Write the figures with good black ink and then cut other pieces of white tracing paper into strips of the same width. {108}
Now lay the piece of tracing paper with the numbers on it, upon a piece of glass 4 1⁄4 × 3 1⁄4 inches, and fix it in the centre, then place thicknesses of tracing paper over this so that space 2 has three thicknesses of tracing paper, space 3 five thicknesses, space 4 seven thicknesses, space 5 eight, space 6 nine, and so on up to 10. Now if a piece of ordinary sensitive albumen paper is put into a frame behind this screen, it will form an actinometer, and when, after exposure to light the figure 5 is just visible, five tints are registered.
This actinometer will with practice enable the operator to judge when the carbon tissue has been exposed sufficiently long; the only guide that can be given, is to err on the side of overexposure rather than underexpose.
The carbon tissue being exposed, it is removed from the frame, and placed in clean cold water; then as soon as it begins to uncurl itself, the copper plate is placed in the water, and the two are brought into contact, the tissue adjusted in position upon the copper, then removed from the water, and a squeegee applied vigorously to the back of the tissue, applying the squeegee first one way and then the other.
Now allow to stand a few minutes, then immerse it in water at a temperature of 90° or 95° F., and in a few minutes the pigmented gelatine will begin to ooze from under the paper backing, which may now be stripped off and thrown away. The copper plate may now be raised and gently laved with the hot water, or it may be allowed to remain until the whole of the soluble gelatine is dissolved away, leaving the picture upon the copper plate, which is rinsed in cold water and dried.
For the purpose of copper plate work, the image must be thoroughly well exposed, every detail being impressed, else it will be impossible to get a good photogravure; therefore, until the operator thoroughly understands the use of the actinometer it will be as well to make a few prints upon a piece of opal glass.
Carbon tissue does not keep in a sensitive condition more than a month, under the most favorable conditions, the general time perhaps being a week. The first sign of deterioration is generally shown when trying to mount upon the copper, after soaking in cold water, the tissue refusing to adhere after the application of the squeegee. The best way to test the sensitive tissue is to take a small piece and immerse it in cold water for a minute, then put it into water at 95°, and, if the coating dissolves off the paper the tissue is all right; if it does not, it is useless. Tissue will generally be found at its best about {109} three days after sensitizing, and will gain rapidly in sensitiveness daily afterward.
Carbon tissue can be made at home, the following being the formula:
| Gelatine (soft) | 10 | ounces. |
| Sugar | 4 | ounces. |
| Water | 30 | ounces. |
Dissolve the gelatine at as low a temperature as possible, then add the sugar and churn thoroughly; then add sixty grains of India-ink, previously dissolved in a little water, again churn, then strain through muslin into a dish (which dish is placed in another containing hot water). Now take two sheets of paper, grasp one end tightly and drag them rapidly through the tissue compound, and directly they are lifted clear of the solution let an assistant grasp the bottom and you dropping the top, will reverse the sheets and keep the gelatine on the surface of the paper; when the gelatine has set, divide the two sheets and hang up to dry; when dry the subsequent proceedings are the same as for commercial tissue.