| No. 1. | ||
|---|---|---|
| Bichromate of Potash | 1 | ounce. |
| Water | 20 | ounces. |
| No. 2. | ||
| Chrome Alum | 1 | ounce. |
| Water | 20 | ounces. |
The temperature of the drying box must not exceed 80° F., and the time occupied will be from thirty to forty hours, during the whole of which time the temperature must be steadily maintained, but not exceeded.
When the skin is dry, with the point of a penknife separate the edges from the glass, and then lift the skin away, when it is ready for exposure to light under the negative. If it is desired to keep the skins a few days before using, they may be stored between sheets of dry brown paper in a drawer, or in a tin tube. {158}
It will be just as well to remark here that the dry heliotype skins are sensitive to light; therefore, the drying oven must be in a room illuminated by yellow light, or by artificial light, and the skins must not be exposed to daylight until they are in the printing frame.
PRINTING THE PICTURE.
The heliotype skin is now ready for exposure under the negative, which is done in a printing frame 24 by 18 inches, provided with a plate glass front.
The negative must be reversed (i. e., instead of being taken direct in the usual way, a reversing mirror is used). The margin of the negative must be protected by a mask, with an opening cut the size and shape of the intended picture, the outside edges of the mask being sufficient to cover the rest of the skin.
To print, the back of the negative is cleaned, then laid with the film uppermost, in the centre of the plate glass front of the printing frame; the mask is then laid in position, the opening in the centre being adjusted so that exactly the amount of picture required will be printed upon the skin. The rest of the skin is protected from the action of light by the opaque margin of the mask. Now lay the skin, with the matt surface (i. e., the side that was in contact with the ground side of the glass plate during the process of drying) next the film of the negative; upon this lay a piece of dry black velvet, then a sheet of thick felt, then the back of the printing frame, fasten up, turn it over and see that the front of the glass is clean; then the frame is ready for exposure to light.
EXPOSURE TO LIGHT
should be made in a good diffused light, and its duration timed by means of an actinometer (see page [107]).