in tins at any India-rubber warehouse, in benzole or benzoline.
When the solution is made it must be filtered through paper, or strained through five or six thicknesses of fine muslin, as the presence of any grit between the surface of the pewter and the skin would at once ruin the picture.
The India-rubber film being dry, place the pewter plate in a tray of clean water, the water being about a quarter of an inch deep; now trim the skin with a sharp pair of scissors to a little smaller than the pewter (20 1⁄2 by 14 1⁄2), then lay the skin, face up, upon the surface of water, and at once lift the pewter plate from the water, keeping the skin in position by grasping the two top corners with the two forefingers. Now lay the pewter on the bench, and {160} with a nice, soft squeegee bring the skin into contact with the India-rubber film. This mounting must be done rapidly, as if the skin gets a chance of absorbing water it will frill, and be spoilt. The back merely requires moistening, and the squeegee, in expelling the water, brings it into contact with the India-rubber film. Now, with the squeegee, remove all the water from the plate, and then brush round the edges with some India-rubber solution, so as to reduce the chance of water, to be used subsequently, from getting under the skin.